Image Credit: Fantech
More Q&A Spotlight
The tighter the house, the more it needs mechanical ventilation. That’s become a rule of thumb for energy-efficient builders, and designers often turn to heat-recovery ventilators to get the job done. These relatively simple (but not necessarily cheap) devices use the temperature of outgoing air to moderate the temperature of incoming air, thus lowering the energy penalty for providing fresh air to the whole house.
Cathy O, who with her husband is building a house in Climate Zone 5 on Cape Cod, Massachusetts, is familiar with the arguments in favor of using an HRV. But the cost of the unit was a problem, and as she researched their options she was told HRVs may not even work that well.
“We want fresh air circulating in the house, to have a healthy home, and we’d also like to have enough air for the wood stoves to run,” she writes in a Q&A post at GreenBuildingAdvisor. “A quote to install an HRV system was way too high for us, so my husband thought he could install the product the company recommended himself. But now when I’ve talked to other HVAC specialists, they say the products don’t even work that well and are a waste of money.”
Cathy and her husband are planning to build their house as tight as possible, sealing all the air leaks they can find and adding an inch of rigid foam insulation on the exterior.
“Does the house need an HRV?” she asks. “Would a minisplit or two do the same work of providing fresh air and some heat? Should we just open the windows from time to time (as recommended by our chimney installer)?”
An equally interesting issue is the advice that Cathy O has, or hasn’t, been getting from the specialty…
Weekly Newsletter
Get building science and energy efficiency advice, plus special offers, in your inbox.
This article is only available to GBA Prime Members
Sign up for a free trial and get instant access to this article as well as GBA’s complete library of premium articles and construction details.
Start Free TrialAlready a member? Log in
13 Comments
Spot ERVs
Another very reasonable option to full size HRV/ERVs is to install the regular HVAC system with a Panasonic Whisper Spot ERV. Differences in prices can be $1-1.5k installed. You can get the best of both worlds.
Panasonic Spot ERV?
Armando,
I don't understand how the Panasonic Spot ERV could be the best of both worlds.
Why would you want to "recover moisture" (during heating season)?
http://shop.panasonic.com/resources/consumer_electronics/ventilation_central/pdfs/FV-04VE1-E.pdf
Simple, Dr. (John) Watson...
When installing a forced-air system, you could install a central fan, supply only ventilation system, but they can be costly in energy, especially if you don’t use two stage equipment. Using an ERV, it substantially reduces the amount of energy used for ventilation. An ERV is perfect for a home in cooling climate. It tempers the incoming air in the summer and reduces some of the moisture coming in (not to be used as a dehumidifier), and in the winter time it tempers and slightly humidifies the cool, dry air coming in. Aha!... no static electricity. Oh, yes... the indoor humidity is controlled by the HVAC system, when the IAQ thermostat tells it to.
Typically a full HRV/ERV system can be upwards of $2-2.5K installed, were a Panasonic Spot ERV can be around $800 installed. You can get them with two speed control, humidity sensor, motion sensor w/ time delay, built-in lighting, built-in heater, jada, jada…
As a personal preference, I do admit, in large homes is easier to do, or accept, central-fan supply ventilation, since the HVAC equipment is larger, variable speed and larger amount of fresh air to bring in, and I do like to have positive pressure in my homes.
That li'l Panasonic ERV...
...is not a wintertime ventilation solution for Cathy O (or most of the populated US, for that matter) due to it's limited frost handling capacity. Read the manual- while it does have a defrost mode, it's not recommended for wintertime use in all but the warmer US climate-zones 1-3 and very temperate west coast maritime zone 4, and maybe the warmer half of the rest of zone 4. (See the map on page 7.)
Bigger-deal centralized ERV systems are designed more robustly and can handle Cathy O's climate zone 5 wintertime extremes without damage or loss of function.
But does she need it? It depends somewhat on whether she bothers to direct-vent her wood stoves and other combustion appliances, and the overall tightness of the house. An exhaust-only ventilation approach can work, but might not be the best approach in a tight house with two woodstoves that didn't have ducted combustion air. Even if the low cfm exhaust ventilator does not depressurize enough to cause a backdrafting condition while the stoves are burning, it could be sucking in ash particulate the rest of the year, reducing rather than enhancing indoor air quality.
To be fair, my bigger concern will always be direct-vented/sealed combustion more so than the type of ventilation system used. ANY tight house is better served by avoiding the use of room air for combustion air in any heating or water heating appliance. Building tight is always right, but using room air for non power-drafted heating appliance combustion carries health risks, except in fairly leaky homes. The tighter the house, the more important this is.
Exhaust only make-up air
I am planning the renovation a 920sf house in Zone 5--Indiana--and I'd love to use exhaust only for ventilation. We hope it will be sealed up tight, but here's a question whose answer may be obvious: where does the make up air come from? The fan is simply drawing the make-up air in through those leaks we never sealed, right?
So then, to make sure we get enough ventilation, I need to get the ACH from the blower door test and figure from there how much the exhaust only fan needs to run. Is that the process? Thanks!
No exhaust only ventilation for me
The size of your house is perfect for a Mini-split ac/heat pump for your heating and cooling needs, plus a small HRV or ERV for your ventilation. A good HVAC contractor will size and select the correct equipment.
Personally, I do not like exhaust only ventilation, since it negative pressures the house, then bad things can happen.
Drawbacks of Exhaust Only Ventilation
I work in the weatherization industry, focusing on existing residential buildings. So as the BPI is about to adopt ASHRAE 62.2, this is a very relevant topic. We will be needing to install continuous ventilation in some form or another in a huge percentage of houses that we work on. While it is way better to seal up a building envelope and have calculated and somewhat controlled ventilation, there's still a fairly large energy penalty associated with adding exhaust only ventilation. When you are trying to save energy, deliberately inviting unconditioned air in to the home seems silly. Much of the time that makeup air is coming from places with very poor air quality (crawlspaces, attics etc). Additionally creating negative pressure in the home can lead to a whole host of problems many of which have been mentioned (back drafting appliances or wood-stoves), but some of which haven't, We here in Portland OR. have a radon issue (as do many other parts of the country). Radon is the 2nd leading cause of lung cancer and a very serious health issue. Any presence of Radon is bad, and all homes should be tested to measure the threat, but it can be assumed most homes here have some radon intrusion. If you are creating even slight continuous negative pressure, you are simply inviting more radon in to the home. The tighter the home, the more the fan affects the negative pressure.
Although exhaust only systems are the least expensive way to comply with 62.2, there are better ways. One thing you guys didn't touch on (and I'd like to know more about) is passive HRVs like Venamr's FAE 125. We can install these for around half as much as a stand alone HRV, and it basically has all of the pluses of the central fan integrated supply ventilation without nearly as great of an energy penalty.
For our climate, ERVs are out of the question (we want moisture out of the house), does anybody know of a spot HRV or any other more affordable option for introducing fresh air with balanced pressure and less energy penalty?
Fresh air
It seems crazy to me that people would spend all this money to build a new house with a tight envelope, including rigid foam on the exterior--these details don't come cheap--then balk at the price of an ERV. We all need clean, fresh air to breathe! Spend the money, you won't be sorry! And in addition to an ERV (Mine is made by UltimateAir), because of the wood burning stoves, you'll need way more air when they are used so you'll have to install some means of make-up air. Don't skimp on this either because you'll never get those stoves lit--or if you do they could back draft. And finally, something no-one ever talks about, a house lets go of a tremendous amount of moisture in the first year after construction from the concrete and the wood framing. You've got to get that moisture out and an ERV is made for exactly that purpose.
Why is an ERV "out of the question" if one wants moisture out?
If it is a truly small and tight house (I didn't see the SF in the article) with a heating load in the teens, then a minisplit would be an excellent fit.
The woodstoves had best be tiny and equipped with combustion air from outside.
Operation of any of:
Range Hood
Clothes dryer
Master bath fan
While a woodstove is burning will be cause for concern.
In my own 3400 SF 1600CFM50 home (in other words, not real tight), operating the hood or dryer within a week of having had a fire brings down the smell of woodsmoke...not unpleasant, but an indicator of where some makeup air originates.
Re; Curt Kinder's Response
I am talking about existing homes and we don't want to keep moisture in buildings here in Portland OR. Why would we want the humidity exchange that an ERV offers? In our climate it's high RH in the winter but not in the summer. We are so mild that A/C units are fairly rare. ERVs offer no advantages over HRVs in a vast majority of houses I go in to.
I have never met anyone who is a fan of those spot ERVs for out climate. So I was curious if anyone knows of a spot HRV or any other inexpensive product that offers fresh air without the energy and creation of negative pressure that exhaust only ventilation offer?
http://www.venmar.ca/40-air-e
http://www.venmar.ca/40-air-exchangers-fae125.html
Have you ever got any pricing info on the passive Venmar's??
What about using those with bathroom outputs??
I am facing a similar problem that i do not have a central air system
( i have a re-circulative system i have come up with using 4" PVC and 4" fans to mix all up )
But we use bathroom fans ( 2 panasonic ) and i would to setup the HRV to recover energy from those .
Using a passive HRV + bathroom exhaust(s) should be an economical option for most small tight house nah ?? Easy to setup on a timed switch just as for lights.
( i currently run it on a 5-10-30-60min timed switch )
what do you think ?
Would balancing air be a problem ? the Venmar FAE seems to have an adjustable input dampener.
What about depression created from kitchen output fans ??
Also wonder about frosting
Also wonder about frosting problem during winter time for zone 5-6-7 ??
how do you defrost a passive HRV ?
Response to Jin Kazama
Jin,
The Venmar AVS FAE 125 is intended to be hooked up to a forced-air furnace. The exhaust air stream is pulled from the supply-air plenum, while the fresh air stream is introduced to the return-air plenum.
As far as I can tell, the manufacturer does not recommend that these units be hooked up to Panasonic exhaust fans. If installed that way, the air flows couldn't be balanced -- the air flows wouldn't even be simultaneous.
More information here:
http://www.venmar.ca/DATA/DOCUMENT/44_5_us~v~Installation_Manual.pdf
The installation manual addresses the need to defrost the core by including a control that periodically runs the furnace blower during cold weather: "When the outside temperature is below the freezing point, heat recovery creates frost in the module. If the furnace blower is set to operate intermittently, the heat coming from the stale air will prevent unit frosting."
.
Log in or become a member to post a comment.
Sign up Log in