Commissioning is the process that proves a new or substantially altered commercial heating system is safe, efficient and performs as designed before it is handed over. It is far more than switching the boilers on. Done properly, it gives you a documented baseline for the life of the plant, and the evidence you need if something later goes wrong.
This guide follows the handover process from the client's side: what to ask for, who should do the work, and what evidence to receive. Before you start, have the design specification, the installation documents and a Gas Safe registered engineer appointed for all gas and boiler work. Nothing here is a task for the building owner or in-house staff to carry out themselves.
Commissioning must be carried out by competent, qualified people. All gas, boiler and flue work must be done by a Gas Safe registered engineer, and you can check their registration, including the commercial gas categories they are qualified for, on the Gas Safe Register. Ask to see ID cards and confirm the entries yourself before work begins.
Next, set out the roles in writing. Four parties usually matter:
If you are unsure which party you are, take advice early. Duty holder responsibilities do not disappear because a contractor is on site.
Mistake to avoid: assuming that installation completion equals commissioning. A system can be fully plumbed, wired and apparently running without anyone having measured or recorded how it performs. Before work starts, ask for a written commissioning plan and programme, so time for testing, balancing and witnessing is built into the project rather than squeezed out when the completion date approaches.
Commissioning is measured against something, so you need that something in hand. Collect these documents before the commissioning engineer arrives:
Then check that what has been installed matches what was specified. Walk the plant room with the schedule in your hand and compare boiler models, pumps, valves, flue components, controls and sensors. Substitutions happen on site for good reasons, such as availability, but each one should be agreed and documented, because a different pump or flue component can change performance and manufacturer approval.
Also ask the installer which Building Regulations requirements apply to the project. For non-domestic buildings this can include Approved Document L, and some work may need to be notified to building control or carried out under a competent person scheme. Requirements change and depend on the scope of the work, so confirm the current position with the installer or your local building control team rather than assuming. Whatever route applies, ask for the resulting certificates to be included in the handover pack.
Gaps at this stage are easier to fix than they will be once the system is live and the building is occupied.
Before any boiler is fired, the water side of the system needs to be inspected, tested and cleaned.
Start with a visual check of installation quality: pipework supports, insulation, labelling, isolation valves, and whether there is safe, practical access to everything that will need maintenance. Poor access is a long-term cost, so raise it now. The installer should then pressure test the pipework and provide a record of the test.
New systems contain flux, swarf, jointing residues and general debris. These must be flushed out before the boilers see the water, or they will circulate into heat exchangers, pumps and valves. Ask which cleaning and treatment standard the contractor is following. BS 7593 covers the treatment of water in heating systems, but check the current edition and any manufacturer water-quality requirements, as these can be stricter than general guidance.
After cleaning, inhibitor should be added to the manufacturer's requirements. Ask for the following to be recorded:
Mistake to avoid: skipping a thorough flush where new plant is connected to old pipework. Sludge and scale in existing pipes can damage new boilers and pumps within a short time, and may invalidate warranties. Where the existing pipework is retained, ask how it was cleaned and how the contractor proved the water was clean afterwards. Some designs also call for separation, such as a filter or plate heat exchanger, to protect new equipment.
This step is the safety core of commissioning, and it is entirely the work of a Gas Safe registered engineer. The engineer carries out a gas tightness test and purge of the supply pipework, following the relevant gas industry procedures, which are published by IGEM. Ask for the test results to be recorded and for the engineer to confirm which procedure was followed.
Before first ignition, the engineer should also verify:
Ask to witness the emergency shut-off test. It is quick, and it is exactly the kind of device you will want to know works when it matters.
Do not attempt any of this yourself, and do not ask in-house maintenance staff to adjust anything on the gas or flue side. Gas Safe registration is a legal requirement for this work, and the duty holder is responsible for making sure it is observed.
If you suspect a gas leak at any point: leave the area immediately, do not operate electrical switches, and call the National Gas Emergency Service on 0800 111 999. Do this before anything else, and keep others out of the building until you have been told it is safe.
Only once the gas, ventilation, flue and safety checks are complete should the engineer start each boiler, following the manufacturer's commissioning procedure. Combustion is then measured with a calibrated flue gas analyser. Combustion analysis means sampling the flue gases to establish how completely the fuel is burning. The engineer should record carbon monoxide (CO), carbon dioxide (CO2) and oxygen (O2) readings, along with the calculated combustion efficiency, and compare each against the manufacturer's stated limits.
The engineer should also verify gas inlet pressure and burner settings. Any adjustment to these is made only by the engineer, using the manufacturer's data. Facility staff should not touch burner or gas valve settings, however minor a tweak appears.
For multi-boiler plant, usually called a cascade, each unit should be checked individually first and then in sequence, so you know every boiler performs on its own and that the sequencing logic brings them on and off as designed. Ask the engineer to demonstrate firing order and rotation of the lead boiler.
Mistake to avoid: accepting a verbal "all fine". A statement that the boilers are running well is not evidence. You should receive a printed or written record for each boiler showing the readings, the limits they were compared against, the analyser used and the engineer's details. Those figures are also the baseline your maintenance engineer will compare against at every future service, so incomplete records make later fault-finding harder.
A boiler that burns perfectly can still sit inside a poorly performing system. Hydraulic balancing sets the flow through each circuit, zone and terminal unit to the design values, so every area receives the heat it needs without starving or overheating others. The commissioning engineer should set pump speeds and then verify flow and return temperatures and the temperature differential against the design. Ask for the measured values alongside the design values.
Then test the controls, which are where many handover disappointments originate. Check that:
Failure scenarios deserve their own test. Ask the engineer to simulate a lead boiler lockout and confirm that the system changes over to the standby unit and raises an alarm. It is much better to discover a changeover fault on a planned commissioning day than during a cold January morning.
Seasonal timing matters in the North East. Commissioning in autumn or winter tests the plant against real heat demand, which shows up weaknesses a mild day will hide. If commissioning has to happen in summer, agree a plan for out-of-season retesting once cold weather arrives, and put it in the programme with a named responsible party.
A certificate alone does not prove performance, so look at the evidence behind it. The handover pack should include:
You should also receive the operation and maintenance manuals, warranty registration details and the relevant documentation from the Gas Safe registered engineer, plus any building control or competent person paperwork identified in Step 2. Check the pack against the commissioning plan from Step 1 and query anything missing before final payment or practical completion is signed off.
Your site team then needs training. At minimum, they should know how to operate the system safely, how to shut it down, where and how to isolate gas and electricity in an emergency, and who to call. Keep a note of who attended and when.
Finally, put maintenance in place before the warranty period starts to run or the first heating season begins. Manufacturers commonly require regular servicing to keep warranty cover valid, so check the terms. Our commercial boiler maintenance and annual inspection services are designed to pick up from the commissioning baseline, and the FAQ covers common questions about contracts and emergency cover.
Before you close the project, confirm that every record in the handover pack is complete, signed and filed where the next facility manager will find it, with a digital copy stored separately. Then diarise the first annual service now rather than when the warranty window is closing.
Commercial Boiler Solutions is Gas Safe registered and supports commercial properties across the North East and Northumberland, including Newcastle, Sunderland, Middlesbrough, Tyneside, Wearside, Teesside and North Yorkshire. We can help with commissioning support, planned maintenance contracts and emergency callouts. Learn more about our services