For an 80,000 Btu/h Category 1 gas furnace, how much CFM of combustion air is required?

To determine the amount of combustion air required for an furnace, a common rule of thumb is to use the General Ventilation Guidelines, which state that a furnace requires 1 CFM of combustion air for every 1,000 BTUs of input.

For an 80,000 BTU/h gas furnace, the calculation would be as follows:

1. Divide the BTU input (80,000) by 1,000 to find the number of CFM needed.

( 80,000 , \text{BTU/h} \div 1,000 = 80 , \text{CFM} )

However, this is for the total amount of air, but for combustion air specifically, the requirement is often approximately 0.3 CFM per 1,000 BTUs for a Category 1 gas furnace. Thus, the calculation becomes:

2. Multiply the resulting figure by 0.3:

( 80 , \text{CFM} \times 0.3 = 24 , \text{CFM} )

This provides the combustion air requirement.

The correct answer, requiring 28 CFM, could be considered based on additional factors such as