Guide

How to gas rate a boiler, step by step

Gas rating tells you whether an appliance is burning what the manufacturer says it should. It is a meter reading, a stopwatch and one piece of arithmetic, and most of the mistakes happen before the maths.

What you are actually measuring

You are measuring how much gas the appliance burns in a known time, converting that to a flow rate per hour, and turning that into heat input in kilowatts. Then you compare it against the figure on the data badge. If it is burning noticeably more or less than it should, something needs attention before you leave.

The procedure

1. Turn everything else off. Every other gas appliance in the property, pilots included where practical. Anything else burning goes straight into your reading and makes the appliance look overgassed.
2. Get it to maximum rate. Just calling for heat is not enough on a modulating boiler, which will drop back within a minute or two. Put it into the maximum-rate service mode the manufacturer specifies. The sequence differs by make and by model.
3. Let it settle. Give it long enough to stop ramping before you start timing.
4. Read, time, read. Note the meter, start the clock, and take a second reading after a known number of seconds. Two minutes is the usual choice on a U6 because it gives a volume you can read confidently.
5. Do the maths. Subtract the readings for the volume burned, then work it out below or in the gas rate calculator.
6. Compare against the badge. And check whether the badge is quoting gross or net.

The formula

On a metric meter:

flow (m³/h) = volume × 3600 ÷ seconds
gross kW = flow × CV ÷ 3.6
net kW = gross × 0.901

CV is the calorific value of the gas. 39.3 MJ/m³ is the working figure for UK natural gas. It is not a constant: it shifts slightly by region and by day, and the supplier publishes the real number.

On an imperial meter the volume is in cubic feet, the CV in BTU/ft³, and the gross figure is flow × CV × 0.000293071.

Worked example

0.040 m³ burned in 120 seconds, natural gas at 39.3 MJ/m³:

Step	Working	Result
Flow rate	0.040 × 3600 ÷ 120	1.20 m³/h
Gross	1.20 × 39.3 ÷ 3.6	13.10 kW
Net	13.10 × 0.901	11.80 kW

Four reasons a correct gas rate reads wrong

Gross against a net badge figure. The two are about 10% apart, which is roughly the margin that turns a healthy appliance into an apparent problem. UK manufacturers commonly quote net on condensing appliances. Check which one you are comparing against before you start chasing a fault.

Something else is burning. A hob pilot is easy to miss and it will sit in your reading the whole time.

The appliance never reached maximum rate. If it modulated down partway through the two minutes, the average you measured is not the maximum you wanted.

The wrong calorific value. Using a default when the local figure is meaningfully different will move the answer.

Work it out

The gas rate calculator does the arithmetic in both metric and imperial, and shows flow rate, gross and net together.

Flint is a reference tool for competent, Gas Safe registered engineers. Everything here informs your judgement and never replaces it. Always work to the manufacturer's instructions and the current standards, and check every figure against the appliance data badge and manual.