Combination Boilers: The Annual Service

The combination boiler is a clever device that’s often neglected until it goes wrong. It’s essentially two boilers in one — one side heats your hot water, the other heats your radiators.

When I first started in this trade, combination boilers didn’t exist. You had a boiler for the heating, and a separate boiler for the hot water — what people called a “multipoint.” Having installed many combination boilers over my career, I know some brands very well and others less so, but they all fundamentally work on the same principle.

Why September is always the busy season

These boilers should be serviced every year without fail, because they always seem to break down exactly when you need them most. In my experience, the busiest time for boiler servicing was from late September onwards, depending on the weather, once people started turning their heating back on.

The call would usually go something like: “Hi Steve, I need my boiler serviced ASAP, when can you do it?” My first question was always the same — any problems with the boiler? The way someone answered that told me immediately whether I was booking in a genuine service, or walking into a breakdown.

There are a handful of things that need checking on every combination boiler, regardless of brand — top of the range or the cheapest on the market, it doesn’t matter, they all need the same checks.

The flue pipe — a genuine safety check

First and foremost is the flue pipe — the pipe that runs outside. A lot of people don’t realise it’s also an intake pipe, drawing in the fresh air the boiler needs to burn correctly. This is one of the safety checks that could genuinely save a life.

Most boilers today are condensing, high-efficiency models, which produce a visible plume when running — it looks like smoke, but it’s actually steam turning to water. This water is mildly acidic, a bit like a tomato.

Whatever the length of the flue, it needs inspecting — especially at every joint, since that’s the weak point for leaks. Sometimes when the pipe sections are pushed together, the internal seal shifts out of its seating without the installer realising it’s broken. Even a flue integrity test carried out right after installation might not catch this — it’s often only once the boiler’s been running a while that a leak starts to show. I once had a flue that passed every test for three years, then started leaking regardless.

The inner pipe removes the products of combustion; the outer pipe is the air intake. Sometimes condensate leaks from a joint into that air intake, which is metal and corrodes over time — eventually creating a hole in the pipe. That becomes another route for carbon monoxide to enter the room.

I always told my customers to buy a carbon monoxide detector and place it in the same room as the boiler — they’re inexpensive, and genuinely worth having.

The condensate trap

The condensate system is classed as part of the flue for safety purposes. The boiler produces condensate, which is removed via a discharge pipe — first into a trap connected within the boiler, then out through a pipe running to a drain.

I’ve seen these traps split after a few years of use, letting water drip inside the boiler. Over time, that can eat a hole in the boiler casing — becoming yet another route for combustion products, including carbon monoxide, to escape.

The expansion vessel — the most overlooked check

In my opinion, this gets overlooked simply because it’s a bit more involved than the other checks.

To check it properly: switch off the boiler, isolate the power supply, and make it safe. Isolate the flow and return valves, normally located under the boiler — most brands have a small drain-down point for exactly this. You’re only draining the boiler itself, not the whole system.

Once drained and depressurised, attach a pump to the Schrader valve on the expansion vessel — some engineers use a standard car-type pump, others a battery-powered one. You’ll get a reading on the pump gauge, which you should always check against that specific boiler manufacturer’s recommendation.

Important: leave the drain-down valve open while you do this. If you close it after draining, you can get a false reading when pumping the vessel back up.

A boiler serviced properly every year shouldn’t need much of a pressure top-up, if any. But I found that boilers left unserviced for four or five years often had an expansion vessel completely empty — sometimes even full of water. If it’s full of water, that water should start discharging through the drain-down valve as you begin pumping.

Sometimes it would re-pressurise without any issue; other times it wouldn’t hold pressure at all, even after replacing the Schrader valve — which is why I always kept a box of them on the van. They’re not expensive.

The burner and heat exchanger

Some manufacturers recommend not removing the burner from the heat exchanger every single service — some say every five years, some longer, and the guidance keeps changing. It’s worth finding out exactly which brand your boiler is so you can check the manufacturer’s current recommendation. You may also need a new combustion chamber gasket if it is removed.

Why a proper service matters

A genuine combination boiler service is essential to keep it safe and running — and you’d be surprised how many people never have one done until something goes wrong.

I knew an engineer working for one particular large company (no names) whose idea of a “service” was inserting the analyser probe, taking a single reading, and calling it done. He was required to carry out 15 of these a day just to hit his targets — there was no time in that schedule for a real service.

Want to know exactly what a proper service should include?

I’ve put together a free, printable Boiler Service Checklist based on everything above — download it, keep it handy, and use it to check your own engineer’s work step by step.