Gas Rating Appliances — Understanding Rated vs Actual Heat Input

A gas rating is the manufacturer's rated heat input for an appliance, measured in kilowatts (kW). It is printed on the appliance data plate and represents the expected gas consumption when the appliance operates at full rate under standard conditions.

When you are gas rating an appliance, you are checking whether the actual gas consumption matches this manufacturer specification. This is a fundamental part of commissioning, servicing, and fault-finding for every Gas Safe registered engineer.

The gas rating calculation compares these two numbers. If the measured gas rate falls within an acceptable tolerance of the rated input, the appliance is performing as intended. If not, further investigation is required.

These two terms sound almost identical but mean very different things. Understanding the difference is essential for every gas rating calculation.

The whole point of performing a gas rate test is to compare your measured result against the gas rating. The difference between these two values is what tells you whether the appliance is working correctly, over-gassing, or under-gassing.

Use our gas rate calculator to calculate your measured rate, then compare it to the data plate rating.

Every gas appliance has a data plate showing its gas rating and other key specifications. The location varies by appliance type:

Information on the data plate typically includes:

• Manufacturer and model number
• Serial number
• Rated heat input (gross kW / net kW)
• Gas type (e.g., G20 natural gas)
• Inlet pressure (20 mbar for natural gas UK)
• Gas category (e.g., I_2H for natural gas in the UK)
• Nominal heat output and minimum heat output
• CE / UKCA marking

Data plates contain several values related to the gas rating. Here is what each one means:

When comparing your measured gas rate to the appliance's gas rating, the accepted tolerance is ±5% of the rated input.

Over-Gassing (Measured Rate Higher Than Rating)

Under-Gassing (Measured Rate Lower Than Rating)

The table below shows typical gas rating ranges for common UK appliances. Use these as a rough reference — always check the specific data plate for the exact rated input.

When performing a gas rating calculation on a multi-burner appliance like a hob, each burner must be tested individually at full rate. The total gas consumption of all burners running simultaneously should not exceed the total rated input on the data plate.

If your measured gas rate is outside the ±5% tolerance of the appliance's gas rating, systematic investigation is needed. Here are the most common causes:

Measured Rate Too High

• Check burner pressure — may be set too high and need adjustment via the gas valve
• Check gas supply pressure — high inlet pressure can cause over-gassing
• Check injector size — wrong injector fitted (common after LPG-to-natural-gas conversion or vice versa)
• Check gas valve — a faulty gas valve may not be regulating correctly

Measured Rate Too Low

• Check for blocked burners — carbon or debris restricting gas flow to the burner
• Check heat exchanger — a heavily scaled or blocked heat exchanger can restrict flow
• Check gas supply pressure — low inlet pressure means less gas reaching the appliance
• Check for partial valve failure — gas valve not opening fully

Could Be Either Direction

• Check the correct CV is being used — using the wrong calorific value (e.g., LPG value for natural gas) will throw the calculation off entirely
• Check timing accuracy — inaccurate timing during the gas rate test is one of the most common sources of error
• Check meter reading accuracy — misreading the meter or not allowing the appliance to reach full rate before starting the test

For the formula and step-by-step method to calculate your measured gas rate, see our gas rate formula guide or the practical how to do a gas rate walkthrough.