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Old 03-22-2011, 02:05 PM   #1
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How does my gas furnace get air to combust the gas? What if the room is sealed?


In my crawlspace/basement is a room with the gas furnace, the back of this room is open to the crawlspace, which has several crawlspace vents. That means the basement is always cold, and some contractors (on a different job) said that having the furnace open to the crawlspace was a total waste of heat/energy/money.

So I was thinking of blocking off the vents because I've read that crawlspace vents might not be such a good idea after all (because moldy dusty air gets drawn up into living spaces from the crawlspace). I'm also thinking that I'd slap some plywood and insulation up to enclose the room that the furnace is in.

But I'm wondering ... if the furnace is using room air for combustion, won't it "suffocate" if I block off the vents/room and/or prevent the exhaust vent from working properly and then fill the house with exhaust?

Or does the furnace get air from the furnace vent as well as releaseing exhaust (ie; is there two pipes?)

Should I just leave the whole thing alone?

Thanks!

V

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Old 03-22-2011, 02:24 PM   #2
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How does my gas furnace get air to combust the gas? What if the room is sealed?


depends on the furnace . is it an 80% or 90% furnace ? 90% will be vented with pvc . 80% will be vented with metal. a 90% furnace will be vented with pvc. It could have 1 exhaust pipe & take combustion air from the crawl space area. It could have 2 pvc pipes 1 for exhaust & 1 for combustion air. or, it could have 1 pvc pipe (sometimes refered to as a pipe within a pipe) for exhaust & combustion air

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Old 03-22-2011, 02:30 PM   #3
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How does my gas furnace get air to combust the gas? What if the room is sealed?


For furnaces without a specific air intake pipe it used to be assumed that there are enough imperfections in the weatherstripping that enough outside air infiltrates the house for the furnace to use. The house may not be sealed.

Nowadays the code (International Building Code book) specifies how much additional air intake, perhaps in the form of vents in the basement wall of several square inches leading to the outside.

Too much free opening (square inches) to the outside is better than too little.

Yes the furnace may suffocate and furnace exhaust gases fail to rise up the chimney if there is insufficient air intake from the outside. Large amounts of carbon monoxide may also be produced.

Usually some carbon monoxide is always produced but with insufficient combustion air, what would be carbon dioxide produced will be additional carbon monoxide instead.

For a furnace with a specific air intake pipe, either by itself or a pipe within a pipe, no additional vent to the outside is needed.

Forced air furnaces should not have warm air registers or return louvers near them when there is no intake pipe from the outside and should not have them in a basement that has combustion air vents to the outside.
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Last edited by AllanJ; 03-22-2011 at 02:42 PM.
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Old 03-22-2011, 04:33 PM   #4
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How does my gas furnace get air to combust the gas? What if the room is sealed?


Looks like two strikes for our ancient furnace then. The furnace is in this little closet, the back of which is open to the crawlspace ... which has 3 vents to the outside. The air return is just on the other side of the closet wall, and there are at least two warm air registers very close by. However the air return and warm air registers are on the "sort of basement" side and not the "crawlspace" side.

The crawlspace was dug out a bit 60 years ago and a wall divides the dug out space from the "couldn't be bothered to also dig out" space. The dug out space, which I'll call a basement no longer has ventilation holes in the walls and is sort of part of the house. The crawlspace side is a crawlspace with a door to the basement, but really it's leaky as a sieve between the two areas.

I wished my house didn't suck, but it was built in 1939 and who knows when that ancient Sears furnace was installed. I'm guessing the 50s

Thanks for your opinions!

Vince
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Old 03-22-2011, 06:48 PM   #5
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How does my gas furnace get air to combust the gas? What if the room is sealed?


If your furnace is that old. You need a new 1
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Old 03-22-2011, 07:14 PM   #6
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How does my gas furnace get air to combust the gas? What if the room is sealed?


Quote:
Originally Posted by kenmac View Post
If your furnace is that old. You need a new 1
Based only on age?
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Old 03-22-2011, 09:38 PM   #7
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How does my gas furnace get air to combust the gas? What if the room is sealed?


An 80% would save you $$. A 90% (or better) even more $$.Being that old probably has a bad H. X.. If you keep it have the H. X . checked & take care of the combustion air issues
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Old 03-23-2011, 01:30 PM   #8
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How does my gas furnace get air to combust the gas? What if the room is sealed?


Quote:
Originally Posted by kenmac View Post
An 80% would save you $$. A 90% (or better) even more $$.Being that old probably has a bad H. X.. If you keep it have the H. X . checked & take care of the combustion air issues

I'll start taking a look at this. I remember my Dad "saved money" on a furnace (he lives in Mississauga, a city just west of Toronto, so it gets COLD). When I looked at the energystar rating on it, it was near the bottom of the efficiency scale. He's had it for 20 years...by this time a more efficient furnace would have easily recouped any initial savings.

Thanks! I guess this is a case where an actual HVAC professional looking at the situation is required.
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Old 03-24-2011, 12:00 PM   #9
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How does my gas furnace get air to combust the gas? What if the room is sealed?


I agree its definitly time to upgrade to a more economical and effecient systrem.Granted there may be nothing wrong but its wise to keep up with the times.It can save you a lot of money and the furnaces nowadys are a lot more automated in the operation.Theres no standing pilot burning anymore and they fire up a lot easier as well as not wasting the fuel while its burning!

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