How to gas rate a boiler: meter reading, formula and worked example

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

1. 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.
2. 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.
3. Record the start reading, then time a known volume or a clear movement on the test dial.
4. Convert the measured volume to cubic metres if needed. One cubic foot is about 0.0283 cubic metres.
5. Calculate gas rate in cubic metres per hour: volume in cubic metres divided by seconds, multiplied by 3600.
6. Calculate heat input in kW: gas rate multiplied by calorific value in MJ/m3, divided by 3.6.
7. 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.

Enter these figures in the gas-rate calculator to compare metric and imperial readings, gross and net heat input, or print the revision chart.

Imperial meter worked example

Suppose a test dial measures 1 cubic foot in 45 seconds. One cubic foot is 0.0283168 m3, so gas rate = (0.0283168 / 45) x 3600 = 2.265 m3/h. With a gross CV of 39.5 MJ/m3, heat input = 2.265 x 39.5 / 3.6 = about 24.9 kW gross.

Read the dial volume before timing it: a full turn is not automatically 1 cubic foot. The billing register on an imperial meter can count hundreds of cubic feet; that is different from the volume marked on a test dial. Enter the actual timed volume in the calculator.

What gas rate should a 30 kW boiler have?

If 30 kW is the gross heat input, a revision CV of 39.5 MJ/m3 gives 30 x 3.6 / 39.5 = 2.734 m3/h. At that steady rate, 0.01 m3 takes about 13.2 seconds. These are calculated example values, not a pass limit for every boiler labelled 30 kW.

The model name may describe heat output, while the data plate lists heat input on a gross or net basis. A combi can also have different heating and hot-water ratings. Use the input and operating mode specified for the test before comparing a measured gas rate.

Using a smart-meter reading

Use the cumulative gas-volume register in m3, rather than a cost display or daily energy total. Subtract the start reading from the end reading and time the interval. For example, a rise of 0.080 m3 over 120 seconds gives (0.080 / 120) x 3600 = 2.40 m3/h.

The display must give enough resolution and update often enough for the approved test method. Follow the meter and appliance instructions; a display that has not updated does not establish that no gas has passed.

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.

Common gas-rating mistakes

• Mixing units in one line — cubic feet, cubic metres and seconds have to be reconciled before the formula, not halfway through it.
• Comparing a gross calculated input against a net data-plate figure, or the reverse; for natural gas, net is roughly gross divided by 1.1, so a "10% low" reading is often just the wrong basis.
• Timing too small a movement, so a fraction of a second of timing error becomes a large error in the rate — time a clear, measured volume.
• Starting the clock before the appliance has settled at the rate you are testing.
• Calling a reading "high" or "low" without checking the manufacturer tolerance, when it is actually within spec.