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ducting fresh air to wood stove–danger?

Megunticook | Posted in Energy, Heating & Insulation on December 17, 2002 06:25am

Hi,

I installed a new wood stove (Vermont Castings Encore) into a tight new home and plan to connect the stove’s air inlet to a 3″ duct that terminates at a vent to the outside. This is recommended by Vermont Castings for tightly constructed homes for two reasons–better stove performance and to avoid having negative pressure potentially affecting the draft of the stove, particulary if you’re running exhaust fans and the like. I gather it’s even possible to reverse the draft and suck smoke into the house this way.

But the local VC dealer told me that at a recent conference he attended there was major controversy about housefires attributed to fresh air inlets, particularly when the ducts were PVC. According to him, the Canadians are no longer requiring fresh air ducts in their code for new construction because of this. I guess in a downdraft situation flames can potentially shoot into the fresh air duct, igniting the PVC. His personal advice was to not connect the vent directly to the stove.

Anyone else heard of this? I tried searching on the net but found nothing. It sounds plausible. In my case, the duct is galvanized metal for the first three feet, then PVC. It runs down through a concrete slab, then underneath, then back up and out through an exterior wall (2×6 with blown-in fiberglass insulation). The vent cap is a standard NuTone stainless steel dryer cap with the flap removed.

I bought the “outside air adapter” from Vermont Castings and would like to hook it up to the duct, but would like to know more about this problem. I could run the duct close to the stove’s inlet but not connect it–but this would of course bring cold air into the room, defeating the tight construction objective.

Opinions?

Thanks,

Ed

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Replies

  1. calvin | Dec 17, 2002 07:11am | #1

    Ed,  A friend in his 70's house, has a hard pipe vent from the outside to just in front of his stove.  A grill with damper is set in the floor.  He opens it when he runs the stove, shuts it down as he shuts down the stove.  You would have to live with the draft while it's burning.  He would burn full bore, until the embers were out, then close it down completely.  Not much complaint from him on the way it works.

    __________________________________________

    Remodeling Contractor just outside the Glass City.

    http://www.quittintime.com/

  2. User avater
    rjw | Dec 17, 2002 07:15am | #2

    FWIW, I haven't heard that, and I read a lot of stuff in this area.

    I suspect (but don't know for sure) that a requirement for makeup air to wood stoves may have been eliminated because of another requirement for make up air in general - such as a Skuttle make up air valve (see, http://www.skuttle.com/216.html ) should eliminate the risk of negative pressure and back drafting.

    You might try the National Fire Protection Association, they will have the latest poop on fire hazards.

    ________________________________________________

    "I may have said the same thing before... But my explanation, I am sure, will always be different."  Oscar Wilde

  3. Piffin | Dec 17, 2002 07:29am | #3

    You need to supply air for combustion

    You don't need direct hookup. The problem with open indirect with a gap, ytou are then providing a cold drfat to the floor of the house to chill you when the fire is out.

    With yours in concrete or ground, I don't see much danger

    You should contact Vermont Castings directly with a well written letter, requesting a directive in writing from them.

    .

    Excellence is its own reward!

    "The first rule is to keep an untroubled spirit.

    The second is to look things in the face and know them for what they are."

    --Marcus Aurelius

    1. User avater
      Megunticook | Dec 17, 2002 03:50pm | #4

      Thanks for your opinions. Nothing on the NFPA web site, but will try to give them a ring later. I'll also contact VC. Did find a couple of interesting discussions of this issue on the web, offering opposing views:

      http://www.woodheat.org/outdoorair/outdoorairmyth.htm

      http://www.chimneysweeponline.com/hooa.htm

      This is obviously a controversial topic. I would like to know more specifics about any cases where housefires have been started by outside air ducts--I would guess that there are only certain circumstances and construction configurations that allow this to happen.

      1. User avater
        rjw | Dec 17, 2002 04:28pm | #5

        Interesting articles; thanks for the links.

        The issue I have about the woodheat sites argument is that it discusses the theory, and raises questions, but those questions only address potential problems without considering whether there are solutions to the problems discussed.

        Its general discussion conforms to what I have learned about air pressure etc in homes, and it points out that a passive air source might not work if improperly located.  It doesn't mention the use of barometric dampers in a fresh makeup air supply, such as the Skuttle I mentioned in my earlier post, which would answer most of the concerns raised, and careful planning and testing could answer the rest.

        Note, I've never done draft testing on wood stoves or fireplaces; I have done draft testing on furnaces and water heaters, and would note that newer homes in my area (NW Ohio) tend to be built tightly enough that draft performance is marginal in winter conditions (closed doors and windows) and often unacceptable when competing vents are in operation (e.g., bath and kitchen fans.)  Especially with water heaters.

        Put a wood stove in a tightly built house and you are almost certain to have makeup air problems and potential backdrafting problems on furnaces and water heaters; with potential dangerous CO entering the house because of that backdrafting.

        ________________________________________________

        "I may have said the same thing before... But my explanation, I am sure, will always be different."  Oscar Wilde

  4. EricGunnerson | Dec 28, 2002 05:02am | #6

    I live in Washington State, where outside combustion air is a requirement.

    I just put a stove in early this month, and it's supplied with aluminum flex tubing - the same kind you use on driers - painted black.

    I looked through the links, and I can't see any reason you wouldn't want to use outside air. Even if it's not a big issue with spillage, I don't see why you'd want to use air you've already heated for combustion when you could use outside air. You will lose a bit of heat to the uninsulated intake duct, but my guess is that it's minor compared to the heat you'd lose to combustion.

    I do agree with others that you should talk to your manufacturer, but it seems to me that any downdraft that could shoot flames 3' into the air inlet could also shoot it out into the room if you didn't have an inlet

    1. DavidThomas | Dec 28, 2002 08:59am | #7

      In addition to what others have said, I'll add: Install a CO detector. A good one. Bob and I have posted that info in the past.

      Have a proper stack/chimney. Not too short. Terminating well above the ridge height. A minium number of bends (one 90 at the stove, maybe one 45 in the run, at most). VC should have diameter/length/height guidelines for you. Don't skrimp and consider overbuilding (taller stack). Too much draft is not a problem. Too little draft is a safety, odor, and nuisance issue.

      I would definitely install an outside-combustion air duct. And not use PVC near the stove (duh). And consider stepping up from Al to Fe or SS.

      David Thomas   Overlooking Cook Inlet in Kenai, Alaska

      1. User avater
        Megunticook | Dec 30, 2002 03:52am | #8

        Thanks for the tips. Actually the slab is already poured so the galvanized steel duct is "set in stone."

        I've got good draft with an excellent chimney setup. The CO detector is a good idea, I've considered that, although it doesn't necessarily address the backdraft/duct fire danger.

        Still haven't heard any details on these cases where housefires have started--if anybody has, I'd appreciate hearing them.

        1. JMartindal1 | Dec 30, 2002 04:40am | #9

          we didn't use fresh air makeup, but we did have a shroud around the stove, and ran a duct up to the cold air return on the furnace. put a block out  switch on the heating element. When the house thermostat called for heat, we used the stove heat instead of elect. worked for us in VA.

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