The calculation of a “gas rate” ends up with a figure in kilowatts – the heat input to a gas appliance – obtained by measuring the consumption of gas at the meter for a timed period and then plugging the numbers into a formula to obtain the gas rate.

What we’re actually doing is:

- Measuring the amount of gas burned in a timed period (usually around 2 minutes).
- Working out from this how much gas would be burned in 1 hour. This is the “gas flow rate” or “gas discharge rate”.
- Multiplying this by a number which converts this flow rate to a boiler power. This number is the calorific value or CV of the gas. The standard figure colleagues and I were taught to use was 10.76 kilowatts per cubic metre per hour (or 1040 Btu per cubic foot in imperial units).

An example is the gas rate measured on a Worcester Bosch 24i combi boiler at maximum output with two hot taps turned fully on:

Initial meter reading = 8.436

Final meter reading = 8.524

Gas burned = 0.088 m^{3} in 122 seconds

Gas rate = 3600/122 x 0.088 x 10.76 = 27.94kW gross, or 27.94/1.11 = 25.17kW net.

The manual gives 24.49kW net maximum – pretty close, but is the boiler slightly over-gassed (under is better than over)?

If we stop at stage 2 above, we simply leave out the 10.76 figure:

Gas flow rate = 3600/122 x 0.088 = 2.597m^{3}/hour.

The manufacturer’s manual helpfully gives us the specified gas flow rate: 2.59m^{3}/hour – almost exactly the same as we measured.

If we divide the manufacturer’s net input of 24.49kW by the gas flow rate of 2.59m^{3}/hour and multiply by 1.11 we get 10.5 – this is the figure the manufacturer has used for the calorific value – not our 10.76.

National Grid gives the range of calorific values for gas passing through its pipelines of 10.42 to 11.94 – quite a range.

Moral of the story: gas flow rate is what you actually measure at the meter. If the appliance manufacturer specifies this figure in the Installation & Servicing manual, compare this with your measurement for best accuracy. The question you’re answering is “is the appliance pulling gas at the right rate?”.