No Hot Water? Work Through This Before You Call
Five checks that fix a surprising share of no-hot-water mornings for free.
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.
What the fault code is really telling you, what's safe to try, and when to stop.
Valve, spindle, tail or the panel itself, each one leaks differently, and only one of them means a new radiator.
A sealed system does not consume water. If you are topping up, it is going somewhere, and where matters.
Immersions, multipoints and wall heaters fail in predictable ways. What to check before replacing anything.
Sediment makes cylinders rumble, slows heating and kills elements. Flushing it is routine maintenance.