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More Green Building Curmudgeon
Before I even get started, I want to point out that I am no expert on ventilation. I have learned a lot from (and rely on) many experts, including Paul Raymer, Gord Cooke, John Krigger, Joe Lstiburek, Armin Rudd, and Terry Brennan, among others. I depend on them to fuss about the details of how much ventilation a house needs.
I do, however, have to deal with ventilation requirements when I work with my clients to certify their buildings. Most residential certification programs require that a home or apartment meet the ASHRAE 62.2-2010 whole house ventilation standard. This can be accomplished through exhaust-only, supply-only, or balanced systems.
In the humid Southeast, we tend to discourage exhaust-only systems, but some developers and contractors meet the requirement by using a continuous bath vent fan – not the most efficient method, but it is simple.
Supply-only systems normally have an outside air intake into the HVAC return plenum with a controller that turns on the HVAC blower motor and opens a damper when air is needed.
Balanced systems in my climate use energy-recovery ventilators (ERV).
Supply-only system problems
When I test out and inspect a home for certification, checking for an operating ventilation system is a piece of the action. For an Energy Star home, the ventilation system’s flow rate must be tested. Other programs only require that the system be installed and sized properly, without any other field verification.
Most supply-only systems I run into include a motorized damper connected to a control with a ventilation cycle function. I have yet to see an HVAC installer who figures out the CFM coming in through the supply duct, calculates the required ventilation rate, and sets the fan control to meet ASHRAE 62.2. They just put in the damper and controller and assume everything will work out fine.
In several cases, I have seen the dampers installed so that they default to the open position – sometimes causing duct systems to fail leakage tests, particularly if they don’t have a good outside damper. In these cases, no one has any idea if the ventilation system actually does what it is supposed to do.
ERVs: Are they as good as we think they are?
In doing some research on ERVs, I learned that if they are connected to whole-house duct systems, the air handler blower must run to move the outside air throughout the house – the ERV motor can’t move enough air through a duct system.
After several discussions with ERV manufacturers’ support staff, I have come to the conclusion that they should be installed with dedicated duct systems to avoid the complexity of control systems and the energy use involved with running the blower motor.
This leads to the question: where do you put the supply and exhaust ducts in the house? Some people suggest drawing exhaust air from bathrooms and supplying fresh air to bedrooms or living areas. This has some appeal in that you can eliminate bathroom fans. However I have read other opinions that don’t like the idea of exhausting bathrooms into ERVs. What’s a poor, ventilation-ignorant boy to do?
Another minor beef I have with ERVs is how they are treated in the RemRate energy modeling program. The new Energy Star target HERS Index, required for some green certifications, can be a challenge to meet. On several recent projects for which I have done energy models, I discovered that upgrading from supply-only ventilation to an ERV lowers the HERS index by 7 or 8 points, allowing a project to meet the required target index and achieve certification.
I appreciate the fact that ERVs are more efficient due to the motor size and the fact that they do actually recover some of the energy in the heated or cooled indoor air. But do they really increase the energy efficiency of a house by 10% or 15%? The conspiracy theorist in me thinks that there might be some collusion between the ERV manufacturers and the energy modeling software companies. (Only kidding, guys).
So how much ventilation do we really need?
I’ve heard Terry Brennan suggest that ASHRAE 62.2 is too low a ventilation rate. I’ve heard Armin Rudd say it may be too much. Joe Lstiburek recommends installing systems that provide 1.5 times the required rate, but setting them to run initially at ½ the rate, letting the occupants adjust it themselves.
I kind of like this last idea, because the designed rate seems to me to be somewhat arbitrary. It has to be either too high or too low for most people. If you have a bunch of dogs, smoke tobacco, fry turkeys indoors, and don’t vacuum your carpet, then you probably need a high rate of ventilation – maybe even more than the ASHRAE 62.2 rate. If you don’t wear shoes in the house, don’t smoke, don’t have pets, and keep your turkey frying outdoors, then you need less.
Then there’s the thought of occasionally opening windows and doors when the weather is nice. I know that we can’t rely on passive ventilation to guarantee a specific number of air changes, but can’t we please just take a little responsibility for our homes?
I understand that a lot of very smart people have been working on the ASHRAE ventilation standard for a long time, but I am concerned that we just aren’t getting the anticipated results from all the effort that goes into this issue. Like I said, I’m no expert, but something about this whole thing is starting to bug me.
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25 Comments
ERV for 62.2
Carl, Play with your infiltration rates and the ERV in REM. My experience is you have to add the ventilation to the model, before you get credit for savings from a tighter home. Say 6 ACH at 50 or below.
I like the idea of a dedicated duct system for ventilation. Each house is unique and should be treated as such.
As an alternative, leave the bathroom spot ventilation. Exhaust from the bedrooms. If people spend 8 hours a day at work + travel time, 2 hours doing things outside the home and 5 - 8 hours sleeping, how much time is left to spend elsewhere in the home? Exhaust where the bodies spend time.
Consider also how to use your ventilation strategy to move conditioned air from a mini split installed ductless or with minimal ducts.
Not usually my call
John - Thanks for the input, but, unfortunately, I rarely get much input on project decisions. I'm the guy who comes in after everything is designed and they ask me to make it green or get it certified. To meet the ESTAR 3 target index, about the only thing I can recommend is to stick in an ERV. I don't get in early enough to suggest better HVAC systems or improved ventilation strategies. Hopefully sometime soon I'll get to give my input earlier in the process.
The projects that have used ERVs so far, HERS index does drop dramatically as compared to non-recovery systems.
erv bathroom exhaust
What kind of problems are you referring to when an ERV is used for bathroom exhaust? The problems I could think of is an extreme situation where a person is taking hot showers all day long or has a pool/hot tub that overwhelms the house with humidity or if people are using crummy ERVs that ice up/shut down ventilation in cold weather. Both of these situations seem easy to avoid.
No ERV for my bathroom exhaust, please.
We have shoulder seasons here in the SE—about five months or so. This means, in my house, windows are open to the fresh air. Why would I want to waste energy on an ERV during this time? But I still want to exhaust bathroom steam and odors.
erv bathroom exhaust
Kristopher
Assuming you’re in a humid climate pulling wet air from the bathroom while trying to dehumidify the incoming outdoor doesn’t work well. Cross contamination can also be an issue with ERVs and is worth looking into if using one. You probably don’t want bathroom smells being broad cast through the house. See Martin’s blog for more info: HRV or ERV?
Home Energy also had some info: ERVs Get The Yellow Flag.
Pre-set controlers
David, I believe the solution to the issue you raised during the shoulder season when the windows are open is to shut off the ERV system from automatic operation. Separate controls are available that will turn it back on for a preset amount of time, 20 mins for example. These are pretty cheap and easy to install. I've got them in my bathrooms and in the kitchen. It's very easy then to temporarily turn on the ERV (or HRV) system whenever taking a shower or cooking etc.
Spot ventilation
Ventilation is a topic that always has differing opinion (as if all the others don’t…)
Something that I have come across in my efforts to resolve fresh air needs in an aggressively tight house is $300-$400 spot ventilator ERV that looks and installs just like a standard bath fan. (Not to be installed as a bath exhaust fan)
It looks like a much better idea to the bath fan exhaust method that is so often used, relying on infiltration for the make-up air. This unit is relatively easy to install and is priced far less than the traditional ERV’s, and made by the big name in bath fans: panasonic model FV-04VE1
I havent installed it as of yet, but will be shortly.
Response to Joseph Lajewski
Joseph,
The Panasonic FV-04VE1 ERV makes sense for hot climates, but it doesn't perform well in cold climates. The map below explains its limitations.
.
In my location which falls
In my location which falls between the B and C zones, it would perform optimally March through November.
I would still think that even with non-optimal operation December through February, that it would be a better solution to the exhaust only bath fan-- unconditioned makeup air derived through unintentional air infiltration (and probably cleaner air that hasnt traveled through insulation, etc)
Response to Joseph Lajewski
Joseph,
According to Panasonic, "This unit has automatic frost protection, which disables the energy recovery function and allows the unit to work as a normal ventilation fan. (Fresh air will not come into the house during frost protection mode)."
Response to Martin Holladay
Accoring to Panasonic from 32 degrees down to 20 degrees it will operate for 60 minutes as supply and exhaust followed by 30 minuts as exhaust only (defrost mode). Below 20 degrees it operates as exhaust only.
The majority of the heating season would be above 20 degrees which would allow for the fresh air to be tempered thus the energy savings, but there would be a dozen or so days a year that it would operate solely as an exhaust fan, the rest of the year you would see savings and better IAQ.
Response to Joseph Lajewski
Joseph,
Sounds good. As long as you understand the limitations of the ERV -- and you clearly do -- you are all set.
zoned duct systems
I don't know if this is the right blog to ask this on or not, but here it goes. I bought a house a couple of years ago, and due to some upstairs expansion, I had a zoned system installed. One zone consist of the original 3 bedrooms and 2 baths. The new zone goes to a new 600sq ft bonus area over the garage. I noticed that the return ducts do not have dampers, and I surfed around, and it seems that is normal.
In the new area, the 14" return duct runs a total of 100ft from the vent to the unit.. The new room cools and heats somewhat OK, but I'm getting about an inch of dust a week to clean, and I think its because the air is never getting filtered in the unit. I think since all the bedroom return ducts are only 15ft from the unit, it is pulling air from them, and very little is being pull from the new area.
Should I pursue having dampers installed on the return ducts for cleaner air?. Both zones are about the same sq ft. The new area only has the one 14" return duct, the old existing area has two 8" ducts. Any suggestions..
I also have heard that a $400 stand alone air cleaner may solve the problem too..
Whoa let's settle down..
While funny to read there's a lot of unnecessary fear mongering here.
Ventilation standards like ASHRAE 62, CSA F326, etc exist to allow designers to install sufficient capacity for a home's peak loads. Building codes require you to install ventilation capacity, but they do not require the occupant to use it. Think of it like a furnace, you install enough capacity for the coldest day of the year, but you don't run it at max for the rest of the time! Running an HRV all year would be analogous to this. It is up to the user to vary the HRV flow rate based on the home's occupancy. If it's a full house, run at a higher rate, if it's empty, slow it down. If it's overly dry in the winter, slow it down. If you see condensation on the windows, speed it up. Etc. Ideally, you can control an HRV based on CO2 & humidity sensors, which gives you the automated control that you're used to with a furnace.
The argument for using an HRV is twofold: 1) Energy savings and 2) indoor air quality.
1) Energy Efficiency
An HRV/ERV will save you energy whenever it is recovering more heating or cooling energy than it uses while running. That means that the decision to use an HRV is dependent on your climate, and on the efficiency of the unit. If you're using the Passive House level equipment with 90%+ ASRE and
If you're using a more run of the mill HRV, say 70% ASRE and >1W/CFM, then you don't want to run it constantly until outdoor temps are much lower (unit & climate dependent). That's why lower efficiency units are designed to run periodically at a higher rate, like a direct exhaust fan.
If you live in a quite moderate climate, then there isn't an energy motivation for using an HRV.
2) You might look at the above and say, "sheesh that's complicated". Indeed. Most designers look then to the 2nd benefit of an HRV: control over both exhaust & supply air flow rates AND locations. Direct ducting an HRV system guarantees that your home will have excellent air exchange. Supply-only and exhaust-only systems MAY provide excellent air exchange, if properly operated. HRV's are more 'set it and leave it'. Even if you forget it on all year, you'll have saved energy in the cooling & heating seasons, and ensured good air exchange the rest of the time. Keep in mind that the high efficiency HRV's consume only 100-200W while running, that's worth good indoor air quality to me. I bet you waste more energy leaving lights on from time to time.
The idea that bathroom exhaust air contaminates the HRV supply side is bunk. Code requires HRV's to have less than 5-10% cross flow. And the argument that you have to run your furnace fan with the HRV to ensure air exchange isn't true (when tied to forced air). If you've direct ducted the exhaust side it'll work just fine with the furnace off, especially if your HRV has two independent fans. When the furnace does kick on, it'll give the supply side a boost. Keeping in mind the climate specific HRV applicability, your furnace will be running fairly often anyways during the heating or cooling season, which is when you want that HRV running. If the furnace hasn't turned on in a few days, turn off the HRV and open the windows. If you want to maintain bathroom exhaust ability throughout the summer, and have the windows open, you can select an HRV with 'summer bypass', which bypasses the heat recovery core.
Stuart Fix, P.Eng
ReNu Building Science Inc.
Response to Stuart Fix
Stuart,
You wrote, "An HRV/ERV will save you energy whenever it is recovering more heating or cooling energy than it uses while running."
That's not quite true -- especially when you are comparing an HRV with an exhaust-only ventilation system. As you point out, an HRV can draw between 100 and 200 watts. That's a very significant electrical load (especially considering the fact that many homeowners leave these appliances on for 24 hours a day). Remember that a Panasonic FV-08VKS1 exhaust fan moves 80 cfm while drawing only 11.3 watts. That's only 5% to 12% of the electrical draw of an HRV.
To accurately determine whether running an HRV is cost-effective requires a more complicated analysis than you have provided. Fortunately, that analysis has been done. Read about it here: Are HRVs Cost-Effective?
Response to Martin Holladay
Martin, I believe it's intrinsically true that:
"An HRV/ERV will save you energy whenever it is recovering more heating or cooling energy than it uses while running."
This is the whole point of an HRV.
Yes, your 11W Panasonic exhaust fan will consume less fan energy than an HRV when running. But it's not so simple, let's crunch some numbers:
From the house's perspective:
Qconsumed = Qaux + Qlost - Qrecovered
Qconsumed = Qaux + 1.08*CFM*(Tin-Tout) - 1.08*CFM*ASRE(Tin-Tout)
At what outdoor temperature does the HRV recover enough heat from the exhaust stream to equal the fan energy it consumes? Qaux = Qrecovered, solve for To.
Pick the RecoupAerator, run at 150CFM. Qaux is 150W (510Btuh), ASRE=0.93, Tin = 68F
To = 68F - 510Btuh / (1.08*0.93*150cfm) = 64.6F
Pick a less efficient HRV, both in ASRE and fan energy. 150CFM, Qaux is 300W (1000Btuh), ASRE=0.70, Tin=68F:
To = 68F - 1000Btuh / (1.08*0.7*150cfm) = 59.2F
So the RecoupAerator is providing a net energy benefit when run at outdoor temps lower than 18C. A lower efficiency unit will not meet net benefit until a lower outdoor temperature. *Assuming you're heating your house to 68F/20C.
Let's compare this to 2 of your Panasonic direct exhaust fans, 160CFM, 22.6W (71Btuh), 18C outdoors:
(you could easily do this for 1 panasonic and the Recoup run at 80CFM, which would benefit the Recoup due to its higher fan efficiency at low flows)
Panasonic:
Qconsumed = Qaux + Qlost
Qconsumed = Qaux + 1.08*CFM*(Tin-Tout)
Qconsumed = 71Btuh + 1.08*160cfm*(68-64.6) = 658.5 Btuh (193W)
Recoup:
Qconsumed = Qaux + Qlost - Qrecovered
Qconsumed = Qaux + 1.08*CFM*(Tin-Tout) - 1.08*CFM*ASRE(Tin-Tout)
Qconsumed = 546btuh + 1.08*160*(68-64.6) - 1.08*160*0.93(68-64.6) = 587 Btuh (172W)
So even at 18C outdoor temperature, your 2 Panasonic exhaust fans are consuming about 20W more energy than the RecoupAerator would, a wash. *Assuming you're heating your house to 20C.
At 0C/32F outdoor temperature:
Panasonic:
Qconsumed = Qaux + Qlost
Qconsumed = Qaux + 1.08*CFM*(Tin-Tout)
Qconsumed = 71Btuh + 1.08*160cfm*(68-32) = 6291.8 Btuh (1.8kW)
Recoup:
Qconsumed = Qaux + Qlost - Qrecovered
Qconsumed = Qaux + 1.08*CFM*(Tin-Tout) - 1.08*CFM*ASRE(Tin-Tout)
Qconsumed = 546btuh + 1.08*160*(68-64.6) - 1.08*160*0.93(68-64.6) = 981.5 Btuh (287.6W)
The benefits of the HRV are clear during cold outdoor temperatures. You can run the numbers for hot weather as well, when using air conditioning, and you'll see the benefit of the HRV.
Now when the outdoor air is between that 18C-25C range, yes, your highly efficient Panasonic fans will use less energy than most ERV's, simply because they have a higher fan efficiency. If your ERV had the same fan efficiency then there'd be no energy difference.
Consider that the better HRV's, like the RecoupAerator, are filtering your air to the MERV12 territory. Your Panasonic will exhaust air, drawing 'fresh' air into the building through whatever path it may find. Certainly not giving you any control over indoor air quality. Adding MERV 12 filtration to a fan system goes a long way towards explaining the difference in fan efficiency.
The super efficient exhaust fan is externalizing the cost of indoor air quality, if you had equivalent filtration on a supply-only system, you'd see similar efficiencies to the higher-end HRV's even in the milder temperatures.
Response to Stuart Fix
Stuart,
Your latest analysis is more sophisticated than your earlier statement, but it appears to me that your calculations assume that heat is provided by electric resistance. In fact, heating energy may be cheaper than that.
Your analysis of "cost-effectiveness" also fails to take into account the cost of the ventilating equipment.
In any case, John Semmelhack's analysis (in the blog I linked to) accounts for all of these factors. His analysis shows that you have to be very close to the Canadian border for an HRV or ERV to be cost-effective.
One more point for Stuart Fix
Stuart,
There's a quirk to exhaust-only ventilation systems that many analyses don't account for: namely, that an exhaust-only system changes the amount of air flow through random cracks -- slightly increasing infiltration but reducing exfiltration.
Bruce Harley discusses this factor in an interview published in the January 2013 issue of Energy Design Update: "People often miss the basic fact that, if you employ supply or exhaust-only ventilation, you use only half of the electricity of other systems, and the heating and cooling loads are smaller than you'd think And unbalanced air flow has the funny characteristic that, when there's enough natural air leakage (at least 50% of air that is flowing through the fan), the fan gets half its makeup air from adding new air leakage. If 100 cfm is leaking in and 100 cfm is leaking out of the building shell in winter, and you add a 50 cfm exhaust fan, the flow in and out will balance. The added fan slightly reduces the pressures in the house, so that the lower part of the house has greater leakage and draws in more outside air; meanwhile, the top floor has less positive pressure, which actually decreases the exfiltration by the same amount. (This is the Palmiter 50% rule.) It amounts to 50% heat recovery for the same cfm air flow, when the weather is severe and you have to pay to heat or cool the air. In mild weather, when you need ventilation the most, the house may be almost neutral in terms of stack pressure, and the fan will move all 50 cfm, but there's far less cost to heating or cooling that air [during mild weather than severe weather]. The one place where this approach breaks down is in a really, really tight house. In that case, you may want to look at operable air inlets, but in the vast majority of cases, that strategy isn't needed."
ERV with a pollen filter?
Carl, Great post! I love the idea of designing ventilation for 1.5x ASHRAE 62.2 but actually setting it for .5 then letting occupants decide what they want/need. I also like the idea of separately ducting your ERV, I think the same should be true for your Dehumidifier in our hot/humid South. Is there a good off the shelf ERV that couples with a MERV 10+ filter on the incoming air to filter out the outdoor pollutants. I'm in Austin TX and pollen is a big issue for allergy sufferers. I'd love your thoughts on filtration for ERV's and HVAC systems in general. Appreciate all your thoughtful posts. Best, Matt Risinger
More than money
Much of the discussion has focused on the financial component. That's fine, but let's not lose sight of the other green, the environment. If the HRV/ERV saves energy, that's good in and of itself regardless if the amount of money saved completely pays for the capital expense of the equipment. This is particularly true if the energy saved is coming from non-sustainable sources. The financial component is important and the easiest to quantify, but let's remember that money isn't everything.
HRVs save energy? No, they use energy.
I find the calculations by Stuart Fix to be odd at best. Running an HRV never saves energy - it uses energy. If you turn off an HRV, energy consumption goes down regardless of the outside temperature (of course, air quality may suffer).
The amount of auxiliary energy use vs. the energy recovered isn't the issue -- it's the total energy use that matters. HRVs are only an energy saver in comparison to running a less efficient ventilation system - they provide ventilation with less energy waste To suggest that they save energy when operating below some outdoor temperature isn't true unless you have two ventilation systems installed that you are choosing between at any given moment, but if that we're the case, then the calculations would involve comparing total energy consumption of each system and not simply looking at auxiliary energy vs. recovered energy for the HRV. Ventilation costs energy.
Filters, (Guns?), and Money
Lots of great comments, some of which were way too technical for my pay scale. I do wonder what actually happens in homes when occupants can easily control an ERV/HRV. I'm sure that some are left on all the time, some are turned off all the time, and some are managed properly, but we don't know how many of each are out there. I think there is some parallel to the number of programmable thermostats that are never set up properly, becoming a waste of money invested in them. It takes careful management of a house, including ventilation, to make sure it operates efficiently and I don't believe that most people are properly trained, and even if they are, unless they are motivated to manage their equipment properly, it will be left on or off, wasting energy and/or degrading air quality,
As to Matt's question on filters - some ERVs and HRVs come with integrated filters, although in hot/mixed humid climates, you might consider using a central ventilating dehumidifier. It doesn't include heat recovery, but it will keep humidity lower in humid seasons. They can be set of specific amounts of outside air and to keep the RH below a preset level.
As to Daniel's last comment on saving money - It isn't always clear that ERVs and HRVs do in fact save money, particularly if they run when not needed or are set to a higher ventilation rate than necessary for the occupants.
Response to Michael Blasnik
Thanks for your comments, Michael. I wrote something similar in my blog, HRV or ERV?: "HRVs and ERVs are not space-heating devices, heat-delivery devices, or energy-saving devices. The more hours that an HRV or ERV operates, the more energy it uses — electrical energy to operate its fans, as well as heating or cooling energy to make up for the conditioned air that these devices expel from a home."
Response..
Good stuff all. It is good to note that my calculations above have nothing to do with $ cost, only energy movements. i don't question John Semmelhack's economic analysis. And it is key to note that I live in Edmonton, AB, a 10 000HDD climate and I'm almost solely designing Passive House style buildings. In that context, where the building is so airtight that you may suffocate without ventilation (done so for durability & energy savings), ventilation of some sort is absolutely mandatory, and thus when comparing the available options, HRV's will save you energy over the others.
If you step into another context, of a looser home and more moderate climate, where you may choose to not even install a ventilation system, without penalty, then sure, an HRV can be viewed as you describe.
As with all things green building, context of climate & building type are critical. I'm simply sharing things from the perspective of a very cold climate and a super efficient building (which is the goal). It's key for less technical readers on your site to understand the context of your articles so that they don't just take blanket conclusions and blindly run with them..
infiltration
> an exhaust-only system changes the amount of air flow through random cracks
I second this. Pressure balance (inside vs outside), infiltration and the effect on air quality, wall/ceiling moisture and latent AC/dehumidifier load need to be considered. On a room by room basis. A dedicated (and properly adjusted) supply and return should be used in any room where the door will be closed.
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