How boiler gas rate is measured in the UK
Gas Safe engineers measure the time in seconds required for a gas meter to record a set volume of gas while an appliance fires at maximum capacity.
Under IGEM standards, the gross heat input in kW is calculated from the volume consumed, the gas calorific value (CV, typically 39.5 MJ/m3), and test duration.
Worked example
Clocking a metric gas meter that burns 0.05 m3 of natural gas in 120 seconds with a calorific value of 39.5 MJ/m3:
The gross heat input is 16.46 kW gross (14.81 kW net at 90% condensing efficiency), consuming 1.500 m3 of gas per hour at an estimated cost of £1.03 per hour.
Inputs for the gas rate calculator
Gas meter type
Choose metric for digital m3 meters or imperial for older dial meters marked in cubic feet.
Volume clocked during test
Volume recorded on the meter during the test run (e.g. 0.05 m3 or 1 to 2 ft3).
Time taken to burn volume
Stopwatch time in seconds recorded while the appliance is running at maximum demand.
Gas calorific value (CV)
Declared calorific value of gas in MJ/m3 from your gas supplier (typically 38.5 to 40.5 MJ/m3).
CALCULATOR QUESTIONS
Gas rate calculator questions
What is the difference between gross and net heat input?
Gross heat input (higher heating value) includes the latent heat of vaporization, while net heat input (lower heating value) is approximately 90% of gross for condensing boilers.
How long should I time the gas meter?
Industry standard practice recommends timing for at least 120 seconds on a metric meter (or 1 full revolution of an imperial dial) to ensure measurement accuracy.