A worked gas-rate calculation for an older imperial (cubic feet) meter, converted to heat input in kW.
Plenty of homes still have an imperial gas meter that reads in cubic feet, so it is worth being able to rate an appliance the imperial way. This worked example uses figures chosen to show the method.
Step 1 - time one cubic foot. With other appliances off, run the appliance at full rate and time how long the test dial takes to pass 1 cubic foot (ft3). For this example it takes 80 seconds.
Step 2 - find the gas rate. There are 3600 seconds in an hour, so gas rate = 3600 / 80 = 45 ft3 per hour.
Step 3 - convert to heat input. Multiply the gas rate by the calorific value, taken here as 1040 British thermal units (Btu) per ft3 for natural gas: 45 x 1040 = 46,800 Btu per hour. Treat 1040 as an example value and use the declared calorific value for real work.
Step 4 - convert to kW. There are about 3412 Btu per hour in 1 kW, so 46,800 / 3412 = 13.7 kW gross input.
Step 5 - cross-check. The imperial and metric methods should agree, because 1 ft3 is about 0.0283 m3: 45 ft3/h is about 1.27 m3/h, which through the metric method gives a similar kW figure. Compare your result with the appliance data plate, and remember it may quote a net figure rather than gross. Use the actual metered volume and declared calorific value on site.