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Makeup air, I think I built our house too tight.

vashonz | Posted in General Questions on

We recently finished our 1260sqft, single level house. Lots of attention to air sealing. I never had a blower door test done, based on wingnut evidence.
Some quick details:
-Zehnder CA200 ERV
-Zephyr Ak7136AS 210 – 400CFM vent hood.
-JennAire Induction Cooktop
-Electric (non-heat pump dryer)
-Kuma wood stove with makeup air kit installed.
-Mitsubishi Split units
-Rheem HPWH.

The only “combustion” appliance in the house is the wood stove, which is only used occasionally (two times total as of now).

When using the vent hood on low (210CFM) the house gets depressurized to the point that there is a noticeable change in fan speed/load/noise when opening a door or window. We have taken to opening a door or window when cooking, providing makeup air and allowing the vent hood to work.
This is my ‘wingnut’ air sealing testing, I felt pretty comfortable with the air sealing without paying for the blower door test.

Now that it’s colder, opening a window makes the kitchen cold, we’ve started opening windows on the other side of the house, which works, but makes that side of the house cold.

Of greater concern was last weekend, I had built a fire, catalytic burner engaged, was burning nicely. Vent hood turned on, and the fire visually went out. Opening a window caused the fire to start burning normally again. I checked the outside air inlet with the fire burning, it had air flow through it (extra scientific back of hand test)

What’s the best/least energy penalty method to have makeup air? Currently looking at 6″ motorized damper, T- into the ERV – exterior ducting. Either switch controlled, or using CT relays arranged in an OR circuit, sensing vent hood and dryer.

I’m also a little concerned that we’re wearing out the exhaust ECM fan on the ERV. Not realizing that the dryer/vent hood needed extra makeup air, the extra load/ DP the exhaust fan has been working against. The ERV has gotten louder (although it may be my imagination). ECM motors should be basically silent, and the exhaust side is louder than the intake.

I can post some videos showing noise change, fire effects if interested.

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  1. Jon_R | | #1

    > best/least energy penalty method to have makeup air

    Deliver the cold air right to the stove - so you don't pay extra to heat it. Consider powered supply (so the duct is much smaller for the same CFM - and no negative house pressure).

    > Either switch controlled...
    Since the goal is to avoid significant negative pressure, consider switching based on a differential pressure sensor. But perhaps it's too difficult to protect the outdoor portion from wind effects.

  2. Trevor_Lambert | | #2

    The motor shouldn't go bad that fast. Good news is Zehnder will replace it under warranty (assuming it isn't actually your imagination)

    1. vashonz | | #19

      Definitely not my imagination, actually replaced the exhaust fan once based on bearing noise. Zehnder sent me a new unit under warranty.
      I was just concerned that something I had done was wearing out the motor excessively quickly. My experience with ECM motors is that they should be basically silent and lower maintenance/ longer lasting, wanted to make sure it doesn't happen again.
      Videos of troubleshooting (spin the motor and listen to noise)

  3. DAVID GOODYEAR | | #3

    Sounds like the house is pretty tight. I have had my share of issues with combustion inside an airtight envelope. My experience is that you will need to balance the make up air and hood to balance air pressure so you will need a powered supply as Jon R mentioned. Or provide enough make up air to slightly pressurize the envelope while the appliance is burning.

    As for wearing out the motors in your ERV...Im not an expert in ECM motors, but in general, when fans move less air, they actually do less work because they move less volume. Put an ammeter across a fan in a vacuum cleaner and measure the current before and after plugging the nozzle. the plugged nozzle uses less electricity becasue the fan starts free spinning. It does less work becasue it is moving less air. maybe cooling of the fan would be effected due to resistive losses but the load on the fan is less.

    1. Yupster | | #9

      True of most fan motors but not ECM motors. They work harder to provide the same airflow as resistance is increased.

      1. Jon_R | | #12

        True for an ERV. But generically, ECM motors can be programmed to provide all kinds of different behaviors - constant flow, constant power, constant pressure, etc.

  4. walta100 | | #4

    Without a blower door test you can only guess how tight your house is. I say pay for a test so you have a real number to work with.

    It seems you have ignored the advice generally given on this site and are now asking how to fix it. High efficacy homes do not have fireplaces or wood stoves, and giant exhaust fans. It seems you doubled down and installed both a giant up and a second down draft fans in your kitchen.

    Please tell me you have CO alarms as you are back drafting your stove and could fill your home with CO.


    1. vashonz | | #17

      Woodstove has a dedicated outside air supply. 4" ducted directly to the firebox.

      Vent hood chosen is 210-400CFM, below the threshold to require makeup air. I was intentional about choosing a lower volume (400CFM vs 12oo+). And decided against a recirculation version, based on the articles and discussions here.

      "Tom Wheeler notes that range hoods that exhaust less than 400 cfm do not require makeup air, and Dave’s own research suggests that even in a tight house—one with an air leakage rate of 1.5 ach50 or less—no makeup air should be needed for a vent hood with a capacity of 250 cfm."
      -This slightly contradictory to what I've found, at 210CFM the hood seems like it needs makeup air.

  5. Expert Member
    BILL WICHERS | | #5

    +1 for carbon monoxide detectors being a good idea in this home.

    If the kitchen hood can cause your fire to go out, it can also cause backdrafting of ANY combustion appliance that is not a sealed-combustion unit (such units have separate air loops for their combustion air and do not require on an air supply from within the home for their burners to operate). That would obviously include your woodstove. While I wouldn't expect an open flame a woodstove to be a big source of carbon monoxide, you do get combustion byproducts that you really don't want to be breathing.

    You need a source of makeup air for that range hood. Some of those articles Walter linked to will help you with that. I would recommend NOT using the wood stove again until you've rectified the makeup air issue. With insufficient air supply, the flue won't draft properly and you could have worse issues than the "fire going visually out".


    1. vashonz | | #18

      CO sensors are a great idea, the smoke detectors have them built in. I have another portable one I'll grab and put inside. Luckily the woodstove is the only combustion source, and is not necessary for heat.

      I'll look and see if I can find a VOC or PM2.5 meter at work.

      Makeup air for the cooktop is a priority, right now it's being solved by opening a window.

      I'll grab a vent capture hood, get some empirical data on how flow changes during different combinations of events.

  6. exeric | | #6

    Having a tight home isn't generally a problem as long as you plan for it. It doesn't sound like you have. Even tight homes generally need makeup air vent just for the kitchen range hood. Range hoods evacuating to the outside are not an extravagence considering the generation of 2.5 particles there. But you do need makeup air for that in a tight home. The standard remedy is a spring loaded damper valve through a wall to the outside. They work fine; you don't need electrical interlocks. Just make sure you're ERV is creating a neutral air pressure and it will function just as you need it to.

    The problem is that doing that brings in cold (or hot) air. Using make up air just for range hoods seems like an acceptable compromise. Using it for a wood stove and and a clothes dryer does not. There are good replacements for wood stoves and dryers that do not use house air. You should consider replacing both of them if you really value the tight and efficient house you've built.

    1. vashonz | | #20

      I thought that we had planned for it: low CFM vent hood, outside air for wood stove. We discussed non-venting dryers, but what we found didn't meet the needs of our family.
      Decisions were made based on what people living here wanted (lots of windows, wood stove, ect) instead of just energy efficiency.

      Replacing the woodstove/dryer is a non-starter for my wife, finding a fix is definitely an easier conversation.

      1. exeric | | #21

        The important thing is that you and your family are happy with the end result. We can become a little doctrinaire at this site but we don't have to live in it. You do. Best wishes.

  7. Deleted | | #7


  8. walta100 | | #8

    I remember Martin had an article about range hoods that I could not find it.

    You could use the stove but you need a window open to provide about as many square inches as the flue provides and more if the range hood is on.


  9. Yupster | | #10

    Sounds like you did a great job on airtightness, nice work!

    You would need a 12" passive vent to keep depressurization to 3 Pa (5 Pa is the maximum for naturally drafted appliances) delivered close to your stove. Not really an option. Powered makeup air is the way to go. For the amount of time a range hood typically runs, an electric resistance duct heater is a good way to condition the air if you aren't comfortable with delivering cold air to the kitchen.

    This is a great article for further reading on powered make-up air:

  10. exeric | | #11

    "You would need a 12" passive vent to keep depressurization to 3 Pa (5 Pa is the maximum for naturally drafted appliances) delivered close to your stove." Reference?

    I read the BS article and can't say that I agree with his juggling of priorities. The idea of diminishing returns in his analysis is painfully absent. I think most people balance the expenditure of labor and parts when undertaking a project. I usually agree with Lstiburek as he is a known expert but his analysis here is just outside of the practical. I'm specifically referring to having to push air as well as pulling air to equalize air pressure. Then he adds on to the absurdity that you have to power the vents and condition the incoming air. He doesn't seem to have heard of diminishing returns on added investment and complexity.

    If you restrict make up air to the essentials, a range hood, you are only operating during a very limited time. Who cares if the air pressure is not completely balanced during those events. You've compensated for the part that needs to be compensated for. It seems like some people will always make things more complicated than they have to be. The only thing I agreed with in his analysis is that the make up air should come from the floor if it's available to do that. He didn't even emphasize that appliances that are available without using house air should be used and those that can't shouldn't be used. It seems like he's acting like a gadfly just to be a gadfly. (At least in that article)

    1. Yupster | | #16

      The people with fires going out in their woodstoves care if there is depressurization, i.e. the OP. :)
      12" Passive vent based on Q = 1.07 x A x deltaP, the formula for flow through a square edged orifice. I didn't have time to look up the correct formula for round but it would be close enough.

      I don't really understand your critique of the article I linked, it all makes perfect sense to me. If you exhaust air, you have to supply air to prevent depressurization (if depressurization is an issue in the particular home). If you have a giant range hood, a passive vent is out of the question because as we've seen, they get big fast. And if you don't want cold/hot air being delivered to your kitchen, you have to condition it. If you can't afford that, don't put a big range hood in or live with unconditioned air being delivered to your kitchen.

  11. Jon_R | | #13

    I think a Lstiburek point is that pulling odors from an attached garage can occur with any amount of negative house pressure. Passive makeup vents always leave you with some negative pressure.

    My opinion - with small amounts of negative house pressure, garage odors are rarely noticeable. And if the wind is blowing right, they happen anyway. As Lstiburek points out, a detached garage is best.

    1. exeric | | #14

      You're probably right but the article was far from clear. Who would ever put in a passive makeup air vent into a shared wall between a house and a garage. I have a shared wall with a garage and I've never once smelled chemicals from it, of which there are plenty in there. The passive vent I did put in (in a different outside wall) short circuits any pressure difference between the kitchen and the garage and stops fumes from coming from the garage when the range hood is running. I do not need a fan at that outside vent to push air to make that happen. I wouldn't be so irritated at Lstiburek if people didn't so revere him as an expert. Some god awful advice is often given out by experts and this article is one of them.

      I'm not saying experts don't often have useful things to say. But hell, don't leave critical thinking at the door just because something is said by an expert. Trust your instincts when you know BS (not building science) is being spouted. I know you do probably trust your instincts but many people do not.

      1. Jon_R | | #15

        > passive vent ... short circuits any pressure difference between the kitchen and the garage

        A passive vent reduces but doesn't eliminate the pressure difference. Elimination *would* require a powered intake.

  12. drewintoledo | | #22

    Please keep us informed of your final decision. I will face a similar predicament.

  13. tallpinescabin | | #23

    I don't understand, if you're ducting outside air DIRECTLY to the wood stove, how can it be that the range hood use affects the flame in the stove? Shouldn't the interior of the stove and the chimney be working in tandem with outside air pressure, and not interior?

    Does a cat stove have a secondary air inlet as well maybe?

    1. DAVID GOODYEAR | | #25

      Stoves aren't airtight. Due to the high temperatures associated with wood burning, woodstoves stoves typically use compressed glass fiber rope for sealing gaskets between doors and stove bodies. These seals are not airtight. Additionally, they break down further over time and become leakier and need to be replaced. With the house depressurized, air will come from any path of least resistance...even through the compressed glass fibers, pulling air through two large ducts....the chimney and the combustion air intake. Unless natural draft pressure is large enough to compensate for the house depressurization, smoke will get sucked into the space. I have seen it. Its even more pronounced in tight houses.

  14. jackofalltrades777 | | #24

    As Walter stated, "High efficacy homes do not have (wood) fireplaces or wood stoves, and giant exhaust fans."

    You would not be having these issues had you followed the above advice. Combustible fuel sources and carbon monoxide can kill inside a leaky house, let alone a tight house.

    As a former first responder, I've seen my share of families snuffed out and found dead because of CO poisoning inside of a home. My advice is to get rid or stop using the combustible devices inside the home that generate CO. Go electric.

  15. Expert Member
    Peter Engle | | #26

    There is a deep psychological resonance for humans sitting in front of a warm fire, and many of us consider that a requirement for a comfortable home in a cold climate. All of us building "scientists" should be able to design a solution that allows use of a wood burning appliance in a tight house without killing the occupants or destroying its energy efficiency.

    And, as stated in many articles here, the PM 2.5 produced by cooking, even electric cooking, is unhealthful and must be removed by a range hood. Even at (relatively) low flowrates, those hoods depressurize houses and cause other effects. We should also be able to design around this issue.

    The OP seems to have been relatively careful in design and construction of the house, and has made design decisions based on the needs and requirements of the various stakeholders. He has recognized that there's a design goof and asked for advice. Criticizing his choice of fundamental design specifications is not productive or helpful.

    1. cjwagner | | #27

      A lot of people talk about designing these high performing systems for comfort. I don't know many things in a house to be as uncomfortable as cooking smoke ect. The ubiquitous toaster is an incredible indoor air polluter. So maybe someone has some real numbers on energy loss from (even high powered) range hoods to overrule comfort but I dont see the clients paying the cost of a high performance home to choose to forgo having one. They should be designed into the system and not maligned as energy hogs. (Again id really like to see some energy data on this)
      edit to say I agree with Peter ^

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