What a Gas Safe engineer actually checks in a boiler service

A service is not a safety check, and the difference costs people money

These two get sold interchangeably and they are not the same job.

A gas safety check asks one question: is this appliance safe to use right now? It looks at the appliance, the flue, the pipework and the ventilation, and it produces a record saying they were safe at the time of inspection. For a landlord this is the annual legal requirement under the gas safety regulations, and the record is the thing usually called a CP12.

A service asks a different question: is this appliance in good order? That means opening it up, cleaning what gets dirty, measuring what drifts, and putting right the things that will otherwise become a breakdown in February.

The overlap is real but partial, and the gap runs in both directions. A safety check can pass an appliance that is filthy inside and six months from failing, because filthy is not the same as unsafe. A service on its own does not produce a landlord's record. Landlords who assume the service covered the certificate find out at renewal; owner-occupiers who assume the safety check was a service find out when the boiler stops.

The Health and Safety Executive's own guidance for landlords puts the maintenance side plainly: appliances should be serviced in accordance with the manufacturer's instructions, and where those are not available, annually. So for most people, "annual" is not a legal interval at all. It is the manufacturer's, and it is very often the condition the warranty is written on.

Before anything else, look at the card

Every registered engineer carries a Gas Safe ID card, and you are entitled to ask for it before work starts. The HSE describes what should be on it: a photograph, the business registration number and the engineer's personal licence number, the company name, start and expiry dates, and a security hologram.

The front tells you the registration is current. The back is the part almost nobody turns over, and it is the part that matters, because it lists the categories of gas work that engineer is actually qualified for. Registration is not one blanket permission. An engineer registered for central heating boilers is not thereby registered for your gas fire or your cooker, and if the appliance in front of them is not on the back of that card, they should not be working on it.

If anything looks wrong, or the card has expired, Gas Safe Register will confirm a registration on 0800 408 5500 or through their website. It takes a minute and it is the single highest-value minute in the whole appointment.

The test that cannot be faked

If you remember one thing, make it this. A modern condensing boiler burns a mixture the appliance sets for itself, and the only way to know whether it is still burning that mixture correctly is to measure the gases coming out. That is done with an electronic flue gas analyser: a probe goes into the flue sampling point, the boiler is run at high rate and low rate, and the instrument reads out how much carbon monoxide is present and what the ratio of carbon monoxide to carbon dioxide is.

That ratio is the number that matters, because it is a measure of combustion quality independent of how much the boiler happens to be firing. The industry action levels, published by Gas Safe Register in its technical bulletin on the subject and in the British Standard covering analyser use, are a carbon monoxide reading at or below 350 parts per million and a CO/CO2 ratio at or below 0.0040 for a condensing boiler with an air/gas ratio valve, which is nearly every boiler installed in the last fifteen years. An older boiler without that valve has a looser action level of 0.0080. Those figures apply where the manufacturer has not published their own; where the manufacturer has, theirs govern.

Two things follow from this that are worth knowing as a customer.

The first is that an engineer without an analyser cannot service a modern boiler, whatever else they do to it. The instrument is not optional equipment or a nice extra. It is the only access anyone has to the question of whether the appliance is burning safely.

The second is that the numbers exist and can be shown to you. Analysers store and print readings. If you want to know whether the test was done, ask what the ratio came out at. It is an ordinary question and any competent engineer will answer it without blinking.

One honest qualification, because it gets overstated: recording those readings became mandatory in 2014 for the commissioning of a new condensing boiler, not for every subsequent service. Whether a full analysis is required at each service is set by the manufacturer's service instructions for that appliance, and most of the major manufacturers do require it. So the test is near-universal good practice and usually a manufacturer requirement, rather than a blanket legal duty at every visit.

The parts that should come off

A service that does not involve removing the case is not a service. What comes off next varies by appliance and by what the manufacturer's instructions call for, but on a typical combi the engineer should be getting to:

• The burner and the heat exchanger. The combustion side collects a fine deposit over the years, and on a condensing boiler the underside of the heat exchanger collects the acidic condensate that comes with the territory. Both restrict the appliance and both are cleaned rather than admired.
• The condensate trap. It is a water trap like the one under your sink, it fills with the same sludge, and it is the single most common cause of a boiler locking out in cold weather. It comes off, gets emptied and flushed, and goes back with its seal intact.
• The ignition and sensing electrodes. These are the two thin probes that light the gas and then confirm it lit. They wear, their gaps drift, and a boiler that fires on the third attempt on a cold morning is very often telling you about these long before it locks out.
• The seals. Case seals, burner door seals and internal flue seals are what keep combustion products where they belong on a room-sealed appliance. They perish, and they are checked on the way back together.

The checks that are about the system, not the boiler

A good service does not stop at the appliance, because most of what shortens a boiler's life is outside it.

The flue, along its whole visible length and at the terminal. Joints, supports, and whether anything has been built, planted or nested too close to the outlet.

Ventilation, where the appliance needs it. Room-sealed boilers mostly do not, but the compartment they sit in may have requirements of its own, and kitchen fitters have a long history of enclosing boilers in cupboards that no longer meet them.

Gas supply pressure and gas rate. Whether the appliance is actually receiving what it needs, measured at the meter rather than assumed. An undersized or partly blocked supply produces intermittent faults that look like a hundred other things.

The expansion vessel charge. This is the one most often skipped and it explains an enormous number of the pressure complaints on this site. The vessel is a sealed chamber with a rubber diaphragm and a cushion of air behind it that absorbs the expansion of water as it heats. That air escapes slowly over years. When it has gone, the pressure gauge climbs alarmingly whenever the heating runs and drops back when it cools, and eventually the pressure relief valve starts passing. Checking the charge means depressurising the system and putting a gauge on a valve like a tyre valve. It takes real time, which is exactly why it gets missed, and if your gauge swings across the dial between hot and cold this is very likely your answer. How to repressurise a boiler covers what the gauge should be doing across a normal week.

The safety devices. The pressure relief valve, the overheat thermostat and the flame supervision are what stand between a fault and an incident, and their operation is confirmed rather than assumed.

System water quality, at least by eye. Black water off a radiator or a magnetic filter that comes out loaded tells you the system is corroding, and a corroding system will take the new heat exchanger with it in due course.

The paperwork, and why it is tied to your money

At the end, two things should be written down.

The first is the Benchmark checklist, the booklet that came with the boiler and usually lives near it. It has a service record section, and it is where the date, the engineer's registration number and the combustion readings are entered. This is not administration for its own sake: manufacturers make an unbroken Benchmark service history a condition of the warranty, and an otherwise valid claim on a five- or ten-year warranty can be refused because nobody filled the book in. If your boiler is in warranty, put the booklet where the engineer will see it, and check it was completed before they leave.

The second is a plain statement of what was found. Anything that failed, anything that is worn but serviceable, anything that will need doing next year. That is the part you are really paying for, because it converts an unpredictable breakdown into a planned expense.

If an appliance is found to be unsafe, that is a different document again, and the engineer must have your permission to disconnect. If it happens, the situation is genuinely serious — see the category page for the fault codes that go with it.

How long it should take, and what a bad one looks like

Time is a crude measure and it is still the most useful one available to a customer standing in the kitchen. A first service on an appliance in good condition, done properly, is generally somewhere around half an hour to an hour, and longer on anything neglected, awkwardly sited, or being seen for the first time.

The signs that you did not get one:

• The case never came off. Nothing meaningful can be cleaned or inspected through the front panel.
• No analyser appeared. Covered above, and on its own decisive.
• Nothing was written down, and the Benchmark booklet was not asked for.
• Twenty minutes, start to finish, including the conversation on the doorstep.
• "It's fine" as the entire report, with nothing about condition, wear, or what to expect next.

A service is one of the few pieces of trade work whose absence is invisible until the appliance fails, which is precisely why it is worth knowing what it consists of. The questions to ask are undramatic — what was the combustion ratio, did you check the expansion vessel, is the Benchmark filled in — and asking them changes the job you get.

What this does not cover

A service is preventive maintenance, not a repair, and it is not a diagnosis of a fault you already have. If your boiler is displaying a code right now, start with what the code means for your make: the boiler fault code pages set out what each manufacturer publishes, and which of them are safe to reset and which are not.