More Musings of an Energy Nerd
Most homes have several appliances or fans that expel air from the home: one or more bathroom exhaust fans, a clothes dryer, a range hood fan, a gas furnace, or a wood stove. The rate at which these appliances expel air is usually expressed in cubic feet per minute. What are typical air flow rates for exhaust appliances? Here’s a guide:
- Gas furnace (atmospherically vented, 72,000 Btu/h): 30 cfm
- Wood stove: Less than 50 cfm
- Power-vented water heater: 50 cfm
- Bathroom exhaust fan: 50 to 150 cfm
- Central vacuum cleaner: 100 to 200 cfm
- Clothes dryer: 100 to 225 cfm
- Range hood exhaust fan: 100 cfm to 1,400 cfm
Common sense tells us that if a fan is expelling 100 cubic feet of air per minute from your house, then an equal volume of air (“makeup air”) must be leaking into your house at the same time to replace the exhausted air. If your house is old and leaky, the makeup air enters through random cracks in your home’s thermal envelope. But if your house is relatively tight, you may need to provide a deliberate source of makeup air whenever high-cfm exhaust appliances are operating. (For more information on this topic, see “Makeup Air for Range Hoods.”)
Some builders use the following basic (but imperfect) rule of thumb: “Most houses don’t have to supply makeup air for exhaust appliances rated at 400 cfm or less.”
This rule of thumb is actually enshrined in the building code. It can be found in the 2009 IRC, section M1503.4, which states: “Exhaust hood systems capable of exhausting in excess of 400 cfm shall be provided with makeup air at a rate approximately equal to the exhaust air rate. Such makeup air systems shall be equipped with a means of closure and shall be automatically controlled to start and operate simultaneously with the exhaust system.”
(Note…
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
30 Comments
Martin,
Great to see you referencing Minnesota Mechanical Code. MN Code is very well thought out, written and reviewed by some very sharp people. I think MN Code should be considered for all cold climate building.. I believe other northern tier states are considering MN Building Code as their framework.
Doug,
I'm not disparaging the authors of the Minnesota code when I share my opinion that no code will succeed, however brilliant its authors, if it is too complicated for residential builders to easily implement.
Martin,
I'm curious, would it be possible to modify the controls an HRV or ERV to change its function depending on whether an exhaust appliance, such as a range hood, is active?
It seems possible (theoretically...) for an HRV or ERV to run in a balanced manner in normal operation, with intake air equal to exhaust air. Why not program the system to boost delivery air based on a call for makeup air? This would seem to be an elegant and energy-saving approach for tight houses with an HRV or ERV already installed.
I understand that hacking an ERV or HRV control system might not be a viable option in the real world today for most people. But if an HRV/ERV manufacturer supported the type of function I described, do you see any reason why it couldn't (or shouldn't) be done that way?
Thanks for another useful post.
Brian,
An HRV or an ERV is a balanced ventilation system, not a makeup air appliance. In fact, most HRV manufacturers explicitly state in their owners' manuals that these appliances are designed to provide balanced ventilation, not makeup air.
Just as it's possible to use a toaster as a space heating appliance, it's possible to imagine a way to hack the controls of an HRV to turn it into a makeup air fan. But supply fans aren't that expensive, so I don't see the point. If you want to install a supply fan, install a supply fan.
This is possible with a Zehnder unit, no hacking required. I have no idea how well or poorly it might work. For each of the four settings, the fan power of the exhaust and supply are programmed separately. So there's nothing stopping you from programming the HIGH setting to 100% power for the supply, but have the exhaust fan at the same percentage as for MED speed. Like I said, I am not endorsing this as a method, but if you wanted to try it would literally take seconds to set it up. Note also that this would restrict your "boost" function to only provide make-up air. You could no longer use it to boost the exhaust in the bathroom, for example.
Think you are safe with "sealed" combustion appliances? My experience suggests otherwise.
A recent build with owners that insisted on gas appliances, against my strong discouragement, has a tankless gas water heater that will not operate with the vent hood running. Presumably the water heater has a built-in safety device which doesn't allow it to run in backdrafting conditions.
This is a pretty big annoyance and highlights a life-safety issue. Occupants can't shower or use any hot water when someone is safely running the cooktop. If or when the safety feature fails, it's unlikely occupants will know, or even change their behavior if they did know.
A make-up air unit is installed per code and verified to be operating correctly by the HERS rater.
If you want to completely eliminate chances of backdrafting, eliminate indoor combustion.
Brian,
Thanks for your comments. The situation you describe is unusual. If I were the builder, I would call in a home performance contractor experienced with pressure diagnostics. There must be a factor in this story that is missing.
If the gas water heater is truly a sealed-combustion unit, and if the house has a makeup air system designed to balance the flow of the range hood exhaust fan, then there shouldn't be any reason why the water heater won't work.
I'd want to make sure (a) that the water heater really is a sealed-combustion unit, and (b) that the outdoor air duct supplying combustion air to the water heater isn't blocked, and (c) that the water heater isn't defective or broken, and (d) that the makeup air system is properly sized, and (e) that the makeup air system was properly commissioned (and the airflow rate verified).
You're right, of course, that eliminating combustion is a sure-fire way to avoid this type of problem.
Much needed post here. I think the most important take away is that it is entirely possible to build a home with no exhaust only appliances except the range hood. Residential range hoods are where the innovation is needed. We need sensor driven (comes on automatically) balanced range hood systems (sold as a system). Get on this range hood people, or I'm going to start developing it myself.
The way the code reads, why are passive inlets allowed? Whether integrated with an air handler or not, the home is not "provided with makeup air at a rate approximately equal to the exhaust air rate." The depressurization is not reduced enough to even bother with, at least on the setups I've tested.
One thing not mentioned in here that I've been thinking about a lot lately is what is an acceptable level of depressurization for intermittent operation? I've tested Energy Star homes with exhaust appliances that can put 40pa of negative on the house, not that they will all be on at once, but the range hood alone is often 18 to 20. I won't even get into Habitat's issues. With Energy Star limiting bedroom to core pressures to 3pa, should it be a goal to keep whole house depressurization under 3pa? Is there a standard that sets up what the tolerance is here? Except for radon, and attached garages, are there other reasons not to spend time at -20pa? This level of depressurization has never seemed right to me, even with sealed combustion or all electric.
Andy,
You raise an interesting question: What, if any, problems might occur in an all-electric home that is occasionally depressurized to -20 Pascals? While it is tempting to jump to the conclusion that "Obviously, that's a bad idea," I don't have any data on the issue, nor have I heard any anecdotes about problems or failures. I welcome comments on the issue.
I'm building a small (all-electric) home that will be very air tight via AeroBarrier. Even though the range hood will be 300 CFM or less I'd like to use a motorized damper for makeup air to make sure it exhausts effectively.
It looks like I'd have to use one of the Broan "Universal" dampers which switch on based on a sensed pressure drop in the hood duct (their direct-wired dampers are only compatible with larger hoods). I suppose that should work but was hoping for a more hard-wired option. Does anyone know of any other direct-wire damper setups that could work with any hood?
Bonus question - ways to make the damper open when the clothes dryer is on?
Thanks all!
Just about all I see are the pressure switches and they seems to work just fine. The ones I see can be set up to respond to negative or positive pressure, not sure what the threshold is. I don't see any reason the same switch couldn't be drilled into the dryer exhaust duct, though I'd still check manufacturer specs.
Ok thanks Andy, I guess I worry about the little probe getting clogged with grease or lint over time...
You could also potentially use a current-controlled switch to detect when the dryer is running. I got one of these from hvacquick.com a few years ago to tame a dryer boost fan that wasn't reliably turning on or off based on the pressure differential. Worked great in that application... not sure it would work with the Broan damper, but it would provide a way to tell when the dryer (or range hood, or whatever) was running.
I filmed a video on the same topic and I would appreciate feedback. The link is attached - https://youtu.be/piEo3FC2Veg.
Todd Witt
Synergy Home Performance
Todd,
I don't recommend this video.
Two comments:
1. I question the judgment of the designer who included a masonry wood-burning fireplace in a new, relatively tight home.
2. Your statement about the return air duct is false: "The cold or warm air that is pumped into the house [by the furnace or air handler, presumably], it gets brought back to the unit [again, the furnace or air handler] to be reconditioned. And that's essentially what the return duct is doing. But all day long, as the unit is running, it's pulling air back out of the house."
No. The return duct isn't pulling air "back out of the house." The air is simply going in a circle, inside the house -- unless, of course, your duct system is deeply flawed.
Notice that I say in the video that the best solution is never to have a wood-burning fireplace in new home. However, people continue to install them and we have found the best way to make them work in a home.
In this house, the air handler is in the conditioned/sealed crawlspace and maybe that is not the best wording. However, in most houses in the Southeast, the air handler is located outside of the conditioned space.
GBA articles are fantastic and helpful, but sometimes by the time the article addresses all the possible scenarios (house tight or not, combustion appliances or not, construction materials, climate zone, etc) then I have a hard time sorting out what does or does not apply for my scenario. So can someone tell me if I have this right? For a tight all-electric house (elec stove/oven, heating/cooling by a mini-split heat pump, ventilation with an ERV/HRV), R40 walls R60 ceiling, east coast Zone5)…
(1) a range hood fan with low cfm (<250) should be vented directly to the outside and is likely to not need makeup air. The ERV/HRV can also pull air from the kitchen but should be several feet away from the stove to avoid picking up a lot of grease.
(2) If you opt against a heat-pump clothes dryer, the exhaust air from the dryer should be vented directly outside, that it is undesirable to dump all that humidity into a tight house, even during the dry winter.
(3) If a range hood <250 cfm is OK without makeup air, then it seems a fair assumption that a dryer venting < 250cfm would also be OK, although both devices active at the same time could be an issue.
Lastly, the only dampers I know of are the little gravity-operated flaps in bathroom fans. I would imagine these can leak a lot of air in either direction under varying conditions. This type of vent/damper seems like a bad idea when you want a tight house. Is this true? Should any vents to the outside (namely stove and dryer) have a motorized damper that seals tightly? (I’m assuming that motorized dampers shut tight – I haven’t studied them yet). Thanks for your help
Dennis.
I agree with your take on all three p0ints.
There may be a case for motorized dampers. To me it's just one more thing to go wrong, and I've never seen any numbers to suggest vents with good conventional flappers are a huge energy problem. The one spot I'd be really wary of putting a motorized damper is on the dryer, because the consequences of a blocked exhaust duct can be so serious.
Thanks Malcom,
And yes, that occurred to me that if a motorized damper were put on the dryer duct that a "failsafe" mechanism should be in place, a limit switch detecting whether the damper is open or a vane detecting air flow, and this triggers an alarm or even an electrically controlled switch/relay on the dryer -- something to prevent it from operating if the damper didn't open (that would take a relay with a hefty rating)
In bathroom fans the dampers I've seen don't come anywhere near closing off the passage. Maybe the dampers connected with range hoods and dryers are better at minimizing air leakage.
I rely on a good vent termination at the exterior. These seem to work pretty well.
https://www.hvacquick.com/products/residential/Ventilation-Accessories/Discharge-Caps/Seiho-SBP-and-SFBP-Series-Aluminum-Dryer-Vents-with-Backdraft-Flapper-and-Tail-Piece
They look really good too.
Check these out: https://www.amazon.com/LAMBRO-289W-Dryer-Vent-Seal/dp/B00JPMU7SY/ref=sr_1_13?crid=CJRDETM34S7D&keywords=heartland+dryer+vent+closure&qid=1551328585&s=gateway&sprefix=heartland+dr%2Caps%2C147&sr=8-13
Dennis,
Like Malcolm, I agree with your analysis on the three numbered points. The vast majority of homes that have a 250 cfm clothes dryer and a 250 cfm range hood work just fine -- especially if there are no atmospherically vented combustion appliances in the house. And of course you are right that a conventional clothes dryer needs to be vented outside -- that's a code requirement, and it's also required by the dryer manufacturer.
As with any rule of thumb, though, there are exceptions. The exception is the small home built to Passivhaus-levels of airtightness -- or close to it. Many Passivhaus builders choose a heat-pump clothes dryer, or use a recirculating kitchen range hood coupled with a ceiling-mounted exhaust grille connected to an HRV system. That's because their homes are so tight that exhaust appliances can be starved for makeup air.
Martin,
Thanks for the suggestions, but don't think they solve the big picture problem of "sealed" combustion appliances in very efficient (tight) homes.
Suggestions: "(a) that the water heater really is a sealed-combustion unit"
Yes, tankless water heater is a "sealed-combustion" unit
"(b) that the outdoor air duct supplying combustion air to the water heater isn't blocked."
No, it's clear and works fine except when vent hood is operating.
"(c) that the water heater isn't defective or broken:"
I suppose this is possible, but my understanding is that this is a working feature of safe operation. It's good that a "sealed-combustion" unit will shut down when it senses backdrafting conditions. The concern is when this safety feature fails.
(d) that the makeup air system is properly sized, and (e) that the makeup air system was properly commissioned (and the airflow rate verified).
The MUA system was installed per code and properly commissioned by Vandemusser, one of the most experienced HERS raters in our region. Founder Amy Musser, who has a PHD in mechanical engineering was present at commissioning. She would have preferred a 10" MUA intake to our installed 8" (or maybe 12" to our 10"?) but said intake size should be sufficient with the overall suggestion of a powered MUA fan to overcome the water heater problem. It was a big expense the clients decided against. I certainly don't like the idea of providing such an expensive solution that will only add to maintenance concerns.
I don't think the problem is "your MUA system is not big and expensive enough" the problem is the "sealed combustion" appliance. When a home achieves airtightness levels that requires a ductblaster or blower door C ring to achieve 50 pascals, is it really surprising that a powerful vent hood can achieve the same depressurization?
I believe Amy said that backdrafting "sealed-combustion" appliances becomes a concern at 50 pascals but I read somewhere that 30 is where some units can begin to be depressurized. This seems to be an easy threshold to reach for tight homes under normal operating conditions.
Obviously the solution is not to build so tight. It's either install a powerful, expensive MUA system or eliminate indoor combustion.
Brian,
You wrote, "I believe Amy said that backdrafting 'sealed-combustion' appliances becomes a concern at 50 pascals but I read somewhere that 30 is where some units can begin to be depressurized."
I can't really confirm Amy Musser's statement, but for the sake of discussion, let's assume that Amy is right. Here's the issue: If someone has installed and commissioned a makeup air unit to balance the exhaust flow of the range hood exhaust fan (and other relevant exhaust fans), then the house can't be depressurized to 50 pascals by the range hood exhaust fan.
True but most systems are unbalanced. How many raters out there even measure MUA CFM? Powerful vent hoods depressurize with typical MUA systems. I bet many sealed combustion appliances backdraft at lower pressures than 50 pascals. 15?
Maybe it's a low threat for most homes but smaller, tight homes are best to get off the gas. Moving forward where owners insist on powerful vent hoods with indoor combustion appliances, I will push for unbalanced MUA as offered in Dr Joe's manure sucking article. Positive pressure, positive vibrations.
https://www.buildingscience.com/documents/insights/bsi-070-first-deal-with-the-manure
Brian,
We're in agreement on most points. It's better to have an all-electric house than a house with gas appliances, and it's better to have a small range hood fan than a big range hood fan.
Way back in my Comment #7, I gave the following advice to anyone installing a make-up air system: It is essential "that the makeup air system be properly commissioned (and the airflow rate verified)."
You're saying, "How many raters out there even measure MUA CFM?" You're right that many don't. But they should.
“[Deleted]”
I'm building a very tight house in 6B with a Zehnder system, induction cooktop, <400 cfm hood, standard electric dryer, central vac, mini-splits, direct vent gas fireplace. All these items are on the first floor with a fully conditioned crawlspace below. Also a sealed combustion gas water heater or boiler in the crawl beneath the laundry room. The kitchen hood is about 50 feet from the laundry room. I'm trying to figure out the best make-up air strategy and I'm seeing solutions all over the map from extremely simple to quite complex. The house envelope is 3840 sf.
1. I'd like to use one makeup air system for all the needs: hood, dryer and central vac. Seems like I could do that with a pressure sensor to control a make-up air damper, or 3 different current sensors to control the damper. Pros and cons? The only drawbacks I've heard to a pressure sensor are that they can drift, and that the house will go under negative pressure for a short time until the makeup system activates.
2. Some suggest just opening the damper and letting the air in passively, others suggest using a fan. Is a fan necessary?
3. To what degree can I just make the air available and it will go where it needs to go, and to what degree do I have to duct it close to every location it needs to go? (Not even possible really in the case of the central vac outlets which are distributed.) Someone else addressed this by putting the vac inside the envelope, but even with a hepa filter that seems to negate some of the benefit of having the CV.
4. Will the Lstiburek trick of bringing the air in from the other end of the house, so it gets conditioned along the way, work well enough to avoid electric pre-heat?
Appreciate your thoughts!
qofmiwok, I had the exact same set of questions. The only thing I could think of as to why all the suggestions and diagrams I see are to duct the makeup air to the kitchen would be to avoid mixing cold air with the rest of the house as much as possible, or to avoid pressure differences across rooms, but those both seem questionable to me. I have yet to find a calculation for what size motorized damper to use, or a good argument of using a powered fan over a damper. I see a lot of videos/articles about people using expensive inline powered fans, silencer ducts to reduce the noise, and a heater to heat it, and I would really question if that's just because they can sell it to high end clients vs a hole in the wall at the opposite side of the house.
Qofmiwok and Carson,
I don't recommend a complicated system with pressure sensors, for the simple reason that they don't work. (If you compare the indoor air pressure with the outdoor air pressure, you'll find that the outdoor sensor is often affected by wind--and a shift in wind direction will lead to a misleading sensor reading.)
Simple is better than complicated. Clothes dryers are in the range of 100 to 200 cfm, and they don't usually cause problems. Ditto for central vacuum cleaner systems--they're usually in the range of 100 to 200 cfm. The 400 cfm range hood is potentially problematic--but only if you have atmospherically vented combustion appliances, which you don't. (You have told us that your gas water heater is sealed combustion model.) So it's not even clear that your house will have problems.
If you want to know where your makeup air is coming from, the simplest approach would be to crack a window. If that's too simple for you, you could install a 6-inch diameter or 8-inch diameter outdoor air duct (ducted to any part of the house you desire) equipped with a motorized damper that is electrically interlocked with the range hood fan.
Log in or become a member to post a comment.
Sign up Log in