A worked metric gas-rate calculation, converted to gross heat input and compared with the data plate on the same gross or net basis.
A gas rate tells you how much gas an appliance actually burns, so you can compare its heat input with the data plate.
Measure
- Turn off every other gas appliance, so only this one is using gas.
- Run the appliance at the specified condition, usually full rate, and let it settle. Start-up surges and modulation distort the figure.
- Read the meter, time the test, and read it again. The volume used is the end reading minus the start reading, taken from the dial you actually timed.
Keep the measurements separate. The meter's volume change and the elapsed time give m³/h; a pressure gauge gives mbar for a different supply check. A meter face may mark different volume increments, so read the marking beside the moving dial before timing it. A guessed increment changes the volume in the calculation, however carefully the time is measured.
Example
The meter advances 0.040 m³ in 120 seconds.
- Gas rate = volume ÷ time. There are 3,600 seconds in an hour, so 0.040 m³ × (3,600 ÷ 120) = 0.040 × 30 = 1.2 m³/h.
- Gross heat input (kW) = gas rate (m³/h) × calorific value (MJ/m³) ÷ 3.6. The 3.6 converts MJ per hour into kW. At a gross calorific value of 39.0 MJ/m³: 1.2 × 39.0 ÷ 3.6 = 13.0 kW gross.
- Match the basis. A gross calorific value gives gross input, but many data plates quote net. For natural gas, gross ≈ net × 1.11, so a 24 kW net plate means about 26.6 kW gross. Going the other way, 13.0 ÷ 1.11 ≈ 11.7 kW net.
- Compare input with input: the plate's heat input, never its useful heat output.
Quick checks
- 0.10 m³ in 2 minutes: 0.10 × (60 ÷ 2) = 3.0 m³/h.
- 2.0 m³/h at 39.3 MJ/m³: 2.0 × 39.3 ÷ 3.6 = 21.83, about 22 kW gross.
- Timed in minutes: gas rate (m³/h) = volume × 60 ÷ minutes. Work it back to check: 3.0 m³/h is 0.05 m³ a minute, which gives the 0.10 m³ in 2 minutes.
- The usual slips with 0.10 m³ in 2 minutes: multiplying by the minutes (0.2), multiplying by 60 without dividing by 2 (6.0 m³/h, as if the volume passed in one minute), stopping at 60 ÷ 2 = 30 without the volume, or taking 2 minutes as 0.02 hour (5.0). Two minutes is 2 ÷ 60, about 0.033 hour.
- Sense check: a typical U6 domestic meter is rated for at most about 6 m³/h for the whole house, so one appliance at 30 m³/h is a slip. Larger meter types exist.
Reading the result
- Well above the matching rated input: excess fuel can impair combustion and overheat components. First check the timed volume, test mode and gross/net comparison; then investigate the supply and this appliance's set-up using its instructions. Do not turn the meter regulator to trim one boiler.
- Well below the plate: under-gassing, a partial blockage, or a governor or injector fault to investigate.
- A matching figure is one check. It does not prove safe combustion or complete commissioning.
On a real job, use the actual metered volume and the declared calorific value.