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Community and Q&A

Makeup air and dehumidification

user-7031237 | Posted in Mechanicals on

Built our 900 sf home using Zip Wall Sheathing. During construction I ran around with my foam gun in one hand and caulk gun in the other sealing everything I could find. Simply trying to do the best job I knew how. The end result is that in the winter with windows closed neither the bathroom fan or the range hood work as the building is so tight. I never would have thought.
I figured out that Broan offers make up air dampers which seems like the solution.But being so tight, with the shower and my wife’s cooking on a propane stove we drive the humidity up and have to run a dehumidifier cycling on & off most of the day during the winter.
Is there a better solution or will the make up air damper suffice?
Secondly we are adding on a master bedroom and bath this summer. I anticipate the same problem with this bathroom fan. Do I just plan for a MUAD or is there a better solution? Thanks!

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  1. user-2310254 | | #1

    It might be helpful to know your location. Did you complete a blower door test to determine the amount of air leakage?

  2. user-7031237 | | #2

    Located in zone 6A southern Maine, no blower door test

  3. walta100 | | #3

    Your complaints are kind of subjective before you try to fix the problem let’s try to measure it.

    1 Get a blower door test, and find out how tight your house is.
    2 Without the dehumidifier on will the humidity in the house get over 55% for more than 3 hours?

    If you have less than 1.0 ACH 50 you have a problem. I think an energy recovery ventilator would be the correct solution. Please note do not install this unit in or near a bath or kitchen.


  4. Expert Member


    What you have now is an exhaust-only ventilation system. In tight houses they rely on passive fresh-air intakes. Once you have these you will need to run the fans periodically to provide the ventilation and keep the humidity down - especially in such a small house where you probably generate the same amount of humidity as you would in a larger one.

    Your other alternative is to install a balanced ventilation system, with or without heat recovery. Adding this while you are renovating would be pretty simple. A description of the two systems and their benefits can be found by searching the terms here on GBA.

  5. GBA Editor
    Martin Holladay | | #5

    User 703etc.,
    First of all, can you tell us your name?

    GBA has long recommended that any bathroom exhaust fan installed in a new house has to be commissioned. To commission a bathroom exhaust fan, you have to measure its air flow rate. Here are links to three articles that discuss this issue:

    Bathroom Exhaust Fans

    Is Your Ventilation System Working?

    A Failure That Stalls the Certification of Many Energy Star Homes

    Let's say your goal is for your bathroom exhaust fan to move 50 cfm. (That's the minimum requirement for a bathroom exhaust fan installed in an Energy Star home.) You measure the air flow rate through the exhaust fan after it's installed. If it's moving 50 cfm, you know that it's working.

    If your house is so tight that enough makeup air can't enter through random cracks when you bathroom fan is operating -- a rare but possible problem -- you'll see the problem immediately when you commission your new fan. Instead of moving 50 cfm or 80 cfm, your fan will only be moving 10 cfm or 20 cfm. So you will be alerted to the problem during the commissioning process.

    The solution is to install a passive air vent -- basically, a hole in the wall -- in an inconspicuous place in an exterior wall.

  6. lance_p | | #6

    Being in 6A, if your humidity problem is only in the winter, incoming outdoor air will have much lower moisture content and will lower your indoor RH.

    The most energy efficient method, and maybe the most comfortable, would be to use an HRV. This will temper incoming air with the heat of outgoing air so you don't have any cold drafts. The cost of an HRV is likely high compared to the energy saved, but most new homes built to code need an HRV. You could cycle the HRV on and off to keep humidity in check.

    An ERV will do the same thing but will recover some of the moisture in the outgoing air, which will allow it to be used more often for a given level of winter dehumidification. An ERV also doesn't require a condensate drain (an HRV does), which makes installation easier and cheaper. This may be a good option given the small size of the house. Panasonic makes a nice small ERV that could work nicely:

    An HRV won't help in the summer when exterior humidity levels rise, so you'll be dependent upon air conditioning and/or a dehumidifier to keep humidity levels in check. An ERV is much the same, but can reduce the moisture content of incoming air which can reduce the latent load on your air conditioner.

    Whichever way you go, if your house is that tight you should be looking into some sort of ventilation. A makeup air damper might be a good idea as well just to allow your range hood to perform effectively.

  7. Jon_R | | #7

    > will the make up air damper suffice?

    You can simulate a passive air vent by opening a window about the same number of square inches. Try it before you decide on this solution.

    Just to be clear, make sure your solution considers all of these issues:

    a) excess negative pressure (effecting various exhaust flows)
    b) humidity
    c) indoor air pollution
    d) energy efficiency during operation

  8. user-7031237 | | #8

    Martin and guys Thanks for your responses! Martin I apologize for not giving my name, I am new to your site and did not know the protocol. Tom Wright it is!
    How do I know I need make up air without a blower door test? The only problem occurs in the late fall and winter when the house is closed up. When the Panasonic bathroom vent is turned on it starves for make up air - the exterior plastic 6" round damper on the range hood bows IN to a point then it bongs back out letting air in, sucks closed, then bongs out. Sounds like a wood pecker in the kitchen! Open a window problem goes away.
    Interior humidity only occurs when closed up in the winter months ( we are wide open in the summer). Yes well over 50% from showers and cooking. How do I know? Table top humidistat and window condensation. Great, Integrity double hung windows but we get puddling on the middle sash joint. Running the dehumidifier set at 50% seems to keep everything in check. We empty probably a gallon every 2 days. When we are not home, minimal humidity.
    Not concerned with indoor pollution at all, maybe not smart??? Just the make up air and dehumidification.
    HRV or ERV or stick with my make up air damper and dehumidifier?
    Thanks to all!

  9. GBA Editor
    Martin Holladay | | #9

    Q. "How do I know I need make up air without a blower door test?"

    A. Re-read my response (Comment #5). I wrote, "You measure the air flow rate through the exhaust fan after it's installed. If it's moving 50 cfm [or whatever your target airflow is for the particular fan you have installed], you know that it's working." If you are worried about makeup air, be sure all the windows and doors of your house are closed when you conduct this test.

    You should also read the articles I linked to. One of them, Is Your Ventilation System Working?, explains several ways to measure fan airflow.

    You should also read Simple Methods for Measuring Air Flow.

  10. Expert Member
    Dana Dorsett | | #10

    The small Panasonic ERV can't be operated in winter without voiding the warranty. (The core is susceptible to frost damage.) See the map on page 8 of the manual:

    On the map Maine is in " Zone C: Unit can perform optimally between April through November." December through March is peak time when the ventilation is needed.

  11. lance_p | | #11

    Dana, I don't see where the warranty is voided? There doesn't appear to be any warranty information in that manual. Did you see that somewhere else?

    I didn't realize that ERV wasn't really all that winter compatible, though it does continue to operate. The modes are defined on Page 7:

    1. Normal Operation above 32F (0C)
    2. Defrost Operation between 20F (-7C) and 32F (0C), 60 Min of ERV ventilation, 30 min defrost/exhaust only ventilation
    3. Exhaust Only Operation below 20F (-7C), samples outdoor air temp every 60 min

    So it will continue to operate throughout the winter, but it will do so as an exhaust only ventilation fan during sub 20F weather.

    In a 900 sqft house in Southern Maine it may still be an option? According to the average daily high in Agusta is 28F:

    So only at night or during cold snaps would the unit stay in Exhaust Only mode for extended periods.

  12. user-2310254 | | #12

    The O4VE1 has a circuit board that doesn't handle moisture well. That may be why it is not recommended for winter time operation in what Panasonic labels as zone c. The company does make a cold-weather version of the 100 Intel Balance ERV, however.

  13. lance_p | | #13

    I like the Panasonic Inteli Balance 100. I'm planning to use the cold climate version for our build here in Ottawa. I thought the minimum of 50 CFM might have been a bit much for a 900 sqft house. Compared to the 04VE1 it's also about twice the price, though it is quite a bit more efficient as well with sensible recovery at 81% @ 53 CFM.

    It would also operate in normal exchange mode 100% of the time above 14F, with a much less obtrusive defrosing cycle below that... it doesn't actually stop exchanging until -22F, and even then it's still exchanging for 4 min out of 60.

  14. Expert Member
    Dana Dorsett | | #14

    Lance: Perhaps I mis-remembered the details on the FV-04. The core is susceptible to frost damage, and I didn't think it was warranteed against frost damage. It automatically adjusts the mode of operation self-protection to minimize frost risk, but it's not something you want in a zone 6A location, and only aggravates a make-up air problem:

    "The supply port damper shall close below 20°F
    (-7°C) to prevent freezing of the core. "

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