What does a boiler service include?

A boiler service is a set of checks laid down in your boiler maker’s service instructions. The exact order changes from model to model, but every full service covers the same ground. Knowing what should happen helps you tell a real service from a quick look.

Before the engineer starts

Asking questions

A good engineer starts by asking how the boiler has been behaving. Has the pressure dropped? Does the hot water swing hot and cold? Any noises, leaks or fault codes? Your answers point them to likely problems.

Visual checks

They look at the boiler and its surroundings. Is it in a suitable place with the clearances the maker asks for? Are there signs of overheating, scorching or soot? Is the pipework sound? Is there water dripping anywhere? Are any air vents the boiler needs left clear?

Gas supply

The engineer checks the gas working pressure at the boiler with a gauge, with the boiler running. Low pressure can cause poor burning and lockouts. They may also check the meter and that the gas supply is tight with no leaks.

Inside the boiler

Removing the case

The front case comes off so the sealed part of the boiler, the combustion chamber, can be inspected. This is gas work and must be done by a Gas Safe registered engineer.

Burner and heat exchanger

The burner is where gas and air mix and burn. The heat exchanger passes that heat into the water. Both can collect deposits. The engineer inspects them and cleans as needed, following the maker’s instructions. On some models the burner comes out, and the maker asks for a new seal or gasket when it goes back.

Seals and gaskets

Seals around the combustion chamber keep fumes inside the sealed part of the boiler. Damaged or perished seals are replaced.

Electrodes

The ignition electrode makes the spark that lights the gas. The flame sensing electrode tells the boiler the flame is lit. Both wear and get dirty. Worn electrodes cause failed ignition and lockouts.

Fan

The fan pushes fumes out through the flue. The engineer checks it is running cleanly.

Condensate trap

Condensing boilers make acidic water as they work. It collects in the condensate trap and drains away. The trap can clog with sludge, and a blocked trap stops many boilers. It is cleaned out at the service.

The flue

The flue carries fumes outside. The engineer checks it where it can be seen inside the home and looks at the terminal outside. The terminal should be clear of plants, nests and new obstructions, and in a safe position relative to windows, doors and vents. Building work nearby, such as a new extension or rooflight, can change that.

In newer flats, the flue may run through a ceiling void. If it cannot be inspected, the HSE flues in voids guidance says the engineer must treat the boiler as at risk. See our post on flues in voids.

The combustion test

This is the key measurement. With the boiler running, the engineer puts a flue gas analyser probe into a test point on the flue. It reads:

• Carbon monoxide (CO)
• Carbon dioxide (CO2)
• The ratio between them

These figures show whether the boiler is burning safely and as the maker intended. The maker’s instructions give the expected ranges. If the figures are out, the engineer investigates. The readings should be written on your service record.

Water side and safety devices

Check	Why
System pressure	Too low or too high points to leaks or faults
Expansion vessel charge	A flat vessel makes pressure rise and fall
Pressure relief valve	Safety valve that lets water out if pressure gets too high
Pump	Moves water round the system
Controls and thermostat	Make sure the boiler responds properly
Hot water flow and temperature (combis)	Shows scale build up in hard water areas
Magnetic filter (if fitted)	Catches sludge before it reaches the boiler

Why hot water gets tested in London

London’s water is hard. Thames Water says all the water in its region is hard. In combis, scale builds up in the plate heat exchanger that heats tap water. Running a hot tap and checking flow and temperature shows if it is starting to struggle. Our combi boiler service page explains what the engineer is looking for.

System and regular boilers

If you have a hot water cylinder, the service also covers the cylinder thermostat and motorised valves. On an unvented cylinder, the safety valves and expansion vessel are checked to the maker’s instructions. See system and regular boiler service.

After the checks

Putting it back together

The case is refitted and sealed. The boiler is run and tested again.

The record

You should get a written service record showing:

• The date and address
• Boiler make, model and location
• The engineer’s Gas Safe licence number
• What was checked
• Combustion readings
• Any faults found and advice given

The Benchmark checklist at the back of most boiler manuals has a service section that should be filled in too. Keep both. Makers may ask to see them for a warranty claim.

Faults found

If the engineer finds something worn, they should explain it in plain words and give a price before doing any repair. If they find something dangerous, they follow the gas industry unsafe situations procedure. That can mean turning off the appliance, with your permission, until it is fixed.

A service is not a gas safety check

Landlords need a gas safety record every year. That checks each gas appliance and flue is safe. It does not include stripping and cleaning the boiler. A service does. Our post on the difference between a gas safety check and a service explains more.

Older systems in Streatham and similar areas

In older houses with open vented systems, like many in Streatham, sludge in the radiators is common. The service is a good time to clean the magnetic filter, if fitted, and check the water. Our Streatham area page covers the housing there.

Booking

When you book an annual boiler service, give the make and model, where the boiler is, and any problems. Clear the area around the boiler and the gas meter before the visit.