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Old 03-07-16, 09:05 AM   #9
gtojohn
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yes combustion air from outside the conditioned space is definitely the way to go. Each Cubic foot of gas requires 10 cubic feet of air, so 100kbtu would exchange all the air in your house several times over. Thats why outside combustion air is part of the energy code. Mine are located in the unconditioned open air attic crawl space so there's no loss there. What I suggest is preheating the combustion air which presumably you are bring in from outside, which is colder because you are running the heater. You already have a flue and need combustion air why not preheat it? Testing a double wall flue last week, 3' from the furnace still very hot, couldn't keep my hand on it. about 4' from it it was warm to touch, maybe 100-105f. If I wanted to get max heat out of it I'd use single wall vent. Unfortunately winter's over here, my hvac has been off for almost a month, so might not happen for me until fall. Maybe someone in a colder climate might want to try it before then.
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