Before You Start
Before you begin a gas rate test, make sure you have everything ready. This is a procedure that every Gas Safe registered engineer should be able to carry out on every service, commissioning, or fault-finding visit.
What You Need
- ✓Gas Safe registration — you must be registered to work on gas appliances
- ✓U-gauge or digital manometer — for checking gas pressures if needed
- ✓Stopwatch or timer — accuracy matters, use a dedicated stopwatch or phone timer
- ✓Calculator — or use the free gas rate calculator
- ✓Appliance data plate details — you need the rated heat input (kW) to compare against
How to Do a Gas Rate: Step-by-Step
Prepare the Property
Turn off ALL other gas appliances in the property. Only the appliance under test should be consuming gas. Close the gas cock to every other appliance where possible.
This is critical — if another appliance fires up during your test (a gas hob, a second boiler, a gas fire), your reading will be wrong.
Identify Your Meter Type
Check whether the gas meter is metric (displays volume in cubic metres, m³) or imperial (has a test dial measuring in cubic feet, cu ft).
This determines which measurement method you use. Most modern meters are metric, but many older properties still have imperial meters with a test dial.
Tip: If the meter displays in m³ with 3 decimal places, it is metric. If it has a small rotating dial labelled “0.5”, “1”, “2”, or “5” cu ft, it is imperial.
Record the Starting Reading
Metric meter: Note the meter reading to 3 decimal places (e.g. 12345.678 m³). The last 3 digits (usually in red) are the ones that matter most for short tests.
Imperial meter: Identify your test dial size. This will be 0.5, 1, 2, or 5 cubic feet per revolution. The size is printed on the dial face.
Turn on the Appliance
Fire the appliance at full rate — maximum burner setting. For a boiler, open all hot taps to create maximum demand. For a gas fire, turn it to its highest setting.
Wait 2-3 minutes for the appliance to fully stabilise before you start timing. The gas rate during the initial warm-up period is not representative.
Start Timing
Metric method: Start your stopwatch and note the exact meter reading at that moment. Run for a minimum of 2 minutes (120 seconds). Longer timing periods give more accurate results — 3 to 5 minutes is ideal.
Imperial method: Start your stopwatch as the test dial pointer passes a marker. Time one complete revolution of the test dial.
Record the End Reading
Metric: Stop your stopwatch and note the final meter reading at the same moment. Record both the reading and the elapsed time in seconds.
Imperial: Stop the stopwatch when the test dial completes exactly one full revolution. Record the time in seconds.
Precision matters: A 1-second error on a 60-second test is a 1.7% error. On a 120-second test it is only 0.8%. This is why longer timing is more accurate.
Apply the Formula
Use the gas rate calculator or calculate manually. See the full formula breakdown on the gas rate formula page.
Metric Gas Rate Formula
Volume (m³) = End Reading - Start Reading
kW gross = Volume × (3600 ÷ seconds) × 39.5 × 1.02264 ÷ 3.6
kW net = kW gross ÷ 1.11
Imperial Gas Rate Formula
m³/h = (dial size in cu ft × 3600 ÷ seconds) × 0.0283168
Then apply the same formula as metric from m³/h.
Compare to the Data Plate
Find the manufacturer's data plate on the appliance. This is usually inside the front panel or on the underside of the casing. Compare your calculated net kW to the rated heat input on the data plate.
Your result should be within ±5% of the rated value. For a 24 kW boiler, the acceptable range is 22.8 kW to 25.2 kW.
Pass or Fail
Pass
Gas rate is within ±5% of the data plate rating. Record the result on your service report or benchmark checklist.
Fail
Gas rate is outside ±5%. Do not leave the appliance in service without investigating the cause. See the troubleshooting section below.
The Gas Rate Formula Explained
If you want to understand how to calculate gas rate manually rather than using a calculator, here is a breakdown of what each number in the formula means.
3600
Converts seconds into hours (3600 seconds in one hour) to give you a flow rate in m³ per hour.
39.5 MJ/m³
The calorific value (CV) of natural gas in the UK. This is how much energy is in each cubic metre of gas.
1.02264
The volume correction factor. Adjusts your measured volume to standard temperature (15°C) and pressure (1013.25 mbar).
3.6
Converts megajoules (MJ) into kilowatts (kW). 1 kW = 3.6 MJ/h.
1.11
Converts gross kW to net kW. Gross includes latent heat in water vapour; net is the usable heat. Most data plates show net kW.
For a deeper dive into the formula, including worked examples for metric, imperial, and LPG, see the complete gas rate formula guide.
Pass or Fail Criteria
The industry-accepted tolerance for a gas rate test is ±5% of the manufacturer's rated input on the data plate.
Quick Reference: ±5% Ranges
| Data Plate (kW net) | Minimum (-5%) | Maximum (+5%) |
|---|---|---|
| 15 kW | 14.25 kW | 15.75 kW |
| 24 kW | 22.80 kW | 25.20 kW |
| 30 kW | 28.50 kW | 31.50 kW |
| 35 kW | 33.25 kW | 36.75 kW |
| 40 kW | 38.00 kW | 42.00 kW |
What to Do If the Gas Rate Fails
If your gas rate reading is outside the ±5% tolerance, you need to investigate before leaving the appliance in service. Work through these checks systematically:
1. Check Inlet Gas Pressure
Connect your manometer to the meter outlet or appliance inlet test point. Natural gas inlet pressure should be 20 mbar (working). Low inlet pressure is one of the most common causes of a low gas rate.
2. Check Burner Pressure
Connect your manometer to the burner pressure test point on the gas valve. Compare to the manufacturer's specified burner pressure. Adjust if necessary (and if the valve allows adjustment).
3. Check for Blocked Injectors
Partially blocked burner injectors restrict gas flow, causing a low gas rate. Remove and inspect them. Clean or replace as needed.
4. Check the Gas Valve
A faulty or sticking gas valve may not open fully (low rate) or may not regulate properly (high rate). If burner pressure cannot be adjusted to the correct value, the gas valve may need replacing.
5. Check Incorrect Injectors
Verify the correct injectors are fitted for the gas type. LPG injectors on a natural gas supply (or vice versa) will give a significantly wrong gas rate.
6. Check Heat Exchanger and Flue
A blocked or partially blocked heat exchanger or flue can cause the appliance to lock out or cycle, affecting your gas rate reading. Inspect for debris or blockages.
Frequently Asked Questions
How long should a gas rate test take?
The test itself takes around 5-10 minutes per appliance. You need 2-3 minutes for the appliance to stabilise at full rate, then a minimum of 2 minutes (120 seconds) of timed measurement on a metric meter. Imperial test dials are quicker as you only time one revolution. Including setup and recording, allow around 10 minutes per appliance.
What if my gas rate is too high?
A gas rate reading more than 5% above the data plate rating means the appliance is over-gassed. Check the burner pressure — it may be set too high. Also check the gas valve is not passing or sticking, and verify the correct injectors are fitted for the gas type (natural gas vs LPG). Over-gassing is a safety concern and must be rectified before leaving the appliance in service.
What if my gas rate is too low?
A gas rate below 5% of the data plate rating indicates under-gassing. Common causes include low inlet gas pressure (check the meter pressure), partially blocked injectors, a faulty or incorrectly adjusted gas valve, restrictions in the gas supply pipework, or a blocked condensate trap affecting the flue. Check inlet pressure first — it should be 20 mbar for natural gas.
Do I need to gas rate every appliance I service?
Yes. Gas rating is a mandatory part of any gas appliance service, commissioning, or breakdown repair under Gas Safe regulations. It confirms the appliance is burning gas at the correct rate for safe and efficient operation. You should record the gas rate on your service report or benchmark checklist.
What tolerance is acceptable for a gas rate?
The accepted tolerance is plus or minus 5% of the manufacturer's data plate rating. For example, a boiler rated at 24 kW net should read between 22.8 kW and 25.2 kW. If your reading falls outside this range, you must investigate and rectify the cause before signing off the appliance.
Disclaimer
This guide is for reference purposes only. Please rely on a professional, accredited Gas Safe registered engineer and accredited tools for all measuring. Gas work must only be carried out by competent persons registered with the HSE Gas Safety approved scheme. Always follow the manufacturer's instructions and current gas safety regulations.
Related Tools and Guides
Gas Rate Calculator
Free online calculator for metric and imperial gas rate tests
Gas Rate Formula
Full formula breakdown with worked examples
Imperial Gas Rate Calculator
Calculator for test dial meters in cubic feet
LPG Gas Rate Calculator
Propane and butane gas rate calculations
Gas Rating Appliances
Appliance-specific gas rating guidance for boilers, fires, and cookers