Annual boiler service: what is actually checked, in plain English

A boiler service should take 40 to 60 minutes. If someone is in and out in 15, you have been ripped off. If they are there for two hours doing things you did not agree to, you might also have been ripped off. Here is what a proper annual service actually involves so you know what to expect and what to question.

Why it matters

Three reasons:

1. Manufacturer warranty requires it. Most boiler warranties (Worcester 10 to 12 years, Vaillant 10 years, Ideal 7 to 10 years) require proof of annual servicing. Skip a year and the warranty can be voided. If your boiler fails out of warranty, you pay full repair price.

2. Catches problems before they become expensive. A failing diverter valve, a slowly scaling heat exchanger, a worn electrode: all spotted at service for £100 to £200, all becoming £400 to £800 repairs if left.

3. Safety. A poorly burning boiler can produce carbon monoxide. The combustion analysis done at service is the only practical way to verify safe operation.

Skipping service to save £130 and ending up with a £3,500 boiler replacement five years early is a bad trade.

What a proper service includes

A correct annual service should cover:

Visual inspection

• Check the boiler casing for damage or signs of leaks
• Inspect the flue (inside and out) for blockages, soot, or damage
• Check the condensate pipe for damage and proper flow
• Check the gas supply pipework for damage or signs of leakage
• Confirm the position of the boiler complies with current regulations (clearances, accessibility)

Combustion test

• Run the boiler and measure CO and CO₂ output with a flue gas analyser
• Compare to manufacturer specifications for that boiler model
• Adjust the gas valve if needed to bring combustion back to spec
• Issue a printout (or photograph) of the combustion readings

This is the single most important part. A boiler running outside spec wastes gas and can produce carbon monoxide. Without a flue gas analyser, an engineer cannot do a real service.

Internal inspection and clean

• Remove the front cover and inspect all visible components
• Inspect the burner for soot, debris, or wear
• Inspect the heat exchanger for visible scale or damage
• Inspect the spark electrode (and replace if heavily worn)
• Inspect the flame sensor
• Clean the condensate trap (a U-bend that catches condensate, can clog with debris)

Pressure and water checks

• Confirm boiler pressure is correct (1 to 1.5 bar cold)
• Check the expansion vessel pressure (should be slightly below boiler pressure)
• Check the pressure relief valve for any signs of leakage
• Verify the system is correctly inhibited (for newer installs with magnetic filter, check the filter contents)

Function tests

• Hot water demand test (verify the boiler responds correctly to hot tap demand)
• Heating demand test (verify the boiler responds correctly to thermostat call)
• Verify domestic hot water temperature is correct (usually 55 to 65 degrees)
• Verify central heating flow temperature

Documentation

• Service report including the combustion readings
• Recommendations for any issues found
• Service log entry in the boiler manual or a digital record
• Receipt for the customer to keep

A proper service produces paperwork. If you do not get any, ask why.

What a service does not include

Just to be clear about what you are not getting for the standard fee:

• Repairs. If the engineer finds a fault, the repair is quoted separately. Service finds problems, does not necessarily fix them.
• Power flush or chemical flush. Different job, different price.
• Boiler descale. A serious descale is a separate hours-long job, not part of a 40-minute service.
• Replacement parts beyond simple consumables. A new pump, diverter valve, PCB, or expansion vessel is a separate quote.

If a service report says "needs new pump", that is the engineer doing their job. A separate quote for the pump replacement is normal. If a service quote is suspiciously cheap (£50) and the engineer arrives and finds £400 of "necessary" repairs, you are dealing with a sales operation, not a service operation.

Realistic timing and cost

A professional boiler service takes:

• 40 to 50 minutes for a standard combi
• 50 to 70 minutes for a system or regular boiler with cylinder
• Up to 90 minutes if the boiler has not been serviced for 5+ years and needs a deeper clean

Cost in London 2026:

• Standard combi service: £100 to £150
• System or regular boiler service: £130 to £180
• Same-day combined service plus minor repair: usually quoted as a package

If you are quoted under £80 for a service, look at what is included. Many cheap services skip the combustion test, which means they are not actually a service to manufacturer specification.

What to ask before booking

Five questions worth asking your plumber before booking a service:

1. Are you Gas Safe registered, and what is your registration number? You can verify any registered engineer on the official Gas Safe Register website.

2. Do you do a flue gas analysis as part of the service, and will you provide the readings? A real service includes this. If the answer is no, find a different engineer.

3. What does your service include? A specific list (combustion test, clean, function tests) is the right answer. Vague answers about "checking everything" are a flag.

4. What does it cost, fixed price? Should be a fixed quote, not a "starting from" with mystery extras.

5. What happens if you find a problem? A proper engineer will diagnose, quote separately, and let you decide. You should not be pressured to authorise repairs on the spot.

When to skip a year

I will not pretend you need a service every year regardless of circumstances. Honest exceptions:

• The boiler is brand new (under 12 months) and was commissioned by an accredited installer with a recent combustion check
• You have moved into a property where you have been told the boiler was serviced within the last year and you have the paperwork
• You are about to replace the boiler within the next month or two

Outside those, annual service is the right thing.

Service plans vs pay-as-you-go

British Gas, Domestic and General, and similar companies sell monthly service plans for £15 to £40 a month. They include the annual service plus some breakdown cover.

Honest comparison:

• Monthly plan: £180 to £480 a year, x 10 years = £1,800 to £4,800
• Pay-as-you-go: £130 service x 10 years (£1,300) + 1 to 2 repairs over 10 years (£300 to £600) = £1,600 to £1,900

Plans cost roughly twice as much over 10 years for boiler-only cover. Worth it only if you have a very old boiler likely to need expensive repairs the plan would cover, or if you genuinely will not arrange a service yourself.

For a new boiler under manufacturer warranty, paying for a separate service plan is paying twice.

What I include in my services

For my customers in London, the standard service includes everything in the "proper service" list above:

• Full visual and physical inspection
• Combustion test with printed (or photographed) readings
• Burner clean if needed
• Condensate trap clean
• Pressure and expansion vessel check
• Function tests on hot water and heating
• Service entry in the boiler manual
• Written report of any concerns found

Cost: £120 fixed for a standard combi, £150 for a system boiler. Includes a 10 percent discount on any repair quoted on the day.

If you would like to book a service, send a WhatsApp with the boiler make and model, your postcode, and any concerns about the boiler. I can usually offer same-week appointments anywhere in London.