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Building Science

Make-up Air in Tight Houses

When homes are well air-sealed, negative pressures and supply air pose unique challenges

A combustion air vent, typically used for spaces that have natural-drafting fossil-fuel equipment. A similar system could be used as makeup air for a tight home, though that vent would probably have some type of damper system, so air is drawn into the vent only if the structure becomes negatively pressurized.

A few years ago, I performed a blower-door test on a new house with a small footprint of 1130 sq. ft. The results indicated an air leakage rate of 91 CFM at the test pressure of 50 Pascals—that’s .33 ACH50, which was unexpected for this code-built house. I actually measured the house volume again and ran the test two more times before I was convinced I had accurate results.

Later, the contractor contacted me to report the homeowners were noticing air coming through the exterior light switches when the dryer was operating. The dryer was producing close to the same negative pressure as my blower door every time it was being used—a negative pressure of about 50 Pascals. Is this a problem?

As with just about every building science question, the answer is: It depends. Let’s start with the risks.

Negative pressure in a hot-humid climate can lead to comfort issues. The exfiltrating air may be replaced by hot and humid outside air that will require conditioning, increasing the required energy needed to maintain comfort. Depending on where the outside air is entering, there could be the issue of condensation on exterior walls, air conditioning ducts, and even cold waterlines. This can affect both the durability of the structure and indoor air quality.

Frost on interior
In a cold climate, negative pressures can cause frost to form inside a home.

Cold-climate homes can also have issues related to negative pressures including decreased comfort, increased energy costs, and extremely low indoor humidity levels during the winter months. I’ve even seen the formation of frost on the inside of exterior walls where air is leaking into a home.

Homes in all climates may see backdrafting of fossil fuel–burning appliances if the home goes negative. Another concern is the pulling of air from attached structures such…

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9 Comments

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Randy,

    Like Allison I feel like make-up air is a topic hovering in the background waiting for our attention. What I am completely at sea on is what an appropriate response might be. Temperamentally I'd prefer to see effective passive air-intakes used, but maybe this will be solved through control changes to unbalance ERV/HRVs - or perhaps each appliance that can cause de-pressurization should be designed to deal with it?

    Luckily the problem isn't as pressing in the benign climate I build in where the energy penalty for cracking a window or simply having a few passive air-intakes isn't very high.

    1. Expert Member
      KYLE WINSTON BENTLEY | | #3

      Malcom,

      I've thought about this a bit lately too - I think with the coming era of the smart home, there ought to be an easy way to tie appliances together, so that everything just works behind the scenes, similar to how we just set the thermostat and the rest functions without us.

      An oven could trigger an associated ERV to go into unbalanced mode to supply more air, which boost to the kitchen. A bathroom exhaust could do the same, etc. I think bringing in air in a central location where it could still be conditioned is a good idea, but it has to be transparent or an occupant will never bother.

      1. Expert Member
        MALCOLM TAYLOR | | #4

        Kyle,

        It brings up a number of issues around how much integrated tech we want, or should require in a house. I see a real gulf between the average houses people live in in North America, and the houses GBA readers might aspire to. I'd like to see a couple of acceptable solutions ranging from what you describe to the other end of the spectrum where they don't rely on smart devices at all.

        1. Expert Member
          KYLE WINSTON BENTLEY | | #5

          I like to think of a solution similar to the 'connected fire alarms' level of complexity. They're hard wired with low voltage cable, and that seems sufficient. Similarly for thermostats, they're hard wired to the unit and that isn't overly complex. If we could somehow manage to have an interface between ovens, ervs, range hoods, bathrooms fans, and the like, with a common RJ45 or similar connection, I think it could go a long way in transparent automation.

          1. Northernbuilt | | #6

            Malcom and Kyle,
            This is the conversation I was hoping for. Should we be conducting a worse case negative pressure testing to determine building pressures after the build and incorporate properly sized make-up air systems to reduce risks, or are these risks simply perceived? Are the exhausting devices providing proper performance? If a dryer is designed to exhaust 150 CFM, what happens if the house limits it to 75 CFM? Dry time will be extended, but is there also a risk of the equipment failing early because of less heat dissipation? Is there a fire risk? Like Gary said, less than 100 CFM50 could be a problem. I think it's important to ask these questions.

    2. DennisWood | | #7

      I have been working on an HRV retrofit using two EC fans, both controlled via automation (Leviton 0-10V dimmer + Hubitat)

      As predicted, unbalancing does affect efficiency, but running positive pressure (by about 40 CFM) does completely stop back smoking when loading our high efficiency fireplace (which has outside combustion air feed). I've also got our kitchen hood exhaust inline EC fan "talking" to the HRV which also changes the ventilation behaviour. I suspect it's the future of smarter ventilation :-)

      Quite a few have mentioned that kitchen exhaust only works if used..and found this quite true in our home. To that end, I added ~$190 worth of automation bits so that power use detected on the induction cook top controls an inline EC exhaust fan. Once you have both systems automated, then everything else is just a few mouse clicks and automation rules.

      I have temperature probes in the air streams so you can view live data from the system:

      https://www.hundredgraphs.com/public/denwood

      Attached are the results of unbalancing about stale vs fresh (and vice versa) by about 40 CFM:

  2. billfrombirchwood | | #2

    The Messana ATU (HRV) has an extractor mode which compensates for unbalanced flow. This would require placing a current meter on a kitchen exhaust to raise the contact. It ramps down the exhaust ECM fan and increases the inflow ECM fan.

  3. mark_rob | | #8

    In the two energy positive houses I built I put in Fantech Fancm3000 kitchen exhaust fans, located outside the building envelope to reduce noise and a 2nd Fantech Fanrvf6, also outside the envelope to push the same air the exhaust fan volume back into the house, through a hydronic heat exchanger to heat or cool the air to the building set temp.
    I also installed a Zehnder Comfoair 200 which has a fireplace boost function that makes up the dryer air. Bathroom fans are through the Zehnder HRV and heat and hot water are heat pump with electric boiler backup.

  4. user-6870177 | | #9

    Get a heat-pump dryer, that doesn't vent conditioned air to the outside, and an electric or induction stove-top, which doesn't need a hood to ventilate inadequately burned gas by-products out into unconditioned space. Then add a HRV for fresh air all the time.

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