Natural gas furnace max BTU/Hr.?

Question

How do I find out how many BTU/Hr my furnace can use? It's an old Heil NUGG075DF01. It uses natural gas.

If you don't know about this particular furnace, how many max BTU/Hr is a typical furnace rated for a 3-bedroom single-story house?

The reason I ask this question is because I'm calculating the total amount of combustion air needed by all my natural gas appliances, to make sure I have enough air openings and enough volume of space in the house to provide air through those openings. Because, I'm getting my water heater replaced, and an inspector is going to come, so I want to be sure everything is compliant with code.

Answer 1

when calculating the air requirements keep in mind that the furnace is going to use more air than needed just for combustion, gas furnaces typically require 10 - 20% more air than needed to complete combustion; this is known as "excess air" duh.  Each cubic foot of Natural Gas requires 10 cubic feet of air to burn completely, if the furnace is set up right at 75,000 it means if the furnace burner stayed on for an hour, it would burn 750 cubic feet of gas that will need 7500 cubic feet of air to burn, add a conservative 10% excess air to that and we have a net of 8250 cubic feet of air that must pass through the furnace.