Fresh air intake

Discussion in 'HVAC' started by Jacob Schmidt, Oct 16, 2018.

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  1. Oct 16, 2018 #1

    Jacob Schmidt

    Jacob Schmidt

    Jacob Schmidt

    New Member

    Oct 16, 2018
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    I live in a 1930's era home in a St. Paul, MN suburb. We've been gradually renovating the house which has included new windows, vapor barriers, and bath exhaust fans. The house doesn't have a fresh air intake and I imagine after sealing it up better I'll need to install one, am I correct?

    The furnace is a high efficiency model with a coaxial exhaust and combustion air pipe, so it has it's own air supply. But the water heater and gas dryer use basement air for combustion, and of course the new bath fans depressurize the house compared to before the remodel (no fans). I know I have a back drafting problem on the water heater since some plastic fittings near the exhaust flue opening have melted. The water heater flue is routed up through the old oil furnace chimney.

    Is my best course of action a fresh air intake into my utility room (furnace, water heater, and clothes dryer all in same room in basement)? If so, is there a guide on proper sizing and such? If there's a better option, I'd appreciate the advice.

  2. Oct 19, 2018 #2




    Active Member

    Nov 12, 2016
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    Controlling the positive or negative pressure in a living space can be complicated.
    Sealing up the house makes an air intake necessary. The backdrafting needs to be stopped.

    When your dryer and bath fans (maybe a kitchen exhaust fan too) are all running .. the negative pressure (suction) in the house will be strong, especially as the house is sealed up better.

    One downside to this, is that a strong negative pressure can draw in radon gas, humidity, mold spores, and other pollutants through cracks and crevices in the basement floors and walls. So a slightly positive pressure in the living space, is desireable.

    One thing that's good to do, in any case, is to install a forced-draft water heater, or install a forced-draft unit onto the water heater you already have. These can vent through PVC pipe, which passes through the basement wall or window. No more chimney deterioration due to condensation inside it.

    Or you could install an electric water heater. No venting problems.

    Even doing one of these things, an air intake is still needed, just not as much.

    I'd go with a galvanized-pipe intake, at least 6" dia, which attaches to the return side of the air handler. The other end extends to the outdoors, away from sources of odor, heat, extreme humidity, etc. The outdoor end should have a way to keep insects and dirt out of the intake.

    I'd also make sure the air handler's blower has the ability to run at a very low speed when no heating or cooling is being called for. This keeps a gentle steady flow of outdoor air being drawn into the air handler, and filtered.

    The pipe diameter will depend on the volume of your living space (cubic feet).
    Larger than 6" air-intake pipe might be needed to maintain an "always-positive" pressure.

    When drawing in outdoor air in this way, it's best to use a MERV 12 or better furnace filter. MERV 12 is equivalent to HEPA, and will remove allergens that come in through the air intake.

    An alternative to a simple intake pipe, is an air-exchange device. It would need to be adjusted to maintain a positive pressure in the space.

    best regards

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