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Community and Q&A

Direct-vent fireplace?

Mark Harrison | Posted in Green Building Techniques on

We are building a “pretty-good house” and we are trying for a respectable blower-door test.  We will have a direct vent gas fireplace.

One challenge is airtighting the  FP ducts through the exterior wall.  The combustion system is sealed, so that is no problem, but the fitting that penetrates the exterior wall  is not airtight.   The fitting has very small air passages that I believe are there to ensure pressure equalization. 

  A second challenge is overheating.  We have chosen a fireplace model with a good turndown ratio so that it can run at 14 kBtu, about as much heat output as a really big burner on your cooktop.  But I am still wondering if we will still find this too much heating (in Virginia).

Does anyone have any experience or suggestions regarding a direct-vented fireplace in a tight house?

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  1. Walter Ahlgrim | | #1

    When I look at my manual I see lots of required clearances but no requirement that require air flow outside of the double walled pipe.
    Build required clearances use sheet metal from the framing to the pipe and fire rated calk to make it air tight.


  2. GBA Editor
    Martin Holladay | | #2

    Walter gave you good advice. Check the manufacturer's installation instructions. As long as the manufacturer doesn't require clearance on the outside of the double-walled pipe, feel free to perform air sealing work.

    The usual method for air sealing a round metal chimney is described in my article, "Air Sealing an Attic":

    "Gaps around metal chimneys are sealed with techniques that are similar to those used for brick chimneys. The easiest way to seal around a metal chimney is with two overlapping pieces of sheet metal; of course, you’ll need to cut each piece with a half-moon hole that corresponds to the chimney diameter." I also wrote, "the seams where the metal pieces overlap and the gaps between the metal and the chimney can be sealed with high-temperature silicone caulk."

  3. Trevor Lambert | | #3

    Re: overheating
    The nice thing about a gas fireplace, unlike a wood burning appliance, is that you can just turn it off and back on at will. Unless this is your primary heat source, in which case that's not going to be very practical.

  4. Walter Ahlgrim | | #4

    My gas fire place is rated at 25,000 BTUs input.

    It is a Heat&Glo Cosmo 42. The salesman said not to expect much as a heating device that turned out to be very true. I guess 80% of its affect on the room is radiant the heat is transferred much like a light shining on you and you feel the warmth. It will warm the room air but it has never become uncomfortable.

    We use the gas fire place to set a mood in that it works very well.

    As a heating appliance I would say it is not very efficient or affective.


  5. Jana Pruitt | | #5

    Great Advice.

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