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Lee Roy 10-26-2010 01:34 PM

Combustable air, how much is needed?
We are installing a Patriot 80 oil burner in a 16x14x7 basement room, and running the return through an adjoining room that is 12x24x7 to the upstairs level of the house. How much combustable air should be needed for the unit, and if more is needed from the adjoining larger room, what size hole would be needed to vent the proper amount of combustable air to the unit? There has to be a way to determin the amount of combustable air in a room, and likely how much that the unit requires that may not be in the owners manual.

kenmac 10-26-2010 03:34 PM


Originally Posted by Lee Roy (Post 523305)
proper amount of combustable air to the unit? .

I would not want any. You want fresh air for appliance combustion

Marty S. 10-26-2010 05:32 PM

Need 50 cubic feet of free air for every 1000 BTU including the water heater. One way is to put grill on the walls connecting rooms, one within 12" of the ceiling and one within 12" of the floor. There's a formula for how much air can pass through 1" grill space but I can't rember it right now. One of the other guys will know shortly. Got to have more supply feeding these rooms then return or the negative pressure can make the flue downdraft.
As kenmac stated the other option is to bring it in from outside. Again can't recall the exact procedure but believe it's no less then half the flue diameter. Down side is it makes a mighty cold room in the winter which will freeze pipes if you don't have enough heat blowing in there.

yuri 10-26-2010 05:55 PM

He would have to know the oil burner nozzle size and firing rate and convert that to BTU's, not easy. Best to just put a louvered door into the furnace room and allow the air into the house naturally. That way there is no cold drafts in the furnace room etc. Oil burners are power burners/fan assissted and can easily draw air in thru a louvered door. If he seals the house airtight then he may have chimney problems etc and needs additional outside combustion air. Most homes are leaky enough not to have that problem.

Marty S. 10-26-2010 06:08 PM

Thanks yuri. A quarter million people in and around town and I work on exactly one oil burner. Thought they were purchased like gas furnaces with a specific BTU for the house needs.

yuri 10-26-2010 06:12 PM

The older ones could use 3 different nozzle sizes. Newer ones are more house size specific but he would still need all that info to know for sure.

beenthere 10-26-2010 06:24 PM

Between those 2 rooms. You have enough volume for 71,680 BTUs of input.

So what size is your furnace?

Lee Roy 10-27-2010 12:07 PM


Originally Posted by beenthere (Post 523443)
Between those 2 rooms. You have enough volume for 71,680 BTUs of input.

So what size is your furnace?

The unit says 85000 BTU's, we have weatherized the upper part of the house, really tightened things up from what they were. We ran the return through a cinderblock wall, and now I am framing the hole. My plan is to use two 20x24 inch grates to allow air flow between the rooms. I can also add a smaller one to allow outside air if needed. What is the formula used to determin combustable air and BTU ratio? My local hardware store said they just use a chart. Thanks for your help.

beenthere 10-27-2010 06:21 PM

50 cubic foot per 1,000 BTUs of input.

Then what you lack. You need to add 4 sq in per 1,000 BTUs.

So a 6" pipe is all you would need to add from the outside.

Marty S. 10-27-2010 10:17 PM

One thing they allow here when a little short on combustion air for replacement jobs is to put in a gutted 4x10 supply in the room. Has to have a sticker applied stating "combustion air- do not cover". Won't fly on new construction though.

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