How furnace firing rate is clocked in the US
HVAC technicians clock a gas meter by timing one complete revolution of the smallest test dial (typically 1/2, 1, or 2 cubic feet) with all other gas appliances shut off.
Under the National Fuel Gas Code (NFPA 54 / ANSI Z223.1), the firing rate in BTU per hour is calculated by multiplying 3,600 by dial size and heating value, divided by seconds per revolution.
Worked example
Clocking a 1 cubic foot test dial on a residential furnace that takes 36 seconds per revolution, with natural gas heating value of 1,030 BTU/cu.ft:
The calculated firing rate is 103,000 BTU/hr (1.030 therms/hr or 30.19 kW thermal power), costing approximately $1.34 per continuous firing hour.
Inputs for the US gas rate calculator
Test dial size
The volume marked on the test dial that completes one revolution (0.5, 1, or 2 cubic feet).
Seconds per dial revolution
Stopwatch time in seconds for the test hand to make exactly one full 360-degree rotation.
Gas heating value
Energy content of the utility gas in BTU per cubic foot (typically 1,000 to 1,050 BTU/cu.ft).
CALCULATOR QUESTIONS
Gas rate calculator US questions
Why should other gas appliances be turned off during the test?
Any water heater, pilot light, or gas dryer operating during the test will draw gas through the meter and distort the furnace firing rate calculation.
What is 1 therm of natural gas in the US?
One therm is a standard energy unit equal to exactly 100,000 British Thermal Units (BTU) of heat energy.