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Question about wood stoves

GBA Editor | Posted in General Questions on

Yes, it is an odd time of year to be asking about wood stoves but this topic came up in another forum and I will ask it here. Is there an energy penalty for a wood stove while it is not in use. In an air tight home, it is necessary to have a dedicated combustion air supply from the outdoors. I assume that it is a requirement to shut off this intake and or the chimney damper while the stove is sitting idle to prevent air from freely circulating and removing heat from the home. Even if the dampers are tightly closed, air above the damper inside the pipe will become buoyant and rise right up to the top and be continuously replaced by cold outside air. Is this significant ?? Does it matter?? Does the same thing happen in plumbing stacks??

Thanks
Garth

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Replies

  1. Riversong | | #1

    Plumbing stacks are pneumatically isolated from the conditioned space. They are venting the warm decomposition gases from the septic or sewer system and should not contribute to heat loss unless ceiling penetrations are unsealed (use a roof flashing at the ceiling level).

    A modern EPA-approved air-tight woodstove does not require and should not have a flue damper. As long as door seals are maintained, there should not be sufficient air leakage to cause a noticeable change in the building's pressure balance. And, as long as the chimney is insulated metal or masonry, there should not be enough conductive loss to make much of a difference.

    If you're really concerned, you can install a chimney-top damper with high-temperature silicone seals. http://www.safesidechimney.com/images/damper.jpg

  2. GBA Editor
    Martin Holladay | | #2

    Garth,
    I agree with Robert's answer. However, most wood stove installations I have seen (including those in my own home) are not truly airtight. For example, I have a kitchen cookstove that is over 100 years old. When not in use, the stovepipe damper (which is not airtight) is insufficient to prevent the stack effect from pulling indoor air up the chimney.

    I'm aware of the airflow, however, and tolerate it as a "poor man's ventilation system." Undoubtedly, Passivhaus builders wouldn't accept such airflow.

    So, think everything through and choose your stove to meet your own standards for airtightness.

  3. Steven Landau | | #3

    I think the biggest heat loss will be the conduction due to the metal in & flue pipes going thru the wall. We can insulate for air leakage but not for conduction thru the metal.

    The european ones are available with Outside air intake:
    http://www.evolutiontradegroup.com/detailpopup/Florenz.html but I can't find a thermally broken flue pipe.

    Any hints?

  4. JohnEd | | #4

    what about front door air flow when it is used?

  5. Riversong | | #5

    Why would the flue pipe go through the wall? Flue pipe, also called connector pipe, is for connecting the woodstove to an approved chimney, whether masonry or insulated metal like Metalbestos.

    A chimney should always be interior to the conditioned space and not outside an exterior wall, as used to be common practice. An insulated or masonry chimney exiting through the ceiling/roof will not be a major heat loss because of the mass effect.

  6. SteveP | | #6

    Interesting questions. I think it depends on the stove and the setup. To answer the unanswered question first - yes, a modern "airtight" woddburning stove requires outside air to perform properly. As mentioned, in an airtight house turning on an unbalanced exhaust like a kitchen hood will actually suck air back down the chimney if the door or vents on the woodstove are open (or it is not airtight).

    Modern stainless steel insulated chimneys (GSW, etc.) are well insulated and if properly installed should be airtight as they penetrate the building shell. There would be some conductance along the length, but I think that would be negligible (and difficult to avoid without some sort of high-temp "break").

    Some airtight stoves provide an inlet for outside air and this can be done simply with a small-diameter plastic pipe intake vent that can be insulated and controlled with a valve. Alternatively, you could rig something that "delivers" outside air to the stove's intake vents. What you want to do is avoid using conditioned inside air as you will develop negative pressure in the house and it will have to be replaced by unconditioned outside air filtering in (plus the stove will not draw properly).

    Here's a rather Rube Goldberg solution

    http://www.instructables.com/id/Fresh-Air-Intake-for-a-Woodstove-or-Wood-Heater-w/

  7. Riversong | | #7

    Actually, negative pressure is a good thing during the winter, since infiltration cannot cause condensation. Exfiltration is problematic from a moisture-management perspective. And exfiltration at the ceiling will create a negative pressure zone in the lower part of the house (where the woodstove typically is) and can overcome the stack effect of the flue and backdraft the stove.

    I've designed and built tight, super-insulated homes in which the woodstove was not only the primary heat source (in addition to passive solar) but also the primary exhaust "fan" (requiring no electricity). Imagine that: heat and ventilation when the grid is down! In those homes, I provided a closely-coupled but not directly-coupled combustion air inlet to the woodstove.

    In a house which used the woodstove as a secondary heat source (radiant slab and floor as primary), I direct-coupled the woodstove to an outside air source. To avoid the need to "valve" the intake but still control air flow when the stove was not running, I simply formed a reverse trap in the 4" intake pipe by running it up 3' and then down again before going under the slab to the base of the chimney.

  8. Steve McCarthy | | #8

    So , Robert.....I know this is off the subject ...but , would you choose to live in a "passivehaus"???
    I like living in a house heated (and ventilated) with wood.

  9. frank holcombe | | #9

    Does it matter...? Put an extra log on, sit back and enjoy...!!

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