New construction in Ga. today and run across a house with a walk in spray foamed attic with a 100k btu 80% furnace and no type of outdoor combustion air source. The attic was large but the walk-in attic door was adjacent to a bedroom (no vents on the doors). So I reported that the combustion air adequacy was questionable and to have a licensed hvac contractor confirm that there was adequate combustion air source.
I’ve run across it before with a 44k btu being in a larger spray foamed attic with less concern than this one. But a few factors being the btu of the furnace (single unit/2 zone) the door being next to a bedroom and just curious as to just how large of an attic would be needed and even then where does it continue to obtain air from and it seems it would have to cause negative pressure in the attic or back drafting issues. I don’t think I have a specific question though. Ya’ll just talk about it.
General rule is 50 cubic ft of room/area space for each 1K of BTUs of appliance drawing combustion air (I’ve found it’s easier to calculate 5% of BTUs as necessary cubic feet but that just may be the way my brain works). Tightly sealed foam insulated space practically might impact that but am not sure if that’s addressed in the code (yet). No grill in the door to the bedroom is a good thing and hopefully it’s an exterior/weather-stripped door.