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House ventilation and makeup air

nj_homeowner | Posted in General Questions on
Hi!

We are building a new house in northern NJ, and after reading the great information here about ventilation, I am trying to get ours right.

Our house is a two-story colonial with mostly new construction. I do not think it will be especially tight.  We will have 3 bedrooms, two baths on the second floor.  The first floor has a 400 cfm range hood, a powder room, and a traditional fireplace.  The basement has a conventional clothes dryer and a hobby area with 150 cfm exhaust.

Heat will come from a hydronic system, AC from minisplits, so we have no ducts or central air handler.

I think we might get 3 pairs of Lunos ductless ventilators, putting one unit in each bedroom, and the remaining three spread throughout the living areas.

For the first floor and basement, I am thinking of installing a Fantech Makeup Air System that will provide balanced makeup air for the range hood, powder room, dryer, and hobby exhaust.

I’m having a hard time figuring out what to do with the two upstairs bathrooms, where I would like to install a 150 cfm Panasonic exhaust in each.

I have read here and elsewhere that such fans probably don’t “need” makeup air.  I have also read that people seem generally not satisfied with passive air inlets.

Some kind of makeup air seems like a good idea to me, especially because the cost of adding it now is relatively modest, whereas a retrofit might be tough.

I am thinking of installing a motorized supply air damper, such as those made by Broan, near each bathroom.

I could also use my makeup air system but they are pretty far apart in the house.

What do you think?  Any ideas?  What would be the absolute best solution?

Thank you all in advance.

Brian

 

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Replies

  1. MattJF | | #1

    The Lunos take care of your continuous ventilation, which is generally the only ones you worry about fresh air distribution. Have you determined the required ventilation rate? Unhappiness with passive inlets is generally related to limitations on providing fresh air distribution.

    Everything else is localized intermittent use. The only reasons to provide makeup air is if the local exhaust can't "find" enough air to operate at the desired CFM, you can't depressurize the house due to open combustion, or you feel strongly about filtering a small amount of air. You don't want to needlessly go punching a bunch of extra holes in your thermal envelope.

    How tight of a house are you building? In a really tight house you may not be able to source enough for things to operate. It would be reasonable to provide makeup air for the range hood, although not required at 400 cfm and below. Figure out what the actual CFM of the hood will be based on the planned duct static, it will probably be low to mid 300's. Most homes are fine without interlocked makeup for everything else.

    How big are your baths upstairs? A 150cfm bath fan is a lot. The Panasonic ECM fans will adjust to hit close to the target CFM, so performance is not a huge concern. At most I see needing a slightly larger door undercut or worst case a jump duct to source air from the rest of the house.

    You can figure out how much depressurization you will see based on a blower door result. For example a 2000sf (plus 1000sf basement) 8ft ceiling house has 24,000CF. An ACH50 of 1.0 converts to (50pa*0.004018 inH2O/pa)/(24,000CFH/60min) = .000502 inHg/CFM

    400 cfm * .000502 inHg/CFM = .2 inH2O - factor that into your hood performance

    1. Jon_R | | #2

      Pressure isn't proportional to flow, it's exponential.

  2. Jon_R | | #3

    > I have also read that people seem generally not satisfied with passive air inlets.

    When you check the numbers for higher flows, you typically find that a passive air inlet has to be huge to provide the required CFM without excessive pressure.

    > What would be the absolute best solution?

    An active fan that measured house and outside pressure and controlled the difference to whatever you want (eg, slightly negative in Winter, slightly positive in Summer).

  3. nj_homeowner | | #4

    Thank you for these comments.

    It sounds like maybe I can use the makeup air system for all of the exhaust fans... I could measure the pressure differences when construction is complete, and then adjust accordingly?

    Is there a problem having makeup air provided from another, distant part of the house?

    Does nobody use mechanical dampers for bathroom fans?

    1. Jon_R | | #5

      I looked up the Fantech Makeup Air System and evidently it is an automatic pressure balancing system. So a good choice. It will work (pressure wise) from distant parts of the house if they are connected via open doors. There will be some thermal effects.

      A balanced bathroom fan would be nice. A HRV would work, even if the heat exchanger isn't cost effective for the short periods involved..

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