Houses with very low rates of air leakage and heavily insulated walls and roofs don’t allow much natural ventilation. The whole point of building a high-performance house is to control the source and amount of outdoor air that gets inside.
That’s why designers typically include some kind of mechanical ventilation, often in the form of a heat-recovery or energy-recovery ventilator (HRVs and ERVs). But as LH has discovered, providing fresh air for a very small house can be a little more complicated, particularly when factoring in a clothes dryer that vents to the outdoors.
“When we built our little home (720 sq. ft., basically 2 main rooms) 13 yrs ago, we made it very tight and well insulated, but we did not take into account the need for makeup air,” LH writes in a Q&A post. “The first time we ran our dryer, we had air pulling in through the electrical outlets!”
LH’s solution was to leave windows open a crack, and to install a fan to pull outdoor air into the laundry closet while the dryer was running. Because LH lives in the Pacific Northwest, the fan brings in cold, damp air during the winter.
Although wall and ceiling space is limited, LH wonders whether an ERV is a possible solution. In looking over the options, however, LH finds that advice on this issue seems to be appropriate for larger homes with forced-air heating systems.
“One thing I would like to see on this site is more advice for those with small homes,” LH says. “Everything seems to be geared to having a forced-air furnace, which I don’t have. It wouldn’t make sense in a house this size, and I’ve never cared for the kind of heat they give.”
Ventilation for LH’s very small home is the topic for this Q&A Spotlight.
An ERV is probably not what you want
Even in the Pacific Northwest, outdoor air may seem cold and damp but it’s usually much drier than indoor air in absolute terms, Dana Dorsett responds. So pulling in outdoor air doesn’t create an indoor humidity problem. It will, however, create an energy and comfort problem.
One possible solution is to install an unvented heat-pump clothes dryer. Because it’s not exhausting a high volume of indoor, it’s not pulling in a high volume of outdoor air to replace it.
Further, Dorsett says, LH would be better off with a heat-recovery ventilator rather than an ERV. The two systems both incorporate a heat exchanger to reduce energy losses, but an ERV preserves some of the moisture content of incoming or outgoing air. An HRV does not.
“An ERV usually only makes sense in locations that have a fairly substantial latent cooling (outdoor humidity) problem in summer,” Dorsett says. “In the PNW the latent loads are almost always negative even when it’s hot out. An HRV (sensible heat only) makes more sense in your climate.”
Consider Lunos fans
The ventilation standard published by ASHRAE (62.2) calls for about 40 cubic feet of fresh air per minute for a 720-square-foot, one-bedroom house and 50 cfm for a two-bedroom house, Dorsett notes. That’s about half or less the volume of air discharged by a clothes dryer.
“But,” he adds, “if half the dryer makeup air is coming from the HRV it’s still better than pulling it through the electrical outlets (which is an indication of leaky house sheathing, by the way).”
Building Science Corp. believes the ASHRAE standard is too high, he adds. It calculates the required ventilation differently, suggesting that a one-bedroom, 720-square-foot home should have about 22.5 cfm.
This suggests that LH might consider through-the-wall fans, an option that GBA Editor Martin Holladay also suggests. (Holladay also noted, “HRVs, ERVs, and Lunos fans are not makeup air devices. They are balanced ventilation systems that exhaust the same volume of air as they supply. Manufacturers of HRVs and ERVs usually state this fact in their installation instructions, noting that these ventilation fans should not be used as a makeup air source.”)
A pair of Lunos fans would provide adequate ventilation, Dorsett says, and “would also provide a makeup air path for the dryer way better than what you have going now.”
Passive makeup air
Michael Maines has another suggestion: a Panasonic bathroom vent to exhaust air and a makeup air kit from Lunos that lets in fresh air when the building becomes depressurized.
This is a tactic Brian P. uses successfully. “We run one of our bath fans 24/7 at 30 cfm for a 1300+ sq. ft. house and that seems like more than enough ventilation,” he writes. “For a small place like yours, I think anything over 30 cfm would be overkill. Your best options might be something from the Lunos product line: either one pair of the E2, and eGO or two, or even their low volume exhaust fan (if you don’t have a bath fan).” The products are available from .
Brian P. installed four Panasonic passive air inlets in his very tight house to provide enough makeup air for the WhisperGreen fan running at 30 cfm. When the fan is boosted to 80 cfm, the exhaust fan will still depressurize the house.
“We initially installed 2 passive air inlets because the specs say up to 18 cfm, but that is wrong,” Brian P adds. “The reality seems like they are only capable of 10 cfm. The Lunos makeup air kits look nice, but 4x the cost of the Panasonic.”
Brian P. isn’t convinced the arrangement is the best idea in his cold climate because indoor air gets a little dry during the winter. “But,” he says, “it’s probably fine in the Pacific Northwest. If building again, I would go small HRV or consider Lunos.”
Lunos and other potential choices
Maines sees a gap in the market for balanced ventilation equipment for small houses, one that is currently filled only by Lunos.
Although well-made and energy efficient, the Lunos fans may be a problem for light sleepers when used in a bedroom, Maines says, because they turn on and off every 55 to 75 seconds.
“I don’t think Lunos would bother anyone in a living area, but inside a bedroom, sensitive sleepers can hear the fan change direction every 55-75 seconds (55 for thin walls, 75 for thick walls),” Maines adds. “They also put a hole in the wall, so if you have thick, cellulose-filled walls and triple-glazed windows, which block almost all sound, Lunos punch a hole in that envelope. I still think they’re great, just not for every situation.”
Other readers doubt noise would be much of a consideration, point out that other options are on the market.
Trevor Lambert, for example, doubts that fresh air would have to be brought directly into the bedroom in a house as small as LH’s.
“If it was really deemed necessary, I would still default to a small HRV with ECM motors,” Lambert says. “ has 5 speed fan controllers, so getting down to sub 20 cfm should be a trivial matter. My Zehnder’s ‘away’ mode defaults to about 18 cfm, and in that mode it would have to be completely silent in the room for me to tell if it’s on at all, or powered off from 3 feet away. Once I dialed it back to about 10 cfm. I literally can’t hear it run.”
Lance Peters says Panasonic makes two ERVs that are worth considering. One of them, the , has settings for 10, 20, and 40 cfm and can be installed much like a large bathroom fan. A second, the , can be set to cycle on and off if 50 cfm is too much ventilation. That unit can run as little as 10 minutes per hour.
Either of these would be “great choices” for a very small house, Peters says.
Maines adds that he recently spoke with a Zehnder America representative and was told that the CA70 has been discontinued, and the ComfoSpot50 was never available in the U.S. market.
Pros and cons of unvented dryers
The possibility of installing an unvented dryer might be appealing, LH says, but a little research has turned up mixed reviews.
“Long drying times, clothes coming out wrinkly, inability to lightly dry delicate items/clothes getting very hot, and frequent repairs are some of the complaints I’ve been reading about,” LH writes.
Brian P replies that he has had an for more than three years, without the need for a repair.
“I can’t say that wrinkles and drying delicate items have been an issue or something important to us,” he says. “We like it because it takes up less space than separate washer/dryer and didn’t require a big hole in the house for vent (and makeup air issues). Those benefits make it worthwhile compared to a normal setup. In your situation, it seems like switching to a ventless dryer (or combo unit) would be easier than making major ventilation changes.”
Lambert says his does take longer to dry clothes and also is “pretty noisy.”
“If you can put it away from the living areas, it’s not an issue,” he adds. “We have it in our upstairs bathroom, and while I wish it were quieter, I wouldn’t trade it. A regular dryer isn’t exactly quiet either.”
Aaron Beckworth says he has had an LG condensing washer/dryer for about 18 months. He can program wash and dry cycles before leaving for work in the morning, a plus, and clothes come out feeling soft without a static cling.
But, he adds, there are some cons: “The dry cycle is very noisy. It’s not a constant rumble noise, but an intermittent ronking mechanical sort of noise. We are in a very small home. Therefore, we try to always start our laundry on our way out the door. Of course this doesn’t allow particular items to be removed between the wash and dry cycles. The clothes do tend to come out wrinkled, some more than others. Jeans are the worst!”
Our expert’s opinion
Here’s how GBA Technical Director Peter Yost sees it:
It’s interesting that in really small airtight buildings even “small” pressures — such as that created by a clothes dryer exhaust — can pose problems. As Joe Lstiburek is wont to say: no good deed goes unpunished.
Here are the options as I see them, with variables being cost, efficiency, efficacy, level of technology:
Ventless heat pump dryer: Ventless condensing clothes dryers have been on the market for quite some time, but I have heard plenty of user complaints. For example, here is a detailed assessment of the from an owner of a local Passive House-performance-level home:
“I haven’t been thrilled with the dryer. There is no setting that gets the clothes really thoroughly dry, so you can’t just leave it to dry and walk away. When it’s signaled that it’s done drying (extra dry) the clothes are still a bit damp. If you catch it in time, you can do a second extra dry cycle to get them actually dry, but if you don’t, then they mildew…
“Our motivation for not using an externally vented dryer was that we didn’t want to have to re-heat the makeup air, but I think the right way to solve this problem is to actually have the dryer pull the makeup air from outside, heat it, and run it through the clothes, rather than trying to do a closed-loop heat pump system. The closed loop system sounds good on the surface, but the results aren’t good.
“… It appears that Bosch and Miele finally decided the American market was worth pursuing. I’d be curious to know if these dryers perform any better.”
Hang-dry in the same space your new HRV is servicing: My wife and I like hang-drying our clothes, outdoors when weather permits and indoors when not. We have an HRV dedicated to our basement (our radon mitigation system), which acts as a dehumidifier in the winter.
I measured how many pints/pounds of water are left in a typical h-axis load of clothing — about 4 — and then measured relative humidity in the basement when we hang-dry a load. Our HRV easily exhausts most of that latent load and is considerably more efficient to run — even for many hours — than our conventional clothes dryer. I just bump the fan speed up to high (200 cfm; about the same as the clothes dryer…).
Admittedly, in your very small home, this might not be quite as practical, but maybe you can cycle your clothes-washer and hang time so that it does not interfere with household operations or hosting small parties…
And, OK, Vermont wintertime moisture content of outdoor air is likely on average much lower than almost anywhere in the Pacific Northwest. Maybe you would need to plan your clothes cleaning operation around upcoming weather, which makes it more like drying clothes in the summer.
Air inlets: The thing I don’t like about this approach is that all kinds of events can activate these inlets, especially in a small tight home. The thing about opening a nearby window is that it is event-based.
Do your laundry someplace else: You need a bigger, leakier, building in which to dry your clothes — like a laundromat or laundry service. Of course, I offer this with plenty of tongue-in-cheek, but just sayin’…