HRV vs. ERV in Cold Climate
Ine
| Posted in Mechanicals on
I’m building a well insulated house (R-40 walls and R-60 roof) in a 9500 heating degree day climate (Upper Michigan) and will not have air conditioning. Most sources seem to suggest an HRV in this situation but I have had several people recommend an ERV as the better choice. What would I gain with an ERV in my situation, I thought ERVs were for air conditioning climates?
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Replies
Donald,
Either piece of equipment should work fine.
Some cold-climate homes get too dry during the winter. For such homes, an ERV may help a little -- by transferring a small amount of moisture from the outgoing stale air to the incoming fresh air.
Although ERV manufacturers are strong believers in the value of humidity transfer between air streams, most energy researchers have concluded that the results are not very significant. That said, most purchasers of HRVs and ERVs are happy with their equipment.
Most authorities recommend HRVs in all but Mixed-Humid and Hot-Humid climate zones. In a house as tight as yours is likely to be, high winter humidity is going to be the problem. An ERV will cycle the unwanted humidity back into the house rather than eliminate it.
The primary advantage of an ERV is not the minor latent heat savings in cold climates but the dehumidification energy savings in a humid summer climate.
See: http://www.fantech.net/iaq23.gif
Robert,
According to researchers who have measured the dehumidification effects of ERVs during hot humid summers, the effects are minor — so minor they are hard to measure. The source of my information is Max Sherman from LBNL.
Martin,
I've been wondering about humidity and pollen and mildew in ERV's in out hot humid North Carolina climate. In the past I've used HRV's because the foil core made sense to me intuitively.
I never liked the desiccant wheel type we used to get from Honeywell but was really intrigued by the new (to me) Venmar ERV's that seem to use the HRV counter flow principle with a moisture permeable separator. Have you heard of any adverse issues with the moist membrane and imperfect air filters leading to dust accumulation and mildew growing inside the permeable membrane ERV's?
I've seen mildew growing on my stainless steel potting bench and even vinyl soffits down here.
Michael,
I've never heard reports of dust or mildew problems with Venmar ERVs. Of course, every HRV and ERV manufacturer recommends a maintenance schedule that includes cleaning or replacement of the unit's filters; these recommendations must be followed.
To my mind — I'll probably get flak for this — an ERV is an idea that makes sense in principle but turns out not to make much of a difference in the real world. Yes, it makes sense to add moisture to incoming air when the outdoor air is very dry, and it also makes sense (if you own an air conditioner) to try to pull out some of the moisture from the incoming air by using the exhaust air — which should be a little dryer than outdoor air — to take away some of that moisture.
The only problems with the idea are math and physics. In areas of the country where air conditioning is common, one of two situations applies. Either
1. Air conditioners work pretty well at keeping indoor air dry, or
2. Air conditioners can't keep up with latent removal, leading to periods of high indoor humidity.
Situation 2 occurs in Houston. If you are somewhere where the outdoor air is so humid that you have this problem, an ERV ain't going to solve it. There's just too much moisture involved for the ERV to make much of a difference. Researchers can't measure any discernable difference in indoor humidity from using an ERV — because there isn't any.
Hi There!
I have to answer this post due to it's complete lack of insight into the maths and physics of the ERV technology. When people state that "most energy researchers have concluded that the results are not very significant", for some reason they always fail to provide a source for this information.
That air conditioners work fine is not surprising. The question we want to raise is the use of energy. Considering the fact that evaporating 1 liter of water requires about the same energy as keeping a 60Watt lightbulb lit for 11 hours (approx 2500KJ), and adding to this the observation of how much water actually drips from your AC (measure it!), the conclusion is impossible to misinterpret - if your AC condenses 2 liters in a night, you could have had your outdoor lights on for this time instead if you used ERV technology (even taking COP into consideration). This is not fiction or guesswork, but maths and physics at its simplest.
Furthermore, Martin states that "There's just too much moisture involved for the ERV to make much of a difference", which is a complete misconception. Since an ERV works on the principle of vapor pressure difference, the higher the difference in humidity and temperature between the air streams, the larger the driving force of humidity through the membrane, which consequently increases efficiency. The truth is, the more humid the climate, the more energy is saved by an ERV.
A common misunderstanding though, is that an ERV works as a standalone unit, which is only partly true. If there is nothing keeping the temperature and humidity down indoors, there is no differential in vapor pressure between the air streams, and consequently, no exchange of humidity between the air flows. In short, an ERV should work in concert with a properly sized air conditioner.
I sure hope this advisory forum will be better technologically updated in the future, since with this kind of advice, there is simply no way we will reach the climate goals required to maintain our standard of living in the future. We need to cut down energy use, and that is not accomplished by advising people to just use air conditioners and telling them that technologies like the ERV does not work.
For reference:
http://www.ahridirectory.org/ahridirectory/pages/erv/defaultSearch.aspx
(This is an independent test organisation that exists in order to give you, the consumer, a fair chance of comparing performance between eg HRV and ERV models. just choose "Packaged" and a producer and you will see the test results and efficiencies for that model or brand.)
To conclude, ERV is the future of most heat exchange. There is no point in leaving more than 50% of the energy for the birds, no matter where the technology is used. ERVs DOES work very neatly, ESPECIALLY in humid areas. In winter though, one does well to note that a bypass function may be needed for ventilating excess indoor humidity in extreme cases.
Regards,
Johan
Johan,
In a hot, humid climate, either an air conditioner or a dehumidifier can be used to lower indoor humidity levels.
However, an ERV will not lower indoor humidity levels. In fact, ventilating with an ERV will raise indoor humidity levels. The more you ventilate, the worse the effect.
About the best thing you can say about ventilating with an ERV in a hot, humid climate is that (in terms of helping indoor humidity levels) it isn't quite as bad as an HRV.
Johan, your statement -- "When people state that 'most energy researchers have concluded that the results are not very significant,' for some reason they always fail to provide a source for this information" -- does not apply in my case. As I wrote on this page, "The source of my information is Max Sherman from Lawrence Berkeley National Laboratory."
Max Sherman is the country's leading researcher on residential ventilation issues, and was the longtime chairperson of the ASHRAE 62.2 committee focused on developing a residential ventilation standard.
The study that Martin refers to was a simulation, not a field study, used just a single set of operating parameters for the theoretical HRV and ERV equipment, and considered only one hot/humid climate - that of Houston (acknowledging that conditions in south Florida, for instance, would be significantly different). However, it did show that an ERV in Houston would reduce the number of hours of peak indoor humidity (over 70%).
Other studies by equally reputable researchers, such as Stephen C. Turner, P.E. - a mechanical engineer at Brown University, Providence, R.I., a voting member of SSPC 55, Chairman of ISO TC 205 Building Environment Design, and on the R.I. State Building Code Board - publishing in the ASHRAE Journal, suggest that with the proper mix of HVAC equipment, an ERV can lower indoor RH and overall operating costs (though he was describing commercial applications).
Every study has its own limitations and no single study can be used as "gospel". Another review of ERV problems, which included a field study, by Bruce Davis - a building science specialist with Advanced Energy of Raleigh, North Carolina - indicated that most ERV deficiencies are system design and installation problems. For instance, he recommends that an ERV never be integrated with the HVAC system, never include the bathroom or kitchen (separate exhaust fans), be designed for continuous operation (most efficient), have their airflows checked for consistency with specs, and separate the outside intake and exhaust ports to prevent crossover.
Another report indicated that the airflow of an ERV can be reduced as much as 50% after the first month of operation by dust in the filters and media.
It still remains true that the industry universally recommends HRVs for northern climates and ERVs for mixed/humid and hot/humid climates, with appropriate defrost mechanisms for freezing weather. But, the technology and sensible and total efficiency of any particular piece of equipment and how it's integrated with the rest of the heating/cooling equipment and ductwork is what matters most.
Robert,
Your statement — “with the proper mix of HVAC equipment, an ERV can lower indoor RH” — is true.
But so is this statement: “With the proper mix of HVAC equipment, a piano can lower indoor RH.”
It’s the air conditioner and/or the dehumidifier that is lowering the indoor RH, not the ERV. The ERV is making the situation worse.
Not according to the study you reference. In the Houston case, the ERV resulted in fewer hours of peak relative humidity (over 70%). This is, in part, because (as the study states) "Even in many hot, humid climates indoor air has a higher humidity than outdoor for a considerable part of the year."
Remember, moisture transfer in an ERV is dependent upon vapor pressure differential, which is a function of absolute, not relative, humidity.
Note, too, that the leaky Houston house with no mechanical ventilation had considerably fewer hours of peak indoor humidity than the "standard" house without mechanical ventilation. There seems to be a net benefit to letting in outside air.
The cases in which Bruce Davis found the ERV contributing moisture to the house was because it was integrated into the HVAC ductwork with the air-handler fan running whenever the ERV would run. But, when the heat pump cycled off and condensate pooled in the catchment pan, the moving air would pick up this moisture and make indoor conditions worse. Compounding the problem was the bathroom suction duct adding considerable moisture to the ERV outflow, which was partially returned to the incoming air. To make matters even worse, a dramatically unbalanced air flow in one of these "integrated" systems dramatically lowered the moisture transfer efficiency of the ERV.
Robert,
When you wrote, "the study you reference," I'm not sure what study you are talking about. I re-read my posts, and it doesn't seem that I referenced any study.
Martin,
as stated earlier, an ERV was never meant to be a Standalone unit. It is a typical case of synergy between two technologies, since an ERV is inherently a passive unit. It CANNOT lower the humidity by itself, because it needs both a humid AND a dry air stream. Thus, whereas you'd spend $50 on air conditioning without an ERV, you would have spent about $25 if you had used an ERV WITH the air conditioner. That's the whole point of the ERV. It is after all an exchanger..
Along the same working principle, if you have problems keeping humidity down indoors, you can connect an ERV in series with your AC, which then PRE TREATS the air by using the dry and cool (compared to outside) air inside to move away moisture from the incoming air . Thus, the air that your air conditioner needs to cool (and consequently condense water from) will have a lower temperature AND moisture content, which means your air conditioner won't have to condense such vast amounts of water. It is easy to check. Just put a bucket under the draining tube from your AC, and then connect an ERV and see how much less water you get at the same temperature/humidity.
The only time the situation may become as you describe is in winter, if moisture is not vented out but recycled in the ERV. This is the situation when you'd need a bypass.
Martin,
Twice you said, "The source of my information is Max Sherman from Lawrence Berkeley National Laboratory." I assumed you were referring to the report, "Humidity Implications for Meeting Residential Ventilation Requirements", by Iain S. Walker and Max H. Sherman, Environmental Energy Technologies Division, Lawrence Berkeley National Laboratory, April 2007.
That is the report in which he concluded, as you noted, "The use of an ERV did not change the humidity distribution in a hot, humid climate compared to a continuous exhaust system." And it is also the report which I quoted that indicates that an ERV does make a measurable difference in peak humidity events.
Sherman also concluded that "For the hot humid climate of Houston [ASHRAE] 62.2 ventilation increased the average RH by about 5% to 10% RH but reduced the occurrence of high humidity levels above 70% RH.
So, while you choose to emphasize the increase in average RH determined by this single simulation study using a single set of parameters, you seem to ignore the more important finding that the ERV reduced incidents of peak RH, which is what causes moisture problems, occupant discomfort and increased AC or dehumidification energy use.
Johan,
An ERV is hardly a passive device, as it requires energy input and maintenance, has moving parts, requires an internal logic unit to control the defrost or bypass cycle...
And ERV manufacturers often tout the dehumidification benefit of their units, such as this from American Standard AccuExchange™ Energy Recovery Ventilator (ERV): "Saves energy during hot, humid weather, by pre-cooling and dehumidifying incoming air."
And, using an ERV to pre-condition the air supplied to an HVAC system is highly discouraged and can lead to both increased indoor humidity and mold in the ductwork.
This, for instance, from American Aldes:
"We do not recommend ducting the supply air from the HRV to the ducting of the HVAC. During summer months, the outdoor humidity supplied through the HRV may cause condensation on interior surfaces of the HVAC equipment and the supply plenum and ducting. These surfaces will be around 60F, and outdoor air dew points in the summer in much of the US are well above this temperature. These are the very conditions that promote mold growth. How ironic it would be that the very system tasked to provide good indoor air quality by means of ventilation can be the very means of promoting mold growth, by lack of attention to basic psychrometric principles."
This situation is also one of the problem findings in "Use and Misuse of Energy Recovery Ventilators (ERVs) in Hot, Humid Climates" by Bruce Davis which I cited earlier.
Robert,
My information came from conversations and e-mail exchanges with Max Sherman, in connection with an article I wrote for EDU.
Armin Rudd conducted a field study of several dehumidification options in Houston; one of Rudd's conclusions was that indoor relative humidity was poorly controlled in the house with the ERV. I can track down a reference on that study if anyone is interested.
Martin,
In the report, "Residential Dehumidification Systems Research for Hot-Humid Climates", Feb-2005 by Armin Rudd, Joseph Lstiburek, P.Eng., and Kohta Ueno, the conclusion was:
System 5: Energy Recovery Ventilator (ERV) System (Three Homes Tested)
This system will not dehumidify house air, but will lessen the need for dehumidification.
The ERV system did not show good humidity control performance. Its first cost was high, but operating cost was low. The lack of humidity control resulted because, while this system has the capability to lessen the latent load of ventilation air, it cannot dehumidify the conditioned space.
This can be thought of as dehumidification in “ventilation mode” as opposed to dehumidification in “recirculation mode.” For energy-efficient houses with controlled mechanical ventilation, the reduction of sensible heat gain and interior moisture generation were the dominant factors in increasing indoor relative humidity above 60%. The ERV systems were rated to reject about 60% of the latent load from ventilation air and would have shown more improvement in humidity control if ventilation air was a dominant factor.
In other words, while an ERV may be highly efficient at removing moisture from the incoming air stream, and certainly far surpasses an HRV or conventional exhaust/supply fan for that purpose, it can only dehumidify the incoming air and not the air within the building, which requires recirculation.
Another study, Assessing Six Residential Ventilation Techniques in Hot/Humid Climates, published by the Florida Solar Energy Center, and originally published in Proceedings of ACEEE 2004, states:
The best performing system, case 4 (10/20 cycle with dehumidifier), was able to maintain the humidity nearly constant for almost 80% of the test period. The next best performers were case 2 (spot ventilation) and case 6b (energy recovery ventilation).
And that study measured more total condensate from one of the tested ERVs than from either the the dehumidifier or the AC unit. In other words, an ERV is a very effective dehumidifier, perhaps more so than some AC units or stand-alone dehumidifiers.
But you can't dehumidify a house by only removing moisture from ventilation supply air - it requires moisture removal from ambient indoor air (recirculation).
Robert,
Thanks for tracking down quotations from the report.
I think we are in agreement on this issue.
A new concern about ERVs
Anyone following the debate about ERVs versus HRVs will probably be interested in the lead article in the September / October 2010 issue of Home Energy magazine. Written by May Sherman from LBNL, the article ("ERVs Get the Yellow Flag") raises a new concern: some ERVs allow formaldehyde to be transferred from the exhaust air stream to the incoming air stream.
If you're worried about formaldehyde levels, this is one more reason to prefer an HRV to an ERV.
Thank you guys for this very informative discussion. I work with developing ERV cores and have for many years, which means that ultimately, I move in labs and test facilities rather than real houses and buildings.
When I state an ERV is a passive unit, I am referring to membrane type ERVs, which are passive devices, except that they use fans and sometimes some controlling equipment, especially in the sense that they will not actively control the humidity. Or, as Robert stated:
"In other words, while an ERV may be highly efficient at removing moisture from the incoming air stream, and certainly far surpasses an HRV or conventional exhaust/supply fan for that purpose, it can only dehumidify the incoming air and not the air within the building, which requires recirculation."
In European houses, the house envelope is sealed. With mechanical ventilation, we mostly use HRVs, either metal or plastic, here to recycle energy. The only sensible input we have, is waste heat from electrical applicances, sun inlet and humans. The only moisture source inside the house are humans (100ml/hr). If the house is built properly (according to our standards) the air exchange is high, approx 0.35 litres/s and m2 floor area. This means that if the air conditioner has a correct setting, and the ERV is properly installed, the incoming air should always be a little less humid to compensate for the added humidity indoors, exactly like a chiller is set a little below room temperature to compensate for heat leaking in. As I have mentioned earlier though, the trick is to install a by pass. This by pass is there to make sure that when humidity inside is too high (above 70-75%) the air is vented out without recycling in the ERV core. This efficiently lowers the humidity inside since the incoming air is drier. This of course, only works in buildings with a SEALED ENVELOPE. I know this isn't very common in Australia, and perhaps the kind of houses you've seen in the reports have been self draught houses (i don't know the term in US-english). If there is no sealed envelope, I wouldn't expect much result from an ERV..
I would very much like to read that article you mention. Is it available online somewhere?
Correction in the above: The other humidity sources are of course washing, kitchen and bathroom, but as Robert stated earlier, these should have their own outlets, separated from the ERV ducting.
I might also add that the bypass should quite often be applicable in night-time, when humidity and temperature levels may very well go below indoor temperatures, and thus the ERV would heat and humidify the incoming air instead of the other way around..
Johan,
Your post has several half-truths.
"The only moisture source inside the house are humans." This is untrue. You forgot showers, cooking, mopping floors, filling aquariums, drying clothes, drying firewood, and watering houseplants -- just to name a few sources you forgot.
You wrote, "The air is vented out without recycling in the ERV core. This efficiently lowers the humidity inside since the incoming air is drier."
The incoming air may be dryer during the winter in Germany and Sweden, but it certainly won't be in Houston in July. The topic of much of the recent posts on this page has been the use of ERVs in warm, humid North American locations. In Houston, the outdoor air during the summer is generally much more humid than the indoor air. Ventilating with an ERV or an HRV during the summer in Houston raises the indoor relative humidity, it doesn't reduce it.
.
Martin,
I don't know what Johan had in mind, but all those moisture sources you've listed are anthropogenic - caused by human occupancy. The only moisture source, assuming no mechanical leak or wet crawl space, that is not anthropogenic is natural air leakage or moisture diffusion through the envelope - which should be insignificant in a well-built house.
Depends on what the indoor RH is. Normal July outdoor RH for Houston is 90% at 7:30 AM, 58% at 1:30 PM and 66% at 7:30 PM. Depending on efficiency of indoor dehumidification, the outdoor RH may very well be either higher or lower than indoors at different times of day. During the hottest part of the day, outdoor RH may be lower than indoor RH.
And, whether an ERV would raise the RH depends on what it's being compared to. Compared to conventional ventilation (to meed ASHRAE 62.2) or natural leakage, it would reduce it.
We don't agree on this issue. You apparently continue to rely on a single source of theoretical data which is not supported by some of the field data and research of other experts. Consequently, you continue to make blanket statements about ERV's, when the experience with them is quite mixed and mostly dependent upon installation variables.
I am interested in hearing more about the original poster's question... Does an ERV make sense in very cold climates such as northern Michigan or here in the Canadian prairies where it is very difficult to maintain comfortable humidity levels even in very well sealed homes? The difference between the absolute humidity levels of outside and inside air can be significant in these climates during the dead of winter, so it seems that an ERV might make sense here. I have read elsewh ere that there can be problems with the rather fragile cores if the unit does not have an effective defrost cycle.
Garth,
If your house is tight enough to require mechanical ventilation, then excess indoor humidity and not dryness should be the problem in winter. If a house is properly sized for its occupants, and very air-tight, then the anthropogenic humidity (4-5 gallons per day for a family of four) should be more than enough to keep the house at the 30%-40% RH range.
Even cold climates typically have higher outdoor RH in winter than in summer, though that equates to lower winter absolute humidity.
Of course, if the house is too big for the number of occupants, or if the occupants never cook food or do laundry at home, then the house will be dry. Lifestyle and appropriate size matters, in many ways.
HRVs are recommended for norther climates and ERVs for southern climes.
http://www.houseneeds.com/shop/images/fantech_selectsizemapmed.gif
Robert
My neighbor recently built a new modestly sized home to R2000 standards and currently operates an HRV to provide fresh air. His house is very dry in winter...20 to 25% RH indoors. His hardwood floors develop gaps and he complains of dryness to the point that he operates a humidifier for several months. I know this is hard to believe but I have heard of many others who experience the same problem. Perhaps he operates his HRV to much for the size of house he has...
Garth,
A "modest sized house" has become decidedly immodest. In the 50's and 60's, a 1200 SF 3-bedroom house was the middle class dream. I've designed a 900 SF 3-bedroom house for a Farmer's Home self-help building program and I've built a 768 SF (exterior dimensions, 8" double-wall) 3-bedroom home for a community land trust in rural Tennessee. Those are modest homes.
Most new homes are excessive in size, cost, material consumption and energy consumption. And one of the prices paid for such excess is winter dryness if the old ASHRAE 0.35 ACH standard is followed. Partly for that reason, the 62.2 standard has been modified to a per bedroom and per square foot standard, which typically reduces the ACH.
I build healthy houses and never plan more than 0.25 ACH - less than that is often sufficient. One option is to control whole-house ventilation with a dehumidistat instead of a timer.
Garth,
If your neighbor's house is too dry during the winter, either
1. His thermal envelope is leaky, or
2. He is operating his HRV for too many hours per day.
Martin
I agree with your two possibilities by I also believe that there may be a third. I am assuming that your two possibilities have been ruled out.
3. It is extremely difficult to maintain a comfortable indoor RH when outside air has an absolute RH of close to zero for much of the winter.
Garth,
I think you meant to say "absolute humidity" rather than "absolute RH".
But no where on earth is absolute humidity "close to zero". In the most extreme desert environments, RH may drop to as low as10%, but at 120°F, that's more absolute humidity than in a house at 70° and 40% RH.
Calgary, Alberta, which is considered a semi-arid continental climate, has an average Janurary temperature of 16°F and an average winter RH of 55% (summer is 45%).
A 2,000 SF house, with 0.25 ACH ventilation, would lose about 4 gallons of water per day in a Calgary January. The typical family puts 3-5 gallons of water per day into the indoor environment, so that should allow the house to maintain a comfortable RH above 30%.
Increase that house to 3,000 SF at the same ventilation rate, and the house will lose 6 gallons per day and normal activity may not be able to replenish that volume. Just one more reason to build houses sized appropriately to the occupancy load.
With the risk of repeating the previous post, but in other words, I believe the point Garth is trying to make is that in winter, outside absolute humidity can easily be as low as 2 grams per kg air and less (-10C and 100% RH is equivalent to AbsH of 1.6 grams/kg air) which means that if you take that air and heat it, the absolute humidity will be the same but the relative humidity will drop drastically when heated to 20C. For example: if outdoor temp is -5C and 85%RH, and that air is heated to 20C, the resulting indoor RH is only 15%. One has to keep in mind though, that as stated a few times above, there are several anthropogenic sources of moisture indoors that vary greatly over the hours of the day that may affect indoor air quality negatively. The problems arise if the ERV is "too efficient", or rather, that the ventilation rate is so low that excess moisture (indoor RH of 70% and above) is not vented but returned with ingoing air from the ERV to add to the indoor air. This is a typical situation where a by-pass should kick in.
Ultimately though, the key is (as Robert has stated a few times above) to balance air flow, regeneration and HVAC equipment to optimally suit not only your house, but your way of living. An ERV may increase comfort in a cold climate in winter, but it may also provide too high RH indoors which will promote fungal, bacterial and mite growth (above 70-75% RH). Too low RH though (typically less than 10-15%), will result in material issues (eg wood cracks) and health problems such as decreased resistivity to airborne pathogens, dried out humus membranes in throat and nose, itchy skin, irritated eyes and tiredness etc..
Ultimately, I guess my own best recommendation would be to get a couple of loggers for temp and RH (eg Hobo) and do some monitoring of the actual indoor conditions over time. If coupled with weather data, I'm sure you can deduce what's best for you and your house. In the long run, knowing the house will be your best bet to increase comfort and lower energy bills..
I would also recommend you to read all the posts above since they contain a lot of very useful information and a few good references..
Johan
Thank you for restating my question in a much more understandable way. I do agree that monitoring of indoor RH would be not only prudent, but necessary. My only remaining question regards the durability of an ERV core when outside temps reach down to -10C and stay there for several weeks at a time...I am assuming that a reliable defrost mechanism must be built in.
I believe that varies greatly from brand to brand. Something to have in mind though, is to connect an ERV in series with an HRV. The HRV is put on the "cold side" (outdoor) and takes care of pre-heating incoming air and the ERV sits on the "hot side" (indoor) and takes care of pre-drying outgoing air, which means there won't be so much condensation in the HRV. I haven't tried it myself, but I believe this may be a viable solution for cold climates. It would bring the best of both worlds, although it will probably be a little more expensive as well.. If Robert or Martin has heard anything or know of anyone who has used such a unit, I'd be very interested to hear about their experience.
Johan,
Your suggested solution is too expensive and complicated.
Donald: Just install an HRV. Hook up the bathroom exhaust and laundry exhaust to the unit as I originally suggested (and as every HRV manufacturer recommends).
Allan Edwards just found a link to the editorial I referred to earlier:
ERVs Get the Yellow Flag
Max Sherman is pointing out that some ERVs allow formaldehyde to be transferred from the exhaust air stream to the incoming fresh air stream.
Thanks, Allan.
Johan
Your idea of using an ERV and an HRV in series makes sense to me. Perhaps someday, some manufacturer will design a ventilator that would have both types of cores in series, in one unit....best of both worlds at minimal additional cost....
Yohan and Garth,
The problem with the ERV/HRV combination is that, in the winter moisture must be evacuated in order to keep indoor RH below the danger zone. So it is counterproductive to recycle indoor humidity with an ERV unless the house is too large for its occupancy or too leaky, both of which are design flaws that cannot be corrected by additional mechanical equipment.
Martin,
The fact that "every HRV manufacturer" recommends integrating bath and laundry exhaust into their centralized systems is no more guarantee of function than spray foam manufacturers recommending less than code R-values in order to keep installed cost down sufficiently to sell their product.
Again, if a primary purpose of a whole-house ventilation system is to remove excess humidity, then it's counterproductive to use an ERV to recycle this humidity from point sources back into the conditioned space and counterproductive to impose a high-moisture burden on an HRV which would have to run its energy-wasting defrost cycle to compensate for the excessive condensation.
Integrating a bathroom exhaust fan into an ERV is like running gutter downspouts into a foundation perimeter drain - it defeats the purpose of both systems.
Robert,
So I understand how an ERV venting bathrooms would recycle the humidity back into the house but would that also happen with an HRV? And, if separate bathroom and kitchen fans are needed to eliminate moisture, how does one balance the airflow in the HRV to account for the fans?
Donald,
An HRV does not recycle moisture, only heat or coolth (sensible energy), unless the exterior exhaust and intake ports are too close together (should be 10' apart) or there is internal bypass leakage.
Unless an HRV is a pressure-balancing unit, greater interior negative pressure from independent exhaust fans will increase the inflow through the HRV and decrease the outflow. However, in the heating season, interior negative pressure prevents the exfiltration through the thermal envelope which results in condensation and moisture damage.
In a cooling climate or season, it's important for the same reason to maintain a slight positive interior pressure, so some method of increasing supply air when exhaust fans are running is necessary, or an HRV can be used to exhaust moisture point sources and an ERV used for whole-house ventilation.
Robert,
In the cooling season, since I'm building without air conditioning, I'll probably have the windows open so I am mostly worried about the heating season. If I am understanding your last post correctly, I should be able to use an HRV to exhaust moisture point sources and ventilate the whole house? Also, the intake and exhaust motors could be adjusted to create a slight negative pressure.
Donald,
Yes, many designers and builders rely on an HRV for whole-house ventilation, but it will never be as effective at eliminating source moisture (and other pollutants) from such areas as showers and laundry room as will dedicated exhaust fans (assuming they're used and controlled by a delayed-off timer so that they run for at least 10-15 minutes after the room is vacated.
An HRV can often run at a constant low speed with a boost speed controlled by a bathroom timer switch, but that simply increases the outflow from all exhaust ports and draws bathroom moisture throughout the house.
Those who, like myself, believe in the importance of eliminating pollution and moisture at the source, rather than relying on "dilution as the solution to pollution" (which is the function of a whole-house ventilator), use dedicated exhaust fans in shower rooms and kitchen range hood (and laundry room if chlorine bleach is commonly used).
And, since it's redundant to use three or four independently ducted exhaust fans in addition to a centrally-ducted whole house ventilator, I use high-quality bath fans as the exhaust-only whole house ventilator with strategically-located passive make-up air inlets (Aldes Airlet 100s), and both short-term timer switches in the bathrooms and parallel 24-hour programmable timers to control daily ventilation rates.
While this system does not recover the heat in exhaust air, it is less expensive to install and operate and maintain, and offers the advantage - in a cold climate - of providing negative pressure.
Robert,
An HRV does not "draw bathroom moisture throughout the house." It exhausts bathroom moisture from bathrooms, along with exhaust air, just like your exhaust fans. It can also be used to exhaust laundry room moisture from laundry rooms. You can duct an HRV to exhaust moisture from whatever rooms you prefer.
It certainly can, depending on the proximity of other exhaust ports and their relative position to the supply ports. If the bathroom door is left open following a long shower, then competing negative pressure zones will draw bathroom moisture (likely 100% RH) in various air streams in various directions.
If you want to evacuate a humid bathroom, you don't turn on the bath fan and also the kitchen hood. You create negative pressure only in the bathroom. A central HRV system creates multiple negative pressure zones (around each exhaust grill).
If there is any air leakage around a bathroom window or the tub trap hole in the subfloor, then air will flow out of the bathroom towards all exhaust ports in proportion to the relative negative pressures. Even just with the bathroom door open, thermosiphon and forced convention currents will be created with cooler dry air moving along the floor and warmer moist air moving along the ceiling (dry air is more dense than moist air at the same temperature and a lot more dense when it's cooler).
In regards to the article about ERV's Getting a Yellow Flag, by Max Sherman, and Martin's post:
"Max Sherman is pointing out that some ERVs allow formaldehyde to be transferred from the exhaust air stream to the incoming fresh air stream."
I'd like to note that Max Sherman does not have test data regarding ERV's and their transfer of formaldehyde. He states that ERV's "can" transfer formaldehyde, but this is speculation at best, based on chemical simularities between water vapor and formaldehyde (i suspect... as he makes clear reference to that). The article is so cleverly written... i am amazed at it as i read it again... It is very misleading in my opinion. There is no testing of ERV's and formaldehyde transfer. There isn't even a written standardized protocal for such testing. I traded emails with Max on this topic to confirm. The fact is -there has been no testing regarding ERV's and Formaldehyde transfer. I do not know the purpose of this article or any scientific backing for it.
In light of that... Max (and LBNL) will be conducting some testing on this topic. They are (and myself included) discussing the protocal for such a test right now. I hope to report on this testing at a later date.... and just for record - i do think there will be some formaldehyde transfer. But nothing to worry about. Like breathing the windex you just used to clean a window.
Now, just as a note on this topic. Formaldehyde is what i would call a decaying chemical. Concentrations will be highest when the formaldehyde off gassing item is recently made (carpet?).. and will continually off gas less amounts until it is gone... (negligible). Ventilation (in general) will dilute the formaldehyde concentrations in a given volume (house). There are many factors which influence this concentration in any given house to begin with, such as - how tight the house is, how much formaldehyde is being off gassed into it, how big is the house, does it have operable exhaust strategies (bath fans, kitchen fans,..?) how much are they used... and so on. Then we can discuss - does the house have a mechanical ventilator... is it an hrv or an erv? If its an ERV, how much moisture does it recover (which is different summer and winter condition...). So here I lay my opinion out for scrutiny: if a protocal can be written, and all the priror assumptions standardized (or every explicit case tested)... i do not believe that a diluting ERV will be much more "hazardous" to ones health with regard to the formaldehyde concentration in that exact case study... than if just a hrv were used in the same exact installation. If it can be measured... i'm guessing on a scale of 1-10... 10 being very bad... that the ERV is a 2.26 and the HRV was a 2.24. I'm glad we spent all the time, effort, and cost to determine that... Lets do it again for the other 25 different house conditions that could be present.
Anyhow... please look me up in 4-6 months for this answer. I do hope to have it.... but no promises.
Jason,
Thanks for your thoughtful post. I agree -- until the data are in, the is a "yellow flag" issue, not a "red flag" issue.
In a recent article in Home Energy Magazine, Rob Dumont describes the ventilating system in the "Factor 9" home built in Regina SK in 2007. The home is highly insulated, has an ACH50 of 1.2, and the local climate has 10500 HDD.
http://www.homeenergy.org/article_full.php?id=747
The HRV draws stale air from the bathrooms, laundry, and kitchen areas. He writes, "Two different heat-exchange cores are used, one with plastic plates and one with treated paper plates. The latter will allow moisture in the exhaust air to be recycled back into the home in the winter , when the indoor air tens to be too dry"
It would be interesting to hear more from Rob Dumont on this issue...
I, too, would be interested to hear from Dumont. Regina summertime humidity hovers around 50%, while the winter RH is 80%-90%. Is the air too dry or is he exchanging too much air volume or is the house too big for its occupancy load?
A tight house should not be dry in the winter. That was indicative of a leaky house with too much air exchange. In fact, a primary function of whole house ventilation in a cold climate is to rid the house of excess humidity (which is a precursor of mold, dust mites, and formaldehyde outgassing). This is why an ERV rarely makes sense in the north country.
I think that the reason for Dumont's approach lies in the answer to this question. What is the RH of the incoming fresh air (say @ 10 degrees F and 85% RH) when warmed to room temperature? If I knew how to read a psychometric chart, I would answer it myself. I am betting it is less than 10% RH...
Garth,
That outside air warmed to 68°F would result in less than 9% RH. But that's not the point.
A 2000 SF house with 8' ceilings and continuous ventilation at 0.25 ACH with baseline interior humidity at 40% would lose 3.84 gallons per day by air exchange. A typical family of four contributes 3-5 gallons per day to the interior environment, so this house should easily be able to maintain a healthy interior relative humidity.
If it's drying out, either there's too much air exchange, or the house is too large for its occupancy load, or the occupants eat only take-out food, shower at the health club and have a laundry service.
Robert
A detailed description of the Factor 9 home can be found here
http://www.cmhc-schl.gc.ca/en/inpr/bude/himu/inbu/upload/65664EnW.pdf
By my calculations, the ventilation rate is .145 ACH. The house is larger than 2000 SF(3239 SF including the basement area) but does house a family of four.
Garth,
Then that house, even on a 0°/90%RH winter day should be losing only 4 gallons per day through ventilation, and 0.3 gallons per day through diffusion (with a 1 perm wall), with a steady-state interior RH of 35%.
If the house is too dry, perhaps there is more stack-effect leakage than they imagine, or perhaps their strategy of using two HRV cores was baseless.
It would be interesting to get some actual performance data.
I may have found the source of their design concerns.
While their average January temperature is 1.4°F, they used a 1% worst case design value of -33°F. But even at that temperature, the house should lose only about 5.5 gallons per day.
The ASHRAE standard is to use a 2.5% worst case design value, otherwise mechanical systems will be significantly over-sized.
Robert
Just to be clear, I believe the strategy is to swap out the plastic HRV core in the winter months to an enthalpy core...not using two cores at the same time.
Martin
Perhaps you have Rob Dumont on your rolodex?
Garth,
Okay -- I'll send Rob an e-mail and invite him to comment here.
Garth,
I understand his strategy. But a cold climate house built tight should require elimination of moisture not recycling of it.
There seems to be a lot of disagreement in this blog relative to the benefits of ERV's. I am a builder, and will concede that I am not as informed on the subject matter as you folks are. I recently built a home with a fresh air intake and inquired with the mechanical engineer about the installation of an ERV. This home is located in mid-florida, where it is hot & humid in the summer, but can drop to the 40's at night in the winter. He advised against the use of the ERV, and designed the system with a Whole House Dehumidifier in line with the duct work. He was most concerned with (a) the ability of the ERV to remove moisture and (b) it's longevity in this enviroment. He wanted to use the Dehumidifier to work in conjuction with the HVAC equipment to allow us to keep the set point temp slightly elevated, while maintaining humidity levels. Is this what you folks would suggest?
Doug,
You have a fundamental misunderstanding in your confusion between a dehumidifier and an ERV. They are two totally different appliances designed to do two totally different things.
A dehumidifier does not bring any fresh air indoors. All it does is lower the indoor humidity level. At the same time, it makes your house a little warmer.
An ERV does not pretend to be a dehumidifier. It is a ventilation appliance. Its purpose is to bring fresh air into your home and distribute this fresh air throughout your home, while simultaneously exhausting air from your bathrooms and laundry room.
If you want whole-house ventilation, then you should get an HRV or an ERV. If you want to lower your home's indoor humidity, then you should install an air conditioner or a dehumidifier.
Martin,
You're dramatically oversimplifying the distinction. An ERV is most assuredly designed to dehumidify supply air when the indoor dew point is less than the outdoor dewpoint, as is typically the case during AC operation. It is not, however, designed to do the work of a stand-alone dehumidifier or perform the dessication function of an air conditioner, but it is intended to reduce the load on each.
Robert,
In a hot humid climate, operating an ERV will not dehumidify the air during the air conditioning season. The more you run your ERV, the more humid the air in your house will become. Ventilation will increase, not decrease, the indoor relative humidity.
The best that can be said is that an ERV will not be quite as bad as an HRV in terms of the moisture load it adds to the air conditioning system.
Of course it will, but only the supply air and only when the outside dewpoint is less than the indoor dewpoint. That's its function, to exchange both heat and humidity from the warmer & more humid air stream to the cooler and dryer air stream.
You're still confusing the issue. Air exchange is required, both by ASHRAE and by common sense. Any other type of ventilation device will bring in unhumidified outdoor air. An ERV will bring in partially dehumidified air, thus lowering the latent load of the AC unit.
Whether an ERV will make a net positive or net negative contribution to indoor dehumidification depends on the diurnal dew point cycle of the local climate, the hours of ERV operation, and the controls used to regulate its operation.
Robert Riversong wrote on September 8, 2010 that "An HRV can often run at a constant low speed with a boost speed controlled by a bathroom timer switch, but that simply increases the outflow from all exhaust ports and draws bathroom moisture throughout the house.... Those who, like myself, believe in the importance of eliminating pollution and moisture at the source, ... use dedicated exhaust fans in shower rooms and kitchen range hood (and laundry room if chlorine bleach is commonly used).... this system does not recover the heat in exhaust air...."
to which Martin Holladay replied on September 8, 2010 that "An HRV does not 'draw bathroom moisture throughout the house.' It exhausts bathroom moisture from bathrooms, along with exhaust air, just like your exhaust fans. It can also be used to exhaust laundry room moisture from laundry rooms. You can duct an HRV to exhaust moisture from whatever rooms you prefer."
Robert Riversong replied on September 8, 2010 that "[HRV] certainly can '[draw bathroom moisture throughout the house], depending on the proximity of other exhaust ports and their relative position to the supply ports. If the bathroom door is left open following a long shower, then competing negative pressure zones will draw bathroom moisture (likely 100% RH) in various air streams in various directions.
Thus, we seem to have an unresolved debate between Robert and Martin.
Let me explain my own situation. We are building a very tight 3,000 square foot house in Camas, Washington. Local code requires an HRV. My HVAC contractor recommends an HRV with a single indoor supply vent and a single indoor exhaust vent. He proposes that both be located in the wall of the living room, about 8 feet apart. In addition, he proposes separate vents for each bathroom, the kitchen and the laundry room. If we are limited to only one indoor exhaust vent, why not make this the bathroom vent for the master bathroom? We want to recover the heat from the frequently used vent of the master bathroom.
Bob Pond, Washington, October 19, 2010
Bob,
That little disagreement between me and Martin is the least important part of this discussion. The important element is whether an HRV is sufficient for source removal, since it's intended only for whole house dilution, or whether dedicated exhaust fans for bathroom and kitchen are necessary to meet the need for point-source moisture removal.
My position has always been that there are two distinct functions required for indoor moisture control in a tight house: point-source removal (particularly in shower areas) and whole-house dilution to maintain indoor relative humidity below 40% (or less in very cold climates), and that it is too much to expect one device to perform both functions well.
I know of no building code which requires an HRV. Codes require ventilation and source removal, but not heat recovery. That's an option.
And, since a primary function of whole-house ventilation (or pollution dilution) is to provide fresh air uniformly and particularly to those areas where it's most needed, such as bedrooms and living spaces, it makes no sense at all to install a short-circuit HRV that only circulates air from one side of the living room to the other. That would be a complete waste of money and effort and would not meet the requirements of ASHRAE 62.2 for whole house ventilation.
And it certainly doesn't make sense to install a limited-function HRV if you're going to be exhausting air from 4 or 5 rooms with no heat recovery, which will also unbalance the HRV and could create other negative pressure problems if there are any atmospheric combustion appliances that could backdraft.
I would recommend that you either install a proper HRV system throughout the entire house (you may need to get a different contractor), or use exhaust-only ventilation with point source fans with no more air flow than is necessary for evacuation of moisture and odors (no 400 cfm kitchen hood, eg), passive air inlets and a programmable timer on one or more of the bathroom fans for whole-house ventilation.
"The important element is whether an HRV is sufficient for source removal, since it's intended only for whole house dilution, or whether dedicated exhaust fans for bathroom and kitchen are necessary to meet the need for point-source moisture removal."
Thanks for the quick reply, Robert. Why not mix point-source removal (with seldom used, dedicated exhaust fans for secondary bathrooms and the kitchen) and passive air inlets with a DEDICATED HRV for the master bathroom? Wouldn't a dedicated HRV be sufficient to remove moisture from a single bathroom?
With such a hybrid system, couldn't we recover heat from the frequently exhausted master bathroom, supply most of our fresh air in the process, yet rely upon the combination of point-source removal and passive air inlets for the less frequently used functions?
Bob Pond
Bob,
If you just want to salvage some heat from the one bathroom (or several), you could use a Panasonic Whisper Comfort spot ERV, which is about the size of a bathroom fan and doesn't require extensive ducting. But they move only 40 CFM, aren't as efficient as the better HRVs, and they recycle some of the moisture you're trying to eliminate.
You could use a single exhaust grill in the master bathroom for a central HRV, but you might find that it doesn't evacuate the steam from a shower as quickly as you'd like unless it runs at high speed boost, in which case it will probably consume more energy than a local exhaust fan. And it's not a good idea to mix a balanced HRV with lots of exhaust fans that will unbalance the system.
Which is why I prefer a simple exhaust-only system, which effectively evacuates moisture from the places of production, and can also be used for whole house dilution if passive make-up inlets are strategically located and a timer added.
Robert,
I understand your argument for using exhaust only ventilation. However, the home energy consultant who I plan to have blower test my home is concerned that the fireplace (with outside air intake and sealed door) and the attached garage (actually 1/2 of the basement foundation area - we had a small buildable footprint on the lot) would indicate that a balanced system (HRV) would be a safer bet. I know you have wood burneers in your houses with exhaust only ventilation, any thoughts?
Donald,
I agree with your concerns. An good-quality airtight woodstove with outside combustion air works fine with exhaust ventilation, though you need to avoid opening the feed door with the kitchen hood running.
I never use attached - or worse, yet - basement garages, and I would never put a fireplace in a very tight house unless perhaps it was an authentic Rumford with a well-sealed chimney cap.
"You could use a single exhaust grill in the master bathroom for a central HRV, but you might find that it doesn't evacuate the steam from a shower as quickly as you'd like unless it runs at high speed boost, in which case it will probably consume more energy than a local exhaust fan. And it's not a good idea to mix a balanced HRV with lots of exhaust fans that will unbalance the system."
Clearly ventilation of the latest generation of tight houses is a contentious field. I am left with the impression that the owner and the HVAC contractor have to, at some point, flip a coin when deciding how best to proceed. Having invested in a tight house design (SIP walls, spray on insulation of the roof) and a heating system based upon hydronic radiant floors, I just cannot abide a ventilation system that will pump hot air out of the master bathroom for 30 minutes or more per day. The way that I figure it, this is exactly what a heat recovery ventilator is meant to avoid. The selected HRV will move 120 CFM at boost speed which should be more than enough to vent a single bathroom. Yes, at boost speed this will consume more electricity than a conventional bathroom fan but it will recover up to 80% of the energy difference between the exhausted air and the fresh air that it will draw into the house. And this fresh air, when distributed to the living areas, should satisfy household needs.
As you point out, during the infrequent periods when other direct exhaust fans are turned on, the household ventilation will become unbalanced. Unless we open windows or we have other passive air inlets, the pressure inside the house will drop below the pressure outside the house. I suppose that under these circumstances ("negative pressure" in the house) make up air will flow into the house through the ducting of the HRV without the HRV having access to a complementary outward flow of exhaust air with which to heat the incoming fresh air. And I suppose that this will make the fresh air coming from the HRV vents colder than usual. The end result, I would guess, would not be any worse than relying upon a passive air inlet or an open window.
In any case, isn't this exactly the situation that arises whenever a high volume kitchen ventilator is turned on in a house with even the most extensive HRV system?
Couldn't a person with a tight home just get by with one of the 'ventilating' dehumidifiers? It may not be designed for controlled ventilation but it seems like you're going to be running it year-round anyway.
Matt,
The critical determinants would be the actual fresh air volume that such a unit could provide and the operating limits. The Ultra-Aire whole-house ventilating dehumidifiers, for example, can operate only down to 56°F and may have to shut off the outside air intake in cold weather.
Matt,
Assuming your question is about the house we're discussing -- one in northern Michigan -- then dehumidification is not much of a concern for most of the year. Since the heating season in northern Michigan is 11 months long (I'm exaggerating for effect), anyone who has high indoor humidity just needs to operate their mechanical ventilation system to bring in dry air. You don't need a dehumidifier when it's cold outdoors.
Yes, I was referring to the subject house. I was thinking of those rainy spring and fall days where it's too chilly to run the AC to pull out the humidity, and the HRV is just going to bring in more moist air. I didn't realize that the Ultra-Aire was only good to 56° - that's really quite limiting.
Matt,
A stand-alone dehumidifier only costs about $250, and that's all you need to address the conditions you are concerned about.
I think there is a lot of hand wringing and misunderstandings in this thread. Some good info too. I will, however, share the experience we have had with IAQ systems over the years.
1. ERVs have fairly poor humidity transfer percentages: on the order of 50% to 60% exchange efficiency under rated conditions. Having one in a cold climate does not retain all of the humidity of the outgoing airstream and in fact we, and others I am aware of use ERVs in cold climates as they provide better comfort and prevent overdrying in the winter which helps the occupant keep the unit on continuously, as they should.
Too many systems out there get turned off or timed in the winter because of too much exhaust. Even in super tight buildings the exhaust rates of most HRVs even on low speed greatly exceed the moisture removal requirements of the home. If you want continuous duty exchange in cold climates, we like ERVs. That would mean that we would prefer to use an ERV in nearly any residential application that doesn't have truly stupendous moisture issues.
Our shop is supertight and even with several people in here breathing NOW... not even deep freeze time yet... we're barely over 30% RH in the building. And we're using the most efficient ERV in north america.
2. ERVs are often not used in cold climates because most of them will freeze in cold weather. Preheat can prevent this for a ridiculously small energy usage even in extremely cold climates... can even do the passive house earth loop for an energy net gain, if you like, though it's pricier than a basic electric duct heater. Venmar also makes cold climate ERVs which have worked for many years without the need for preheat but I'm not sure what the penalty is in defrost time/lost exchange.
3. I have no idea why anyone would want a separate bath fan. the "back spillage" issue is kind of silly and is a result of poor design. with a typical house sized ERV you have 150-200 CFM of max speed operation available which can easily handle 3 or 4 bathrooms with 50 CFM each. Not a massive blast of exhaust, but plenty to speed the removal of some excess moisture and odors (that's a full turnover of air in a typical 6x10 bathroom in ten minutes and most boost cycles are at least 20 minutes long), and then you settle back down to continuous rates when you are done. And if you want to go truly nuts, Aldes has a ventzone system now so you could hit one bath with a huge amount of exhaust instead, if you like. but I rarely see the need. Save the bath fans, recover heat and a little humidity from the exhaust, do the happy dance. makes first cost and lifecycle economics better too.
In short when you design ventilation you have to work to define the air flow path. negative pressure "competing" should be handled by providing plenty of air plenty of ways to get to the exhausts. door undercuts and nearby supply ports. not hard. Air is lazy.
4. Kitchen range hoods, still needed. all agreed there. just to be clear.
5. ERVs assist with summer cooling if you have another way to lower humidity. that's pretty straightforward. dunno why there have been a dozen posts wrangling over that issue. that's all it does is help maintain the differential relative to equal exhaust without energy exchange.
Thanks Nrt.Rob, for clearing up certain issues. I think you summarised it well, and I for one agree with you on all points except nr 3, where I abstain from comment since I'm not well enough informed to have an opinion. I think it is easy to be lost behind the screen of calculations which will always only be estimates, since the real situation is way to complicated to simulate or model.. Even a psychrometric chart is only an estimation and based on empirical studies rather than scientific absolutes. I guess the best way to get any sort of decent overall picture is to get several opinions in your local area and go on one's gut feeling.
Is there any chance of getting to know some more about your shop and it's equipment?
I'm looking for a small capacity HRV, the equivalent size of the Panasonic FV04VE1 which can be wired for 20/10CFM, since, as I've read in the installation details, at outside temperatures below 20F, the Panasonic ERV in question operates as exhaust only (and between 32 and 20F, operates in frost-prevention mode, cycling intake on and off).
I'm having very little luck finding one online. HRVs I'm finding are, so far, all much larger. Does a small HRV even exist? Failing that, can I disable the humidity exchange on this small ERV somehow (I'm doubtful)?
I suppose that if, because of the frost-protection mechanism, etc., I'm not going to get a balanced system anyway for most of the year, and especially when I really need it, a) I might as well save my money and buy an exhaust-only fan, and b) I'm left wondering what I'm supposed to do about proper intake air.
Minneapolis,
You are overthinking the air intake issue. If you want to ventilate at a rate of only 20 cfm, makeup air will easily find its way into your house. As long as you commission your exhaust fan to be sure that it is exhausting 20 cfm, then (by the laws of physics) 20 cfm of makeup air is also entering you house through the inevitable cracks in your thermal envelope.
That's a given.
...and regarding the heat recovery?
Minneapolis,
If you want heat recovery, you have to pay for it. An HRV costs more than a simple exhaust fan, of course. Higher purchase cost, but lower operating cost -- your choice.
Martin: Let me repost the first two of three short paragraphs from a few inches upscreen. Here you go:
I'm looking for a small capacity HRV, the equivalent size of the Panasonic FV04VE1 which can be wired for 20/10CFM, since, as I've read in the installation details, at outside temperatures below 20F, the Panasonic ERV in question operates as exhaust only (and between 32 and 20F, operates in frost-prevention mode, cycling intake on and off).
I'm having very little luck finding one online. HRVs I'm finding are, so far, all much larger. Does a small HRV even exist? Failing that, can I disable the humidity exchange on this small ERV somehow (I'm doubtful)?
Minneapolis,
I'm partial to the Venmar EKO 1.5 HRV, because it uses ECM blowers and is quite efficient. At low speed, it ventilates at 40 cfm. If you want to ventilate at 20 cfm, just program it with a timer to come on for 30 minutes every hour.
Thanks.
The last post for this topic is over 10 years old. Does that mean that there have been any new innovations in HRV and ERVs over that last 10 years? My 20 year old Venmar Constructo 1.5 HRV died recently and I am about to replace it with the recommended replacement Braun HRV. Reading through all of the information here confirms my understanding that an HRV or ERV exists to provide the occupants of a house with oxygen and to remove excess carbon dioxide. The CO2 level in my house right now is 845 ppm. If I block off the fresh air make-up to my furnace the level will rise above 1300 ppm.
About 20 years ago I worked on commercial building projects that were designed to save energy using the CO2 level in the return air to control air handler fan speed.
All this focus on humidity with HRV and ERVs has somewhat puzzled me. In order to maintain reasonable humidity I humidify to 35% in winter and dehumidify to 50% by subcooling the AC in the spring summer and fall.
At one time I considered getting an HRV/ERV that had interchangeable cores so that I could swap them out during coldest winter months.
When I get the new HRV installed I'll have to figure out the best way to operate it.
If you are using equipment to add moisture in winter and to remove moisture in summer, an ERV is likely a good choice for you. A common issue on high-performance homes in cool climates is excessive moisture in winter, in which case an HRV is a better choice.