Furnace in Hallway Closet

[dwigger (David Wigger, CMI, Retired)]

Here is an earlier discussion on this topic.

Combustion Air Requirements for Gas Furnaces

Is there a quick and easy way to calculate how many cubic feet of combustion air a natural gas furnace in a room needs? This is assuming all the combustion air comes from that room. I know I’m not supposed to be out there making calculations like this. The reason I ask is every once in a while I run across a furnace in a room with out a source of combustion air (no flues or louvers in the doors). I need a rule of thumb to calculate this and help me decide If I need to write it up for a HVAC prof…