How to Gas Rate a Boiler
Gas rating is the timed meter test that tells you whether an appliance is actually burning gas at the rate the manufacturer intended — five minutes on site that catches problems a flue gas reading alone will miss.
When you gas rate an appliance you measure how much gas it consumes over a fixed period at full rate, convert that to a flow in m³/h, then to a heat input in kW, and compare the result against the data badge. It is a standard part of commissioning a new boiler, of the annual service, and of fault-finding when an appliance is not performing.
This guide covers the metric and imperial methods, the formulas behind them, the crucial gross-vs-net distinction, a worked 30 kW example and a quick reference table. All gas work in the UK must be carried out by a Gas Safe registered engineer — this is guidance only.
Key takeaways
- Metric: m³/h = (final − initial reading) × 3600 ÷ seconds, then gross kW = m³/h × CV ÷ 3.6 (CV ≈ 39.3 MJ/m³ for natural gas).
- Turn off every other gas appliance first and run the boiler at maximum rate until it stabilises before timing.
- Most modern data badges quote NET input — compare like with like: net kW = gross kW ÷ 1.11 for natural gas.
- A rate well below badge points at the supply (pipework, valve, regulator or working pressure), not the appliance.
- Don't chase a 2–3% discrepancy — the calorific value of the gas varies slightly by region and day to day.
What gas rating is and why it's done
Gas rating measures actual gas consumption at full rate and converts it to a heat input you can check against the appliance data badge. It proves a new boiler is set up and supplied correctly before you sign the Benchmark checklist, confirms an appliance still burns at its rated input year on year at service, and helps trace faults.
A rate significantly below the badge value suggests a supply problem: undersized or restricted pipework, a partially closed valve, low working pressure or a faulty regulator. A rate above the badge value also needs investigating before the appliance is left in service.
Before you time anything
Turn off all other gas appliances so the meter only registers the appliance under test, fire the boiler at maximum rate, and let it run a minute or two to stabilise. A pilot light or a hob left simmering will skew the result.
The metric method (meters reading in m³)
Note the meter reading, time a fixed period — 60, 120 or 180 seconds — then note the reading again. A longer test averages out meter resolution: 120 seconds is the common choice, and 180 seconds gives a more accurate result on small-input appliances.
Step 1, the flow rate: m³/h = (final reading − initial reading) × 3600 ÷ seconds. Step 2, the heat input: gross kW = m³/h × CV ÷ 3.6, where CV is the gross calorific value in MJ/m³ — about 39.3 for natural gas (a typical UK figure; the actual CV varies slightly by region and day to day).
Worked example: initial reading 12345.678, final reading 12345.780 after 120 seconds. Volume used = 0.102 m³, so m³/h = 0.102 × 3600 ÷ 120 = 3.06 m³/h; gross kW = 3.06 × 39.3 ÷ 3.6 = 33.4 kW gross; net kW = 33.4 ÷ 1.11 = 30.1 kW net.
The imperial method (meters reading in ft³)
Older imperial meters have a small test dial registering a fixed volume per revolution — typically 1 ft³. Rather than timing a fixed period, you time one complete revolution of the test dial with the appliance at full rate: ft³/h = (dial volume × 3600) ÷ seconds per revolution.
Convert to metric with m³/h = ft³/h × 0.0283, then apply the same gross kW = m³/h × 39.3 ÷ 3.6. Example: one revolution of a 1 ft³ dial takes 33 seconds, so ft³/h = 3600 ÷ 33 = 109.1, m³/h = 109.1 × 0.0283 = 3.09, gross kW = 3.09 × 39.3 ÷ 3.6 = 33.7 kW gross, about 30.4 kW net.
You will also see the imperial formula written with a 1.02264 correction factor that corrects the metered volume to standard temperature and pressure. It shifts the result by about 2%, well inside normal tolerance; the simpler form above is what most engineers and the TradePlanr calculator use.
Gross vs net — which figure does the badge quote?
Gross heat input includes the latent heat in the water vapour produced by combustion; net input excludes it. The conversion is fixed: net kW = gross kW ÷ 1.11 for natural gas, or ÷ 1.09 for LPG.
Most modern appliance data badges quote net input, so check which figure the manufacturer states before you compare. Rating a boiler and comparing your gross result against a net badge figure makes a healthy appliance look 11% over-firing — a classic way to chase a fault that isn't there.
Worked example — a 30 kW boiler
Badge states 30 kW net on natural gas. Expected gross = 30 × 1.11 = 33.3 kW; expected flow = 33.3 × 3.6 ÷ 39.3 = 3.05 m³/h; over a 120-second test that is about 0.102 m³ of meter movement. If it has only moved 0.08 m³ (≈2.4 m³/h, ≈26 kW gross), the appliance is under-firing by around 20% and the supply needs investigating before it leaves your hands.
Quick reference and acceptable tolerance
Assuming natural gas at 39.3 MJ/m³ gross and a net badge figure, common sizes rate roughly as follows (net kW → gross kW → expected m³/h → meter movement over 2 minutes): 24 → 26.6 → 2.44 → ~0.081 m³; 28 → 31.1 → 2.85 → ~0.095 m³; 30 → 33.3 → 3.05 → ~0.102 m³; 32 → 35.5 → 3.25 → ~0.108 m³; 35 → 38.9 → 3.56 → ~0.119 m³; 40 → 44.4 → 4.07 → ~0.136 m³. Treat these as sanity-check figures, not pass/fail limits — the badge and the manufacturer's instructions are always the reference.
Manufacturers typically state an acceptable tolerance in the installation or servicing instructions — commonly in the region of ±5–10% of the rated input. Small differences are normal, so don't chase a 2–3% discrepancy. Outside tolerance means investigate, not adjust and walk away: recheck the test was done properly, then look at working pressure at the appliance, pipework sizing and the regulator. Under-firing that traces back to undersized pipework won't be fixed at the gas valve.
On LPG the method is identical, only the constants change: gross CV is about 93.1 MJ/m³ for propane and 121.8 MJ/m³ for butane, with a 1.09 gross-to-net factor. LPG moves far less volume for the same kW, so a longer test pays off. On cylinder installations with no meter, gas rating isn't possible and you check burner pressure and combustion to the manufacturer's data instead.
Frequently asked questions
How long should the timed test be?
60, 120 or 180 seconds. A longer test averages out the meter resolution — 120 seconds is the usual choice, and 180 seconds gives a more accurate result on small-input appliances.
My gas rate is below the badge figure — is the boiler faulty?
Not necessarily. A rate well below badge usually points at the supply: undersized or restricted pipework, low working pressure at the appliance, a partially closed valve or a failing regulator. Check the test was done at full rate with everything else off before condemning the appliance.
Why does my gross result look 11% high against the badge?
Because you are almost certainly comparing a gross figure against a net badge value. Divide your gross kW by 1.11 (natural gas) or 1.09 (LPG) to get net, then compare like with like.
What calorific value should I use for natural gas?
About 39.3 MJ/m³ gross is the typical UK figure. The exact CV varies slightly by region and day to day, which is why a 2–3% discrepancy is normal and not worth chasing.
From guidance to action
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