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Community and Q&A

HVAC fresh air intake

ruthewalrus | Posted in General Questions on

Hi, I have a 1950s ranch house with full basement partially finished in Chicago suburbs.  The home has been extensively sealed air tight over the past decade and has created a negative pressure situation inside the house.  I’ve been keeping a basement window cracked open to allow for some fresh air but in winter it spills a lot of cold air into our basement we use as a family room.

I’m planning to add a 4″ PVC duct outside and straight into my HVAC return duct so I can keep the window closed and pull in some fresh air.

My question is about being overly cautious.  The best place to put the intake snorkel outside the house is only 8 feet away from my Radon exhaust.  Am I gonna suck radon into my house?

BTW, the window I currently leaved cracked is only 10 feet away from the radon exhaust too.

Pic attached, red pipe proposed location of fresh air intake.


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  1. Yupster | | #1

    That's plenty of distance. No problems there. However...

    If you spent all this effort sealing the basement, why just cut a big hole in it? What is causing the negative pressure? Do you have a fireplace or a b-vented water heater that is affected by this negative pressure? Do you currently have a ventilation system providing fresh air or will this duct be your ventilation system?

  2. ruthewalrus | | #2

    Thanks, Yupster.

    No fireplace, but yes we have a 80% efficiency furnace and a traditional gas hot water heater sucking combustion air out of the basement. We currently have no other fresh air intake, this would be it.

    1. user-2310254 | | #6

      Maybe it is time to replace the furnace and water heater and go all-electric.

  3. ruthewalrus | | #3

    Perhaps worth mentioning my main reason for wanting to do this is. Aside from recently learning about the clean indoor air benefits of slight positive air pressure. The past few years I had occasionally been able to smell the inside of the wall smell being pulled into our house. I believe I addressed 99% of this by caulking up my windows from the inside so this direct air vent was going to be my final adjustment to calling the air issues done.

    Also, I was reasonably well assured it would be a fine thing to do after confirming with a local HVAC shop owner that he himself also has a 4" PVC tied directly into his return air duct. He warned against too much positive air pressure inside could cause air leaking moisture problems the other way, but a 4" intake that shouldn't be a concern.

  4. brad_rh | | #4

    Usually the fresh air for gas furnace and WH is ducted to the mechanical room where they are located. Is there a reason not to do it that way?

    1. ruthewalrus | | #7

      Thanks, Brad. My basement is essentially one open area with a few dividing walls. There is no door closed off mechanical room.

  5. Expert Member
    NICK KEENAN | | #5

    Have you actually measured the interior pressure to confirm a negative pressure situation?

    I agree with the recommendation to bring fresh air directly to the combustion appliances.

    1. MAinspector | | #9

      You can duct fresh air right to the area where the appliances are using the "fan in a can" approach which would be wired to your appliances and only turn on when the appliance is operating. Once this is set up you could add some partitions and create a mechanical room to separate any cold incoming air from the rest of your basement.

  6. ruthewalrus | | #8

    Thanks, DC. I've not done a pressure test. But with zero "by design" air intakes isn't it a good idea to add one?

    Also, my mechanical room is just the entire basement, i don't have a walled off space i could singularly vent.

    1. andy_ | | #10

      A few studs, couple sheets of drywall, a door and a weekend will easily remedy that.

  7. CMObuilds | | #11

    We use a 6” insulated duct from an intake hood to the return plenum. An adjustable gravity damper is between the cold air duct and the 6” flex. If youre looking for space makeup air you could do the same thing just not connected to the return I suppose.

    BTW, dont make a house super tight and have draft appliances, not good. Especially since your saying you have negative pressure. Have you had someone check your worst case CAZ depressurization?

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