Draw air from vented attic?
I am looking at a zero-clearance fireplace which is designed to draw outside air for combustion and for air to be heated from the outside. It is suggested that this air be drawn from a vented attic. Is this a good idea?
It seems like it would be possible for this system to pull air into the attic from the house through leaks (supposed to be no leaks, but ….. you know). Of course there could be adequate vents to supply the 300+ CFM for the blowers, but there might not be enough, or this system might pull air in through the ridge vent which might not be a good thing.
Secondly, drawing air to be heated in from the outside puts positive pressure in the house. Could this be a problem in that it would push interior moist air into the walls and ceiling through those same leaks?
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Bob,
What's the brand name of the fireplace? Do you have a link to the installation instructions?
I agree with you: it's better to draw outdoor air from the outdoors, not the attic.
Any chance we can talk you into scrapping the low-efficiency zero clearance fireplace concept for a 70%+ efficiency woodstove/insert with a big fat viewing window? The cfm requirements for air-tight woodburners is in the very low 10s even for bigger ones running a full-fire, and the efficiency of an EPA rated woodstove as a heating appliance is on the order of 10x what you get out of any high-cfm open-hearth approach, with far less sooty particulate emissions to boot.
Most better woodstoves & inserts have outdoor combustion air kit options too, if you have a fairly tight house and are concerned about limiting backdrafting when exhaust ventilation is in use elsewhere in the house.
Martin, I am looking at the Xtrordinair (44 inch unit). http://www.fireplacex.com. I believe the installation details can be found there. The unit does require air to be drawn from the attic, but the grill size of the blower unit does not fit between the floor joist and out the rim joist. IF my unit were on a exterior wall the grill would go out hte back of the unit just fine, but my install is for an interior install (all four sides). Therefore the saler / installer suggests drawing from the attic and says it is done this way all the time. I suppose my problem is that I read from web sites like yours and have been educated/scared into new ways of thinking about things. So many contractors just look at you like an idiot when you bring up issue little conditioned crawlspaces, or moisture transmission. Ignorance is bliss.
Dana, the efficiency of this unit is rated steady state at 75%+
Bob, You may want to check your local building codes. Their requirements supersede the manufacturer's installation instructions and are sometimes at odds with them. Our code does not allow combustion air to be drawn from the attic.
Thanks Malcolm,
The supplier does the installs this way all of the time in a county with building codes. My county has none so there is no one to ask. It obviously "can" be done this way, I just wonder the wisdom of having some device DRAWING air into the attic through whatever openings air will travel. Would it pull moist into the walls and ceilings trying to get into the attic? Is that a concern or would the attic vents in "all cases" be adequate for all the blower needs? The system is also designed and touted as one creating positive pressure in the house. Again, I ask is that a good thing? Would it not push moist household air into the walls and ceiling?
Oh, and I might have misspoke, the blower is only to bring air into the unit to be heated and blown into the room, not for combustion. That seems od to me as well, as why would you wish to bring 20 degree air into the unit to be heated when you can just use room air at 70 degrees?
I like the unit, and I will just have to see if there is a way to duct it to pull directly from the outside. I would think I could make a deduction bezel of some kind to attach to the blower grill and fit between the floor joist in the crawlspace.