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Outdoor air supply for woodstoves…

user-757117 | Posted in Mechanicals on

Assuming a tightly constructed home (2 floors each 1400 ft2 and exhaust only ventilation) and a woodstove in that home with a properly constructed chimney…
Outside air direct to combustion chamber or should the stove pull air from the house?
What is the difference between a sealed combustion gas boiler (for example) and an airtight woodstove with direct outside air supply?

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  1. Riversong | | #1

    There is little difference in terms of probability of backdrafting, except when the woodstove door is opened. In a tight 2-storey house that is under negative pressure from an exhaust-only vent system, the woodstove absolutely should have a direct-coupled outside air supply.

    But there are too many variables to determine whether you would have a problem without direct coupling. How high are the ceilings - 8', 10', cathedral? How tight is "tightly constructed" - did you have a blower door test and, if not, how do you know it's tight? What kind of exhaust-only vent system do you have, what is the total cfm discharge, and are there make-up air inlets? Where is the woodstove - 1st floor, 2nd floor, basement? What precisely do you mean by "properly constructed chimney"?

    Some guidelines for proper chimney design:
    Chimney flue cross sectional area no more than 25% larger than connector pipe (stove outlet).
    Total connector pipe length no more than 75% of chimney height above thimble.
    Total connector pipe length no more than 10'.
    Horizontal connector pipe runs should be short and pitched upwards at ¼" per foot.
    No more than two 90° elbows in connector pipe.
    15' minimum chimney height above woodstove.
    Add 4% to the minimum height for each 1000 feet above sea level.
    Chimney termination at least 3' above highest point of roof penetration.
    Chimney termination at least 2' above anything within 10' laterally (including tree crowns).

    The question is not as simple as you might think, nor is the answer.

  2. user-757117 | | #2

    Thank you for your reply Robert. The scenario is as yet a hypothetical one. I would like to use two wood stoves (not sure why I only cited one originaly), 1 on each floor for primary space heating in winter.
    The house would be constructed new and I plan on building it as tight as possible so I wanted to consider a worst case scenario for possible backdrafting issues.
    As mentioned (and described by yourself) the stoves would be equipped with chimneys built for maximum draft (upper story stove would have ~16ft of straight chimney in the conditioned space).
    I had thought that a direct air supply to the stoves would be the best strategy until I did some reading over here:
    Is it even possible for an exhaust only ventilation system designed to meet the minimum ASHRAE requirements for the home (vaguely) described above to generate negative pressure to say -15pa or lower? Ignore the possibilty of giant a giant range vent in operation.

  3. Riversong | | #3

    I wouldn't worry about the depressurization from an exhaust-only ventilation system, but with that running and a range hood (normal, not giant) and a dryer going, you're likely to generate enough negative pressure to backdraft a chimney.

    And I'd be more concerned about putting a woodstove on each floor, since the upstairs one will increase the negative pressure downstairs, while the downstairs stove will have more draft and will try to reverse the upstairs chimney. With fans running and normal air leakage, you could run into problems.

    I would ignore much of what is said about outside combustion air at The problems mentioned are design failures. If you are building a truly tight house, particularly with two competing woodstoves, it's important to seal the combustion as well as possible and provide outside direct-coupled combustion air.

    Also, if you're building a two-flue chimney, be sure to offset the tops of the flues at least 3" or separate them with a brick whythe, because if only the upstairs stove is running and the downstairs stove is below the neutral pressure plane (in the negative pressure zone), then the flue gasses can recirculate down the flue to the downstairs stove and spill.

  4. user-757117 | | #4

    Thanks again Robert. As an aside, have you ever heard of an exhaust only ventilation system that relies on chimney draft to pull air out of the house during the heating season and then switches to an exhaust fan during the rest of the year?

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