No Hot Water? Work Through This Before You Call

Check the obvious first

Timer clock wrong after a power cut. Thermostat nudged down. Prepay meter out of credit. A tripped fuse on the boiler spur. These sound insulting and account for a real share of winter callouts, check them before you spend money.

Combi vs cylinder

A combi with heating but no hot water usually has a failed diverter valve, an engineer job, but a modest one. A cylinder system that's lukewarm may just have a failed immersion or a thermostat set low; scalding then cold suggests the cylinder stat.

Airlocks

If one hot tap spits and dies after work on the system, an airlock is likely. Running the mains cold tap through a hose into the affected hot tap for 30 seconds often shifts it. If all hot taps are dead, the problem is upstream, stop there and call.

Combi, system or heat-only: know which you have

A combi heats on demand and has no cylinder. A system boiler feeds a pressurised cylinder. A heat-only boiler works with a cylinder and usually a loft tank. The diagnosis differs completely between them, and it is the first thing anyone on the phone will ask, so it is worth knowing before you ring.

Heating works but no hot water

On a combi that is nearly always the diverter valve, the mechanism that switches between heating and hot water has stuck on heating. It is a common and moderate repair. On a cylinder system the same symptom points to the cylinder coil, the motorised valve or the cylinder thermostat instead.

Hot water that runs out too fast

On a cylinder, that is usually a failed lower immersion element, a corroded coil, or a thermostat set too low, you are heating only the top of the tank. On a combi, reduced flow through a scaled heat exchanger produces the same feeling. In hard-water London the second is common and worth checking before anyone replaces a cylinder.

Which system do you actually have?

Every recommendation below depends on this. No loft tank and hot water on demand means a combi. A hot water cylinder in a cupboard plus a cold tank in the loft means a gravity-fed system. A cylinder with no loft tank means unvented, running at mains pressure. Combis fail differently from stored systems: a combi gives you heating but no hot water when the diverter valve sticks, while a stored system gives you neither if the motorised valve or cylinder thermostat has failed.

The five checks, in order

One: the pressure gauge, under 1 bar and the boiler has locked out, so repressurise to 1.0-1.5. Two: the programmer and thermostat, because a dead battery in a wireless room stat presents exactly like a boiler fault. Three: in winter, the white condensate pipe outside, which freezes and locks the boiler out; warm water along it usually brings it straight back. Four: whether it is genuinely every outlet or just one, because a single tap is a tap problem. Five: on a cylinder system, whether the immersion heater still produces hot water, which separates a boiler fault from a cylinder fault immediately.

Lukewarm rather than cold

Water that gets warm but never hot is a heat transfer problem, not a heating problem. In London, at 280 to 320 ppm off the chalk, that is nearly always scale, on the combi plate heat exchanger or on the cylinder coil. The water flows past the surface but cannot pick up heat through the mineral layer. In Greater Manchester, where the supply is soft at 40 to 80 mg/l, the same symptom points at sludge from corrosion coating the same surfaces. Same complaint, opposite cause, different fix, which is why a descale sold in Manchester is usually the wrong product.

When to stop and call a Gas Safe engineer

You can safely check pressure, controls, the condensate pipe and the immersion. Beyond that you are into the appliance itself, and gas work without Gas Safe registration is a criminal offence rather than a bad idea. Call immediately, rather than working through anything, if you smell gas, ring the National Gas Emergency Service on 0800 111 999 first, or if you see staining, soot or scorching around the boiler case, which can indicate incomplete combustion and carbon monoxide.

No Hot Water? Work Through This Before You Call, your questions