Boiler commissioning is a critical step when installing a new heating system or replacing an existing one. In this guide, I’ll walk you through boiler commissioning, including what it is, what causes commissioning issues, and how to fix them safely and correctly.

What Is Boiler Commissioning?

Boiler commissioning is the process of safely setting up, testing, and fine-tuning a boiler to ensure it runs efficiently, safely, and in line with UK building and Gas Safe regulations. Commissioning is not optional—it is a legal requirement for all boiler installations.

During commissioning, the engineer checks combustion levels, system pressure, water quality, correct installation, flue safety, and that the boiler meets manufacturer specifications. Proper commissioning protects your warranty, reduces breakdown risks, and ensures optimum performance.

Why Does Improper Boiler Commissioning Happen?

Improper boiler commissioning happens for several reasons, typically involving installation issues, water quality, incorrect system balancing, gas supply problems, or skipped steps by unqualified installers. Below, we break down each cause and how to fix it properly.

Is Incorrect Installation a Cause of Boiler Commissioning Problems?

Incorrect installation is one of the most common causes of boiler commissioning issues.

What causes incorrect installation?

Incorrect installation is usually caused by poor workmanship, lack of experience, or the boiler being fitted by someone who is not a Gas Safe registered engineer. This can result in:

• Incorrect pipe sizing 
• Poor system layout 
• Flue installed at the wrong angle 
• Gas supply not meeting manufacturer requirements 
• Missing safety components such as inhibitors or filters
  

How to fix incorrect installation

Repipe or resize the gas supply

If the gas supply is too small, the boiler won’t receive the correct gas flow. A Gas Safe engineer must upgrade pipework.

Correct flue installation

A boiler flue must be installed with precise gradient and airflow requirements. Only a qualified engineer can realign or replace it.

Install missing safety components

If the system lacks a system filter, inhibitor, proper venting, or correct valves, these must be retrofitted.

Reposition components

If key parts (such as the condense pipe or expansion vessel) are incorrectly placed, the engineer will reposition them to match manufacturer guidelines.

Is Poor Water Quality a Cause of Boiler Commissioning Issues?

Poor water quality is a major cause of boiler commissioning problems and long-term boiler damage.

What causes poor water quality?

Poor water quality occurs when the heating system contains sludge, scale, corrosion, or debris. This happens because:

• The system wasn’t flushed before installation 
• No inhibitor was added 
• Radiators and pipework are old and corroding 
• Hard water has caused limescale buildup
  

How to fix poor water quality

Powerflush or chemical flush

A deep clean removes sludge, rust, and scale from the system.

Add inhibitor

Inhibitor protects the system from future corrosion and sludge buildup.

Install a magnetic filter

This traps metal debris and prevents new sludge forming.

Fit a scale reducer

In hard-water regions, a scale reducer prevents future limescale damage to heat exchangers.

Is Incorrect System Balancing a Cause of Boiler Commissioning Problems?

Incorrect system balancing affects boiler performance and commissioning results.

What causes incorrect balancing?

Incorrect balancing happens when radiators receive uneven water flow. This can be due to:

• Misadjusted lockshield valves 
• Poor pipework layout 
• Air trapped in the radiators 
• Designer radiators restricting flow
  

How to fix incorrect system balancing

Adjust lockshield valves

An engineer will set each radiator to achieve optimal water distribution.

Bleed radiators

Trapped air reduces flow and heat transfer.

Correct pipework layout

If the system was plumbed poorly, sections may need repiping.

Increase pump speed

Sometimes the circulating pump needs reconfiguring or replacing.

Is Low or Incorrect Gas Pressure a Cause of Boiler Commissioning Issues?

Gas pressure is critical for safe and efficient boiler operation.

What causes incorrect gas pressure?

Common causes include:

• Undersized gas pipework 
• Gas meter faults 
• Blockages or restrictions in the gas line 
• External gas supply issues (e.g., low pressure in the street)
  

How to fix incorrect gas pressure

Resize or replace gas pipework

A Gas Safe engineer may upgrade from 15mm to 22mm or larger.

Check and repair the meter

Defective meters can supply inconsistent pressure.

Clear obstructions

Engineers may remove kinks or replace faulty regulators.

Notify the gas supplier

If external pressure is low, the gas network must correct it.

Is Skipped Steps During Commissioning a Cause of Boiler Issues?

Skipping commissioning steps is surprisingly common among unqualified or rushed installers.

What causes skipped commissioning steps?

• The installer not being Gas Safe registered 
• Lack of training 
• Rushing to finish the job 
• Failure to follow the manufacturer’s benchmark checklist
  

How to fix skipped commissioning steps

Re-commission the boiler

A Gas Safe engineer will redo the entire commissioning process, including:

• Burner pressure checks 
• Combustion analysis 
• Flue integrity test 
• System pressure checks 
• Temperature differential tests
  

Update the Benchmark certificate

This document is essential for warranty validity.

How to Prevent Boiler Commissioning Problems in the Future

Preventing commissioning issues involves a combination of proper installation, good water treatment, and routine maintenance:

• Always use a Gas Safe registered installer 
• Ensure the engineer completes the Benchmark commissioning checklist 
• Request a system flush during installation 
• Fit a magnetic filter and maintain it annually 
• Add corrosion inhibitor every 12 months 
• Use WRAS-approved components 
• Follow annual servicing schedules
  

FAQs 

How long does boiler commissioning usually take?

Boiler commissioning usually takes between 45 minutes and 2 hours, depending on system size and water quality. Larger systems or unflushed pipework take longer.

Is boiler commissioning included in installation costs?

Yes, boiler commissioning should always be included. If an installer tries to charge extra for commissioning, consider this a red flag.

Does boiler commissioning affect energy efficiency?

Absolutely. A properly commissioned boiler runs cleaner, uses less gas, and heats your home more efficiently.

Can I check the Benchmark certificate myself?

Yes. Homeowners and landlords can request to see the Benchmark commissioning checklist and ensure it is filled out correctly.

Does recommissioning a boiler void the warranty?

No—recommissioning does not void a warranty. In fact, it often restores warranty validity if the original installer failed to complete commissioning properly.

Stay warm, The Boilers2Go Team