The complete guide to boiler problems

Six stories, one machine

Nearly every boiler complaint is one of these: pressure trouble, an error code, no hot water, a strange noise, a leak, or radiators refusing to join in. Each has its own detailed guide. This page is the map that tells you which one you are living through, how urgent it is, and who fixes it.

Start with the two-minute triage

Gather three facts before touching anything: what the display says, what the pressure gauge reads, and what still works. Heating without hot water is a different fault from hot water without heating, and both are different again from a boiler that runs but leaves rooms patchy.

Two facts settle most bookings before anyone arrives: the error code, and whether the gauge sits below 1 bar. Photograph both. An engineer who knows them turns up with the right parts on the van.

Know which machine you own

A combi heats water the moment a tap opens and has no tanks. A system boiler feeds a hot water cylinder. A regular (heat-only) boiler does the same, plus a cold water tank in the loft. The distinction matters because identical symptoms accuse different parts: no hot water points at the diverter valve on a combi, but at the motorised valve or cylinder thermostat in a tank household.

Pressure: the sagging gauge

Sealed systems want to sit at 1 to 1.5 bar when cold. Below about 1, most boilers lock out on principle. A drop after bleeding radiators is normal physics, because the escaping air was helping prop the needle up. A gauge that sags week after week with no bleeding means water is leaving through a leak or an internal fault.

Topping up through the filling loop is a five-minute homeowner job, walked through step by step in boiler losing pressure. The habit that matters is noting the date each time you do it. Frequency is the diagnosis.

Error codes and lockouts

An error code is the boiler naming its own fault in the only vocabulary it has. Brands use different codes, but the families are universal: low pressure, failed ignition, blocked condensate, fan or flue trouble, misbehaving sensors. Boiler error codes, decoded translates each family and explains reset etiquette, which is one press, not ten.

One winter special earns a separate mention. A boiler that gurgles and locks out on the first freezing morning of the year almost certainly has a frozen condensate pipe, and the fix is a jug of warm tap water rather than a phone call.

No hot water, no heating, or neither

On a combi the two jobs share one machine yet fail separately. Heating fine while the taps run cold points at the diverter valve; no hot water from your combi works through the whole sequence, including the checks that save a wasted call-out. In a cylinder household the boiler is often innocent, and hot water cylinder problems covers the coils, immersion elements and motorised valves that actually carry the blame.

If your warmth arrives through the floor rather than radiators, underfloor heating fails in its own particular ways and has its own guide.

Noises, translated

Boilers narrate their own decline, and the vocabulary is short enough to learn in a minute.

Kettling deserves particular respect. That kettle-like rumble means the heat exchanger is boiling water in pockets, usually beneath a blanket of limescale or magnetite sludge. It wastes gas, stresses the metal and never improves on its own.

Cold radiators

Cold at the top means trapped air: ten minutes with a key, covered in how to bleed a radiator. Cold at the bottom is the sludge pattern, and when several radiators show it at once someone will soon offer you a power flush. Whether you actually need one has an honest answer, and it is only sometimes yes.

Leaks and drips

Water from under the boiler is engineer territory, full stop: the casing stays shut and a Gas Safe registered engineer decides what is weeping inside. Drips at radiator valves and pipe joints are plumber work, and they quietly explain a good share of mystery pressure loss. Anything more dramatic is a plumbing emergency first and a boiler question second: stopcock off, then follow the water leak playbook.

Homeowner or engineer?

What boiler work costs

As a rough guide, boiler trouble lands in one of three price bands: a service catches faults early, a repair replaces a named part, and a replacement ends the relationship altogether.

Replacement maths is its own subject. A like-for-like combi swap typically runs £1,800 to £3,000 with median quotes near £2,300 at the time of writing, and repair bills stacking up against a boiler past its twelfth birthday are usually the market trying to tell you something.

The rhythm that prevents most of this

Almost everything above is cheaper as maintenance than as rescue. An annual service, described minute by minute in what actually happens in a boiler service, keeps the warranty valid and catches faults while they are small. Autumn is for bleeding radiators and a pressure check; late autumn for lagging the condensate pipe. And the controls decide what the whole arrangement costs to run: thermostat settings that save money is the place to start.

Keep the paper trail too. Service records prove the warranty is intact, and a photographed history of error codes turns a vague complaint into a five-minute diagnosis.