HVAC Math Practice Test 2025 – The Complete All-in-One Guide for Exam Success!

To determine the required volume of indoor combustion air for an 80% Annual Fuel Utilization Efficiency (AFUE) 100,000 Btu/hr gas furnace, it's important to understand how combustion air needs are calculated in relation to the furnace’s output and efficiency.

AFUE is a measure of a furnace's heating efficiency. In this case, an 80% AFUE means that 80% of the fuel's energy is converted into usable heating for the home, while the remaining 20% is lost. Therefore, the furnace is using a significant amount of air for proper combustion, which requires sufficient ventilation to ensure safety and efficiency.

For gas furnaces, a typical calculation for combustion air is that about 1 cubic foot of air is needed for every 1,000 Btu/hr of input capacity, adjusted based on factors such as the heating value of the gas. However, in standard practice, we often see recommendations that can vary.

With a 100,000 Btu/hr gas furnace operating at an 80% efficiency, to ensure adequate combustion, the calculation often leads to:

1. Calculate the required air based on input:

- 100,000 Btu/hr / 1000 = 100 cubic feet of air