What is a gas rate check?

It's the field test that proves the appliance is burning gas at its rated input. Run the boiler on full heating demand, time a period at the meter (2 minutes is standard), calculate the volume consumed, and convert to kW. If it's within ±5% of the data badge, the commissioning is correct.

Which CV (calorific value) should I use?

The UK national average is 39.5 MJ/m³ and is good for a commissioning check. For more accuracy, use the CV printed on the householder's most recent gas bill — it reflects the actual gas mix on their supply.

Why the 1.02264 correction factor?

Gas Safe Procedures (BS EN 1776 / IGEM/UP/1A) require volume to be corrected from measured conditions (meter inlet) to standard conditions (15 °C, 1013.25 mbar). 1.02264 is the standard UK conversion — applied automatically here.

This tool gives you gross input kW, which is what manufacturers print on the data badge. Net input kW (used in efficiency calcs and some European documentation) is typically ~10% lower for natural gas. Always compare the same number on both sides.

How long should the timed test be?

Minimum 2 minutes (120 seconds) on UK domestic meters with 0.01 m³ resolution. Longer tests are more accurate. On imperial dial meters, time at least 2-3 full revolutions of the test dial.

What does the appliance need to be doing during the test?

Running on full heating demand, with no other gas appliance using gas (turn the hob off). A combi on pre-heat or modulating will give a false reading. Set to max rate via the service menu if the appliance has a commissioning mode.

Will this work on an LPG (propane) meter?

LPG has a different calorific value — around 93 MJ/m³ for propane vapour versus 39.5 for natural gas. Enter a custom CV if supported (future update), or calculate separately. The volume correction still applies.

What if the result is outside ±5%?

Check gas pressures first (working pressure at full rate should be 19-23 mbar on natural gas at the appliance inlet). If pressures are correct and you're still off, check for partial blockages, injector sizing, or an incorrectly set gas valve on appliances with adjustable ones.

Free calculator for engineers

Gas Rate Calculator (kW)

Start the timer, read the meter, enter the readings. Get gross kW input and a pass/fail against the rated figure. Volume correction built in.

Meter type

Start reading (m³)

End reading (m³)

Duration (seconds)

Standard procedure: run the boiler on max heating demand for 2 minutes and note the meter movement.

Manufacturer's rated input kW (optional)

The gross input kW from the boiler's data badge — not the output.

Gas rate

—kW gross

Based on a calorific value of 39.5 MJ/m³ (UK natural gas average) and volume correction factor 1.02264. Use the CV on the householder's gas bill for higher accuracy.

How it works

1
Turn off other gas
Isolate all other gas appliances so only the boiler is drawing.
2
Fire the boiler on max
Force the appliance to full heating demand. Let it stabilise for 30 seconds.
3
Time the meter
Note start reading, run the built-in timer for 2 minutes, note end reading. Enter above.

The calculation, properly explained

kW = (m³/hr × CV MJ/m³) ÷ 3.6

The 3.6 comes from unit conversion — 1 kWh equals 3.6 MJ, so dividing megajoules per hour by 3.6 gives you kilowatts.

Before that, the meter volume gets a correction factor of 1.02264 — the standard UK conversion from meter conditions to reference conditions (15 °C, 1013.25 mbar). Gas Safe requires it on every commissioning calculation.

Common mistakes on a rate check

Not isolating other appliances. A cooker on standby, a fire with a pilot light, even a gas tumble dryer — they're all drawing gas. The meter can't tell them apart. Turn them all off before you start the clock.

Mixing gross and net input. UK data badges print gross kW. Some European and condensing-specific documentation uses net. If you're 10% low and everything else looks right, check which figure you're comparing against.

Reading the wrong dial on an imperial meter. The fastest-moving dial (usually 1 ft³ or 0.5 ft³) is the test dial. Pick the main 100 ft³ dial and your 2-minute reading will barely move.

Not giving the boiler time to stabilise. Straight from cold start, a modulating burner ramps up. Let it run at full rate for 30-60 seconds before starting the timer or you'll read low.

When to use this tool

Commissioning. Every new appliance install needs a rate check. This is non-negotiable.

Annual service. Good practice if the appliance is older or has been running oddly. Drift can indicate valve issues, blocked injectors, or gas pressure faults.

Fault-finding. If a boiler lockouts on ignition or short-cycles on high fire, a rate check rules in or out gas supply issues before you start throwing parts at it.

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Frequently asked questions

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What kW output to specify for a replacement.

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