Category - Boilers
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Will Scholfield, Engineer

30 Nov : Updated 30 Nov ● 6 min read

What is boiler commissioning? A complete guide

Boiler commissioning is an essential phase in the boiler installation process that ensures the system is effective and safe. 

Before use, the boiler must pass a series of checks, tests, and adjustments to meet all performance and safety criteria, which is known as boiler commissioning.

This comprehensive guide will lead you through the main procedures and concerns involved in boiler commissioning.

What is boiler commissioning?

When installing a new boiler, it must undergo a crucial phase known as commissioning. Boiler commissioning is essential to ensure your new boiler is installed and configured correctly, guaranteeing you get the most out of your heating and hot water products. It also ensures that precautions are in place to avoid common boiler issues that could lead to the breakdown of your boiler.

Since April 2018, all boilers installed in the UK must achieve at least 92% energy ErP efficiency and have working timers and temperature controls. This legislation aims to improve the energy efficiency of UK homes.

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Who carries out boiler commissioning?

A Gas Safe registered engineer should always carry out boiler commissioning and boiler services. 

During commissioning, engineers will carry out all the required tests, checking the boiler and system and then make any necessary adjustments. 

Moreover, they will ensure the boiler complies with all necessary regulatory requirements and manufacturer specifications.
It is important to note that you should always appoint a Gas Safe registered engineer when you require any boiler-related maintenance.

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What is the boiler commissioning procedure?

The boiler commissioning procedure typically consists of several stages to ensure it is correctly installed, examined, and tested before turning it on. While the particular technique will differ depending on the kind and size of the boiler, as well as manufacturer standards - these are the general steps:

Pre-commissioning checks

Before starting the commissioning process, the installer or technician should verify that the boiler installation meets all applicable regulations, standards, and manufacturer requirements. This includes checking the integrity of the boiler's components, ensuring proper fuel supply and ventilation, and verifying that all safety devices are in place.

We will touch on the boiler commissioning checklist a bit later on, but for now, let's look at the further steps in the procedure.

Initial inspection

It is vital to thoroughly inspect the boiler to ensure it is free from any damage during transportation or installation. The engineer will check for leaks, loose connections, and potential issues affecting its operation.

Mechanical completion

All mechanical components, such as pumps, fans, valves, and controls, need to be installed correctly and properly connected. The engineer will also inspect the integrity of the system piping, venting of the system, and the route of the boiler flue and gas pipe.

Electrical connections

The power supply, control wiring, safety interlocks and sensors are all inspected during the boiler commissioning process. Wiring the electrical components correctly and securing the connections is extremely important when installing equipment. 

Water and fuel system check

The engineer will ensure the system and pipework is water tight and, if it is a sealed system, they will check that it is pressurised to the correct pressure with all air bled from the system. The gas supply will also be tested for tightness making sure there are no leaks. 

Safety checks

All the boiler safety devices will be checked during this phase of commissioning including flame supervision devices, low water pressure sensors etc. This step of the process is of paramount importance to confirm safe operation of your boiler and system.

Functional testing

The Gas Safe engineer conducts a series of functional tests to verify the proper operation of the boiler. This includes testing the ignition system, burner operation, gas working and standing pressure as well as other specific boiler features.

Performance testing

The Gas Safe engineer will also carry out performance tests, sometimes known as flue gas analysis to confirm the boiler is burning the gas correctly and safely ensuring its efficiency, combustion quality, and emissions. At this point, the engineer will also ensure the boiler is burning the correct amount of gas. 


Finally, a commissioning report is prepared to document the entire process, including any adjustments, test results, and safety checks. The report serves as a record of the commissioning procedure and could be required to ensure compliance.

Boiler commissioning checklist

So, now you know the process, let's look at the boiler commissioning checklist. These are all the steps the engineer must take before commissioning the boiler:

1. Cleanse the system

After installing the heating system, cleaning it according to the manufacturer's instructions is crucial. This involves flushing the  system to prevent black magnetite (this is commonly known as sludge - a substance of dissolved and rusted metal from your central heating system's pipework) from flowing through your new boiler's heat exchanger.

Learn more about the different types of boiler flushes in our full guide.

2. Add a corrosion inhibitor and refill

After cleaning the system, add the corrosion inhibitor. This protects radiators, prevents hydrogen gas formation, increases boiler efficiency, and reduces the risk of corrosion and future repairs being required. We recommend installing a corrosion inhibitor to protect your boiler's long-term durability and help you to save money on repairs.

3. Test the gas flue analyser

Perform a flue gas analyser test to measure flue gases. It's mandatory to fill out the benchmark book that accompanies your new boiler installation and include all analyser readings. While boilers come factory set, occasional adjustments may be necessary. Your engineer will ensure the boiler runs correctly by taking readings and adjusting as needed.

4. Take a gas reading to check the boiler’s gas rate tolerance

A Gas Safe Engineer may have connected the boiler to a new pipe. If this is the case, they must perform a gas reading to ensure everything fits correctly. This step also helps confirm that the pipe is not blocked and ensures smooth operation.

5. Adjust the boiler output

Set the boiler output according to the size of your house and the number of radiators. This ensures energy-efficient operation and proper heating for all radiators, avoiding excessive running time and unnecessary expenses.

6. Gas tightness test

Conduct a gas tightness test to check the gas pressure in the pipework from the meter to the boiler. This verifies that the pipes can hold the correct pressure and prevents leaks, reducing the need for future repairs.

7. Register the boilers guarantee

Let your engineer handle the guarantee registration, as they have the necessary apps. Once registered, they should explain the guarantee to you. After completing the boiler commissioning checklist and ensuring everything works correctly, the engineer will explain how your new boiler and its controls operate. 

They will also review warranty details and provide information about regular boiler servicing and maintenance requirements.


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