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Is it advisable for propane furnace to have an exterior air intake for combustion?

Insular | Posted in Mechanicals on

The propane furnace is located in crawl space, which is also open to the basement.

I was considering Martin’s advice to seal up the crawl space and it occurred to me that might be an issue for the propane furnace, which currently does not have an air intake for combustion. (This is a water based furnace so come that is the only reason for air intake).

Come to think of it, even if the crawl space remains vented, shouldn’t the propane furnace be drawing air from the outside?

I am not concerned here just with the code requirements, but want what’s best, even if it goes beyond code requirements.

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  1. GBA Editor
    Martin Holladay | | #1

    The usual advice for a sealed crawl space is to only install sealed combustion appliances rather than atmospherically vented appliances.

    However, if your crawl space connects with a basement, and the basement is relatively large, it may be possible to leave atmospherically vented appliances in your crawl space. To find out whether this option is safe, contact your local gas utility or fuel supplier and request a visit to have your basement assessed for combustion safety.

    Don't cut corners when it comes to combustion safety.

  2. Insular | | #2

    The basement is 250 square feet. I will contact fuel supplier. Replacing the furnace would not be cost effective, so that's why I was wondering whether a vent can be retrofitted.

  3. GBA Editor
    Martin Holladay | | #3

    Here's some more information on combustion makeup air:

    "In an unconfined space, the makeup air for heating appliances is provided by the large volume of air present in the space where the appliances are located — usually, a basement or crawlspace. According to the National Fuel Gas Code, a space is defined as unconfined if it has a volume greater than 50 cubic feet per 1,000 Btu/h of the combined input of the fuel-burning appliances. For example, if a house is equipped with a gas furnace with an input rating of 70,000 Btu/h, plus a gas water heater with an input rating of 40,000 Btu/h, the total input of the appliances would be 110,000 Btu/h. The space where these appliances are located would need to measure at least 5,500 cubic feet to be considered unconfined — equivalent to a space about 25x28 feet by 8 feet high."

  4. Insular | | #4


    Thank you for the article. The author goes on to explain that even if a space is "unconfined" as defined in the NFG Code, it might not provide sufficient replacement air if the house has been made "too airtight", so it sounds like I am back to square one! If I insulate and seal the crawl space very well, I will have to put a whole in the wall to bring in replacement air!

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