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Is there any way to utilize the central AC ducts with fireplace wood insert ? Suggestions for wood

Post in 'DIY and General non-hearth advice' started by Tahoe99, Jan 23, 2009.

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  1. Tahoe99

    Tahoe99 New Member

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    Hello all -

    new one to the forum. My house is a 2 level ranch with about 1400 sq ft each level. The house is an L shape with a big brick fireplace at the very end of one of the L "legs". The firebox is 28" high by 47" wide. I am planning to install a wood burning insert but was wondering/concern about pushing warm air to the other end of the house. I have all the duct work for my central AC in the attic. Is there any way to use it to get the warm air everywhere ?

    PS: To simplify the issue, the lower level of the house is not hooked up to the AC and can be disregarded at this time.

    Thanks all.

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  2. smokinj

    smokinj Minister of Fire

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    to push air through the duct you would need to go with a wood furance!
  3. Tahoe99

    Tahoe99 New Member

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    Smokinj - I see, thanks. Furnace is not an option.

    Czarcar - can you elaborate little more ? How do you blow cold air towards the heat source ? With fans ?

    Thanks
  4. smokinj

    smokinj Minister of Fire

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    yes fans its better to bring the cold air back to the heat source than the heat source to the cold air
  5. semipro

    semipro Minister of Fire

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    MC,
    I'm not sure this will be helpful given your duct configuration but I'll describe how I used the ductwork on my house to distribute the heat from our woodstove.

    We have a 3 level house with the stove in the basement (1st level) and the main living area on the 2nd level. These two levels are served by a common HVAC system and one of the registers lies directly above my woodstove. All other registers are in the floor of the 2nd level and serve a bathroom, a study, the kitchen, and living area. The 3rd level has its own HVAC system.

    I installed a very quiet and high flow bathroom exhaust fan in the ceiling above my woodstove and very near the HVAC register. I connected the outlet of the fan to the ductwork at the register and installed a check valve so air could only flow from the fan into the ductwork. I also installed a variable thermostatic switch inside the fan housing so that the fan only comes on when the temperature exceeds a preset level. We close the register for winter use so that the fan exhaust doesn't just blow back into the basement. Now when our stove operates and the air temp at ceiling level gets hot enough the fan distributes the air to the registers in the 1st level. The flow is small but you can definitely feel the heat coming out of the registers. There is a potential for the fan exhaust to exit the HVAC at the return but it lies lower than the duct distribution system and heat rises. No heat seems to come out there.

    If I had to do it again I think I would just replace the register above the wood stove with the bathroom fan. I don't see any reason why the fan assembly wouldn't act as a decent HVAC register for summer use other than I couldn't control flow from it like I can the regsiter.

    If your ductwork is located in a vented attic space I'd be worried that any heat you pickup from the stove might be lost to the attic. Also, if your registers are located in the ceiling in outlying rooms I would think the heat exiting the registers would just collect near the ceiling in each room.

    Hope all this is helpful to you somehow.
  6. Gooserider

    Gooserider Mod Emeritus

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    Sounds like a potential code violation and safety hazard to me, Semipro... Code prohibits air intakes w/in 10' of a stove or fireplace because they could potentially aid in spreading flames and / or toxic fumes in the event of a problem. With your setup as described I'd be concerned about the potential for sucking up smoke (carbon monoxide) if there were a problem with your stove or chimney (say the door didn't latch properly for some reason) and it backpuffed into the room - you might get a dangerous level of CO distributed to your sleeping spaces before you knew there was a problem... If you are going to do this, make sure you have plenty of CO alarms in good working order....

    Speaking in general, to respond to the OP's question, MOST of the reports that I've seen here (and my own experience) is that most people seem to have poor luck attempting to use their HVAC systems to move stove heat around. This is especially true when any of the ductwork or the air handler is outside the living space envelope, which it sounds like yours is. Stoves put out much lower temperatures than the HVAC system furnace, and the ducting isn't designed to handle it, coupled with the fact that most ducting setups leak like seives means that using them tends not to work well.

    What seems to work better for most people is to carefully map out the normal convection airflows in the house when the HVAC is not running, and use fans to help encourage them, with the best results being to push the heavier, denser cold air TOWARDS the stove, so that the heated air flows in to replace it, rather than the other way around. Unfortunately the stove at one end of an L is one of the toughest setups to deal with, so don't expect miracles...

    Gooserider
  7. semipro

    semipro Minister of Fire

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    Gooserider,
    Truthfully, I hadn't thought of that. I have numerous books on building codes but didn't even think that the vent might violate. However, now that I think about it I like the idea that that the fan will help me recognize more quickly that I have a problem if one occurs. I'd much rather know sooner than later. Our main HVAC return is located (as unmodified) within 10 ft. of the stove so it must have either passed inspection or gone unnoticed or unreported. Its too low to be useful for circulating stove-heated air though.

    We do use CO alarms.

    Our house could be a study on natural convective flow with numerous high ceilings, and other features Its really interesting to observe. Unfortunately, in our case, the heated air naturally flows upward along high ceilings to a third level, for the most part, bypassing our main living level.

    Heat flow through houses is all to often overlooked in the design process as many of us wood-burners know all too well.

    Thanks for you input.
  8. Gooserider

    Gooserider Mod Emeritus

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    When you are awake, I agree that moving the air around might alert you earlier because of odors if nothing else... At night though it's a different story, and this is where the major concern comes in - the basic theory is that w/o the fan, most of the fumes stay in the stove room, and trigger the alarm there, before you get bad concentrations in the sleeping spaces - so you get woken up with time to evacuate or otherwise deal with the problem in relatively clean air. With the fan, by the time the alarm triggers you could have potentially dangerous levels in the sleeping spaces, which might reduce your ability and time to respond rationally to the alarm...

    Gooserider
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