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Combustion air in a sealed attic
This may be a bit off the normal topic, but I was hoping someone smarter than me could give direction.
I was called out today by a homeowner who wants to encapsulate his attic with foam insulation on the roof deck and gables. The problem is, there is a new Carrier gas furnace model 58PHA90 installed in the attic. Its an 80% AFUE with 88,000 Btuh input.
According to the installation instructions, which mirror the mechanical code, the furnace needs 50 cubic ft for each 1000 Btuh input. The math indicates the furnace needs 4400 cubic ft. The attic incorporates about 6000 cubic ft. So, by the code and manufacturer's prescription, there is sufficient air for combustion. Yet, I am still uneasy about it. The furnace backs up against the gable attic wall and it would only take about 4 ft of duct to pipe outside air into the vestibule and render it "quasi" sealed combustion.
Has anyone ever tried this? Does it even make sense? How would it affect the heat exchanger warranty from Carrier? Any better ideas or suggestions ????