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Thread: Wood Stoves

  1. #41
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    Default Re: Wood Stoves

    Quote Originally Posted by lumpy View Post
    I expected I'd get a unit that ducted in outside air for combustion, because why suck warm air up the chimney? But it looks like there are some fire hazards associated with it and it isn't neessarily any more efficient. I'm surprised. This is the kind of stuff I've been reading; I can't assess how sound it is: A non-commercial service in support of responsible home heating with wood - The Outdoor Air Myth Exposed
    That article didn't pass my sniff test. Just because there are combustion air systems that don't work correctly doesn't mean the whole concept is wrong. One, be sure to account for wind when you locate the inlet. And two, almost every natural-draft system I've seen is undersized.

    Building code specifies combustion air sources for fuel-burning appliances. For something the size of a medium/large wood stove (50,000 Btu/hr) you'd need to provide 2000 cfm from the room. That would be one big and leaky-as-feck room, given that a range hood should draw 250 to 400 cfm. And we rightfully worry about that backdrafting stuff.

    If your room isn't big enough or leaky enough to provide that, code requires two ducts (one low and one high) with a cross-section of 25 sq.in. (6" diameter) each. If you have openings direct through an outside wall, these can be half the size, 12.5 sq.in. or 4" diameter.

    Or you can power vent the combustion air, 17 cfm. That airflow is less than half a bathroom exhaust fan, and a 3" diameter duct.

    The code has evolved around appliances that burn oil or gas, because carbon monoxide is bad. Granted, a wood stove will smoke you out before it knocks you out, but I don't see why you should treat it differently just because you can.

    Disclaimer: I'm quoting code numbers from memory and that's bad practice but hey this is a bike blog and I'm not billing anybody for this time.
    Tee Aitch

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  2. #42
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    Default Re: Wood Stoves

    I've run the numbers on this looking at heat loss from a ducted and non-ducted intake sytem in a house we are building with a 90%+ efficient HRV system and air infiltration around 1 m3 / m2 / hr (predicted, not yet tested).

    The ducted system is about 10% more efficient than the non ducted system. The HRV ensures that delta P is minimised and takes care of any concerns about air supply (non-ducted) by continuously supplying 300 m3 per hour of fresh air into the house.

    We are currently undecided about installing the wood heater at all, given that the predicted energy balance of the house comes out to around 200 MJ / annum without it, so it should cost about AUD$1 per day to run (not counting hot water and cooking). The big plus of the wood heater is IR irradiation of the passive storage surfaces designed into the house, which provides significant subjective warmth.

  3. #43
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    Default Re: Wood Stoves

    Quote Originally Posted by Mark Kelly View Post
    The ducted system is about 10% more efficient than the non ducted system.
    Does the 10% figure account for the energy needed to heat the non-ducted air before combustion? The greater the temperature differential between the outdoor air and the indoor air, the more that inefficiency would seem to be. But I'm certainly not an engineer.

    Our final stove walkthrough is this Thursday. Assuming everything goes as planned and we sign off, we should have a 2020 certified Hearthstone Manchester operational before Thanksgiving.



    The install will have fully ducted outside air, which is the installer's standard practice.
     

  4. #44
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    Default Re: Wood Stoves

    Quote Originally Posted by caleb View Post
    Does the 10% figure account for the energy needed to heat the non-ducted air before combustion? The greater the temperature differential between the outdoor air and the indoor air, the more that inefficiency would seem to be. But I'm certainly not an engineer.
    The model was run with air supply at the appropriate temperature (Text = 0 and Tint = 20). The major energy difference is that the internal heater draw unbalances the flow through the ERV heat exchanger. If you don't have ERV installed you will get different results.

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