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jlaky2000 10-06-2011 06:11 PM

Furnace Venting Question
My furnace is situated in the middle of my late 70s house in an interior furnance & water heater room. The flue vent goes up a shaft through the second to the attic (the flue vent continues out the roof). The shaft is 18" square though the flue vent is 6" or 8". This shaft seems to allow a lot of warm air into the attic and I'd like to somehow seal the area around the flue. It would probably save me on my heating bills and ice dam headaches.

My question is, does anyone know why the larger vent would be there? Air intake, maybe? But the furnace room door has louvers to allow are in (a later addition possibly; I've only been in this house for 4 years).


Houston204 10-06-2011 07:48 PM

The furnace should not draw combustion air from the conditioned space.
That oxygen is for you.

jlaky2000 10-07-2011 04:24 PM

Thanks, but doesn't the furnace also draw oxygen from the living space through the return air vent? I was wondering if the furnace needs additional oxygen (beyond the return air vent), so that it need this shaft to the attic. (The corollary is: Am I stuck with this inefficient hole to my attic because my house was designed in the 70s?)

Marty S. 10-07-2011 04:57 PM

Return air air and combustion air are not the same thing. Return is bringing air back to the furnace through the duct work to be conditioned(heated or cooled). Combustion air is to make sure there's enough oxygen for the fire to burn correctly,open air space. The amount of combustion air needed is 50 cubic feet for every 1000 BTU,including the furnace and water heater. If the area of the mechanical room and any area open to it via the louvered doors meet the combustion air requirement then yes you can seal the chase with a tin fire stop on each end.

jlaky2000 10-08-2011 04:07 PM

I appreciate the explanation. That make sense. Do you suppose that it is possible to install a one-way cover for the chase so that the furnace draws fresh air from the attic when it is running, but prevents warm air from rising into attic when it is between heating cycles?

Does anyone have any other thoughts on how to address both the loss of heat and the ice dam problem caused by the chase (which I am now guessing the I need thanks to Marty S.)? I could seal the furnace room to minimize the heat loss from the house, but I'm afraid I'd still get the ice damming.

Marty S. 10-08-2011 05:11 PM

Did you measure the rooms yet? Don't just assume it's needed just because it's there. Combustion air is most often piped in from an outside wall,using a mechanical damper interlocked with the furnace, instead of the attic so that's an option. Cutting a supply register into the mechanical room used to be allowed for up to 50% of combustion air needs but not sure if that's the case any more.
How old is the furnace? A new 90% or better one will use PVC pipe to bring combustion air from outside directly into the burner compartment. It will also pay for itself in fuel saving . Long term that is the best option but the most money up front , it's much cheaper then replacing the roof.

jlaky2000 10-10-2011 06:24 PM

Marty S., thanks again. You are a wealth of information. Do you know where I can find a resource to learn how to measure the combustible air, and possibly create the damper you are talking about? The furnace room wall also abuts the garage, which could be a source of combustible air.

(As for the 90% efficient furnace, ours is only 4 years old but 80% efficient. I did the math when we bought, and I wasn't able to find enough natural gas savings to offset the addtional cost of the furnace & install -- and that was when natural gas was more expensive than today!)

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