A worked gas-rate calculation for an older imperial (cubic feet) meter, converted to heat input in kW and cross-checked with the metric method.
Some older installations still have a meter that reads in cubic feet (ft³). The idea is the same as a metric gas rate: volume over time, then heat input.
Measure
- Turn off every other gas appliance.
- Run the appliance at full rate and let it settle before timing.
- Time how long the test dial takes to pass 1 ft³, using the volume marked for the dial you actually time. In this example it takes 80 seconds.
Calculate
- Gas rate. There are 3,600 seconds in an hour: 3,600 ÷ 80 = 45 ft³/h.
- Heat input in Btu per hour. Multiply by the calorific value, here 1,040 Btu per ft³ for natural gas: 45 ft³/h × 1,040 Btu/ft³ = 46,800 Btu/h.
- Convert to kW. About 3,412 Btu/h make 1 kW: 46,800 ÷ 3,412 = 13.7 kW gross.
Cross-check with the metric method
1 ft³ is about 0.0283 m³, so 45 ft³/h is about 1.27 m³/h. Put through the metric formula (m³/h × MJ/m³ ÷ 3.6), that gives a similar kW figure, so the two methods agree.
Compare with the data plate
- A gross calorific value gives gross input. If the plate quotes net, convert first: for natural gas, gross ≈ net × 1.11.
- Compare heat input with heat input, never with useful heat output.
- Well above the plate means over-gassing, with a risk of incomplete combustion and carbon monoxide: find the cause.
On a real job, use the metered volume and the declared calorific value.