Servicing Your Boiler

What is a boiler service?

A boiler service is an inspection and testing of your boiler performed by a Gas Safe-registered engineer to ensure the system is running safely and efficiently. 

During servicing, the engineer will also carry out basic maintenance, including the cleaning of limescale and soot from internal components, which can increase the efficiency of the boiler and extend its life. A servicing can also identify small faults before they escalate into major problems that knock out your heating and require expensive repairs.

While this article primarily discusses gas boilers, used for heating by around 86% of UK households, hundreds of thousands of us, especially those living in rural areas off the gas grid, have other types of boilers: electric, oil, LPG, and biomass.^[1] All types of boilers could benefit from annual servicing, although the steps undertaken in the service, the costs, and the concerns raised could be different.

What is involved in a boiler service?

During the servicing, the Gas Safe engineer will usually perform the following steps, not necessarily in this order:

• Operate the boiler briefly

  The engineer will turn the boiler on to ensure it’s working properly.

• Visual check of the boiler and pilot light

  The engineer will visually inspect the boiler, including the flame (also called the pilot light). The flame should be steady and blue, not yellow, orange, or flickering, which indicates incomplete combustion (dangerous for the release of carbon monoxide) or inadequate air supply.^[2]

• Internal components inspection and cleaning

  Once the boiler is switched off and electrically isolated, the engineer will remove the casing to inspect internal components, including the burner, heat exchanger, main injector, igniter, flame sensor, condensate trap, fans, and seals for leaks, soot buildup, corrosion, and other faults. The engineer will clean off soot and limescale from the heat exchanger, clean and possibly replace the ignition system, clean filters, and flush the condensate trap.

• Gas pressure check

  Gas supply pressure refers to the pressure of gas entering the boiler’s main valve. It’s usually around 21 millibars, but the ideal pressure for your boiler will be specified in the manual, which the engineer should have to hand.^[3] The engineer tests the pressure with a manometer or pressure gauge at the gas inlet or test point and can adjust it if necessary.

• Water pressure check

  Boilers have an ideal pressure for the water flowing through them and the wider heating system: too low and the boiler might not turn on and radiators won’t get warm; too high and the stress on the pipes and seals is too great. The pressure gauge on the boiler indicates the system pressure, which the engineer compares to the recommended pressure in the manual and can adjust by using the filling loop to add water to the system - a manoeuvre you may have done yourself when your boiler has lost pressure.

• Check of the electrics

  The engineer will check if the boiler’s fuse rating is correct, according to the manual. They will examine wiring for signs of damage, overheating, or loose connections; ensure the boiler is properly earthed to reduce the risk of electric shocks; and test room thermostats.

• Flue inspection and gas analysis

  The flue is the pipe that carries the gas byproducts of combustion outside of the home. The engineer will inspect the entire length of the flue, both inside and outside the home, for leaks, cracks, or blockages, which can form due to freezing weather. They’ll also perform a flue gas analysis, measuring its output of oxygen, carbon dioxide, and carbon monoxide, along with the temperature, to detect hazardous leaks and ensure the boiler is burning fuel correctly and meets environmental standards.

• Check of boiler safety devices

  Boilers are built with safety backstops to prevent accidents and damage to the boiler. The engineer will check these are all in good condition, including the flame sensor (checks whether the burner flame is lit and shuts off the gas supply if not), overheat thermostat (cuts out if the water temperature gets too high), pressure relief valve (opens if the water pressure is too high), and low-pressure sensor (stops boiler operation if the water pressure is too low).

• Case seal check

  After the casing is replaced, the engineer checks that it fits well, doesn’t have cracks, and keeps the combustion chamber airtight, without leaks.

• Gas leak check

  The engineer will use a gas leak detector to ensure no gas is escaping from any part of the system.

• Service report

  The engineer will produce a service report detailing all the checks performed on the boiler and any faults identified. This will alert the homeowner if they need to make any repairs and can be handed to the engineer at the next annual service to ensure they know about any pre-existing issues.

It’s recommended that you service your boiler annually, ideally in the summer or autumn, in preparation for winter and its higher heating demand.^[4]

With annual boiler servicing, you’re maximising your boiler’s:

Additionally, landlords are legally required to arrange an annual service of boilers in properties they rent out, along with a gas safety check for all gas appliances on the property.^[5]

Common boiler problems

Boilers are an essential part of our homes, usually humming away in our cupboards, quietly keeping our bathwater hot and our rooms temperate. But if something goes awry, a boiler can be, at best, a financial headache and, at worst, a source of major household disruption and danger.

A regular boiler service can identify and address common boiler faults and perform basic maintenance that prevents other problems from arising.

Here are a few common boiler ailments, what they might indicate, and how they can be fixed, either by simple intervention by you or by contacting a Gas Safe engineer:

• My boiler has lost pressure

  When water pressure drops in the system below the ideal 1.5 bars, the boiler will turn itself off. Modern boilers will usually flash an error message indicating low pressure, but on older systems, you can simply check the pressure gauge on the front of the boiler for anything below 1 bar. Water pressure may drop if you’ve recently bled your radiators or as a result of small leaks in the radiators, pipes, or boiler itself, or a failed seal or valve. You can restore pressure yourself by using a filling loop to let mains water into the system. You can also check for water leaks. But if the pressure drops again quickly, it’s a sign that something more serious is wrong, and you should contact an engineer.

• The pilot light has gone out

  Sometimes the pilot light (or flame) in older boilers goes out for benign reasons, such as a draught or buildup of dirt or soot on the gas nozzle. Your boiler won’t fire without it, but you can probably reignite it, following instructions in your owner’s manual. However, if the pilot light goes out again, if it burns any colour other than blue, or if it flickers, it’s a sign something more serious is afoot, such as incomplete combustion that could cause the release of carbon monoxide or problems with the gas supply or airflow. You should turn the boiler off and contact a registered Gas Safe engineer.

• My boiler is making weird noises

  If you hear whistling, banging, humming, or gurgling from your boiler cupboard, it’s a sign that something is wrong. For example, a high-pitched whistle, also known as kettling, could indicate there’s limescale buildup on the heat exchanger or in other pipes, impeding water flow. Gurgling or bubbling may mean there’s air trapped in the system or the water pressure is low, so you should first try to bleed your radiators and top up the pressure. If that doesn’t work and the weather is cold, your condensate pipe could be frozen. Vibrating could mean a component is loose or there’s a fault with a pump or fan.

• My boiler switches off

  If your boiler fires up and then turns off after a short period, you may have a problem with low water pressure, overheating, or a faulty thermostat or pump. If it's happening repeatedly, contact an engineer.

Some boiler faults are emergencies. If any of these happen, you should switch off the boiler, open windows and doors for ventilation, vacate the property, and call the National Gas Emergency Service on 0800 111 999:

• !

  Smell of natural gas

  Natural gas is normally odourless, but energy suppliers add a sulphur-like smell so you can quickly detect leaks.^[6]

• !

  Signs of a carbon monoxide (CO) leak

  If anyone in your household develops symptoms such as headaches, nausea, dizziness, and shortness of breath, it could be CO poisoning. Many households also have CO detectors to detect leaks.

• !

  Scorch marks or soot

  This could indicate incomplete combustion, a blocked flue, or overheating, all of which can have serious consequences, including CO leaks and fires.

Other boiler symptoms aren’t emergencies but should be addressed by a professional:

• You don’t have heat or hot water

• Repeated loss of pressure

• Persistent strange noises

• Pilot light burning yellow, orange, or flickering

• Boiler keeps turning off

• Leaking from the boiler or pipes

How much does a boiler service cost?

An annual boiler service costs between £70 and £150, depending on the type and age of your boiler and your location.

Gas combi boilers are the most standard and simple boilers, with all components easily accessible, making them the cheapest to service. £70 to £100 is typical. Servicing fees rise slightly for gas regular (heat-only) and system boilers, as they have additional components to be inspected, including more pipework, hot-water cylinders, and cold water tanks, some of which may be less accessible, for example, in the loft.

Electric boilers have fewer moving parts than fossil fuel boilers, which can potentially reduce the cost of servicing them to between £60 and £100. However, you’ll need to find a heating engineer or electrician to do so, as Gas Safe engineers aren’t qualified to deal with electrical systems.

Oil boilers cost even more to service, potentially up to £180. This is because oil combustion is messier than gas combustion, producing soot and tar that build up on boiler components and must be cleaned as part of the service. Oil boilers also have parts, namely oil nozzles, filters, and lines, that need to be replaced as regularly as yearly. They must be serviced by an OFTEC-registered engineer. Scarcity of qualified engineers may further push up prices in some regions.

Biomass boilers, which burn wood pellets or chips, are the most expensive to service, with a booking costing between £150 and £450. The high price is partly because the engineer must clean ash, soot, and tar from many components of the boiler, from its burner to the flue to the sensors and fans. There are also more mechanical parts to inspect, test, and clean, including fuel feed systems, automatic igniters, fans, motors, and dampers. The entire process can take four to eight hours. Because biomass boilers are still a niche product in the UK, if repairs or replacements are required, the engineer may have to source parts from Europe. Finally, qualified biomass engineers are also in short supply, so they charge higher rates. 

As with many things, those living in London and other high-cost-of-living areas will pay a premium for boiler servicing.

Some boiler cover policies include annual servicing as one of their benefits. Boiler cover is an insurance policy protecting your boiler and sometimes wider heating system, covering repairs and occasional replacement should they break down. Boiler cover is available as a standalone policy, with premiums starting at £4/month. However, you’ll pay more for policies with complimentary annual servicing, usually at least £10/month.

You can also obtain boiler cover as part of home emergency cover or as an add-on to a home buildings insurance policy. Typically, you’ll need to have annual servicing performed to keep coverage of your boiler under these policies.

How long does a boiler service take?

A service for a gas combi boiler usually takes between 40 and 60 minutes. 

The amount of time it will take an engineer to service your boiler depends on several factors:

• Complexity of the boiler: System or regular boilers take longer to inspect than combi boilers due to the presence of hot-water cylinders and cold water storage tanks.

• Condition of the boiler: A regularly serviced, fairly recent boiler in good working order will take less time to service than an older boiler that hasn’t been inspected or cleaned in years.

• Cleaning required: If the engineer has to remove lots of soot and limescale from the boiler, the service will take longer and the price will increase.

• Faults: If the engineer identifies any problems that need to be fixed, the process could take longer.

Benefits of annual boiler services

• Protect your family

  Annual servicing can reduce the risk of dangerous boiler faults such as gas leaks and fires.

• Lower your energy bills

  During a boiler service, the engineer makes sure the boiler is at the correct settings and cleans away soot and scale that makes the boiler less efficient. For example, British Water estimates that limescale buildup in boilers in hard water areas can reduce efficiency by as much as 12%.^[7] A healthy, clean boiler operating at peak efficiency will cut your energy bills.

• Avoid expensive repairs

  Servicing can catch small issues before they escalate into bank-breaking repairs. For example, an engineer may detect a small leak from a pressure relief valve. Replacing the valve could cost £100 to £150, but if it’s not fixed, it could damage the pump or heat exchanger, requiring hundreds of pounds of repairs and possibly the replacement of the boiler.

• Extend the lifespan of your boiler

  A clean boiler operating at maximum efficiency undergoes less wear and tear, extending its lifespan. While a neglected boiler may last 10 to 15 years, one that is annually serviced could last up to 20 years.^[8]

• Keep your warranty valid

  Manufacturers offer warranties on boilers that last between two and ten years.^[9] During that period, they’ll cover any repairs that might arise from a manufacturing problem. However, these warranties are contingent on you maintaining the boiler well through annual servicing.

• Keep your insurance valid

  If you have boiler breakdown cover, either as a standalone policy or an add-on to your home insurance, you’ll need to have proof of annual servicing to claim on the policy.^[10]

• Peace of mind

  Getting a clean bill of health for your boiler in the autumn means you don’t have to worry as much about it failing in the depths of winter.