Gas rating is one of the best ACS revision calculations because the method is simple but the units catch people out. The aim is to work out how much gas the appliance is using and whether the heat input is close to the rating on the data plate.
Time a known meter volume, scale it to m3/h, apply calorific value and compare heat input with the appliance rating.
The formula
Gas rate in m3/h = (volume in m3 / time in seconds) x 3600. Heat input in kW = gas rate x calorific value / 3.6. Natural-gas calorific value varies, so use the current declared value where accuracy matters; 39.5 MJ/m3 is a common revision default.
One unit trap sits right at the end: a declared calorific value like 39.5 MJ/m3 is a gross figure, so this method gives gross heat input. Many modern appliance data plates quote net heat input (for natural gas, net is roughly gross divided by 1.1). Check which basis the data plate uses before calling a reading high or low.
Step-by-step method
- Check scope and safety first. Do not treat a revision calculation as permission to work on gas; live testing belongs to competent registered engineers using the appliance instructions.
- Set the appliance to run at the required rate and note the meter unit. A metric meter reads cubic metres; an imperial meter reads cubic feet.
- Record the start reading, then time a known volume or a clear movement on the test dial.
- Convert the measured volume to cubic metres if needed. One cubic foot is about 0.0283 cubic metres.
- Calculate gas rate in cubic metres per hour: volume in cubic metres divided by seconds, multiplied by 3600.
- Calculate heat input in kW: gas rate multiplied by calorific value in MJ/m3, divided by 3.6.
- Compare the result with the appliance data plate and manufacturer tolerance before deciding what the reading means.
Metric meter example
A boiler passes 0.01 m3 in 16.4 seconds with CV 39.5 MJ/m3. Gas rate = (0.01 / 16.4) x 3600 = 2.20 m3/h. Heat input = 2.20 x 39.5 / 3.6 = about 24.1 kW gross.
Smart meters and imperial meters
A smart meter still needs a gas volume reading you can time or compare over a measured interval. Imperial meters need conversion before the heat-input formula. The common mistake is mixing cubic feet, cubic metres and seconds in the same line.
What the result does not prove
A matching heat input does not prove the appliance is safe by itself. Combustion performance, flueing, ventilation, operating pressure, manufacturer instructions and unsafe-situations judgement still matter. Gas rate is one useful check inside a wider commissioning or servicing process.
Quick answers
What is the gas-rate formula?
Gas rate in m3/h is volume in cubic metres divided by time in seconds, multiplied by 3600. Heat input in kW is gas rate times calorific value, divided by 3.6.
Can I gas rate from an imperial meter?
Yes for the calculation, but convert cubic feet to cubic metres before calculating heat input. Live gas work still needs the correct competence and appliance instructions.
What calorific value should I use?
For revision, 39.5 MJ/m3 is commonly used for natural gas examples. For real work, use the current declared or supplied calorific value for the situation.
Is the calculated heat input gross or net?
Gross, when you use a declared calorific value such as 39.5 MJ/m3. Many appliance data plates quote net input; for natural gas, net is roughly gross divided by 1.1, so check which basis you are comparing against.
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