No Hot Water But Heating Works? Diagnose It
Losing hot water while the radiators still heat is genuinely good diagnostic news: it rules out most whole-boiler failures and points at the specific components handling the hot water side. On a combi that usually means the diverter valve or a scaled plate heat exchanger; on a cylinder system it means motorised valves, the cylinder thermostat or the timer. Here's how to tell which without touching anything you shouldn't.
First, know which system you have
Combi: no hot water cylinder, no loft tanks, hot water arrives instantly when a tap opens, boiler fires briefly whenever you run hot water. System or heat-only with cylinder: a large insulated cylinder in an airing cupboard or loft, hot water stored and finite, usually a timer or programmer with a separate hot water schedule.
The distinction matters because the fault lists barely overlap. Combi hot water is made on demand inside the boiler, so its failures are boiler components. Cylinder hot water is heated through a coil and controlled by valves and stats outside the boiler, so its failures are usually system components — often cheaper and easier to reach.
Combi systems: the usual suspects
The diverter valve directs the boiler's heat between the heating circuit and the hot water plate exchanger. When it sticks in the heating position, radiators stay hot and taps run cold — the textbook version of this problem, and by far the most common cause on combis. Symptoms include lukewarm water that arrives only when the heating is also running, or none at all.
Second suspect: the plate heat exchanger, which transfers heat to your tap water and, in hard-water areas, gradually scales up until it can't. Symptoms skew toward hot water that's weaker and slower rather than absent, worsening over months. Third: the flow sensor or switch that tells the boiler a tap has opened — if the boiler doesn't fire at all when you run hot water, that's the suspect. All three are behind the case and Gas Safe engineer work.
Cylinder systems: the usual suspects
Start with the free checks: is the programmer actually scheduling hot water (separate from heating on most controllers), and has an immersion switch or holiday mode been left off? Then the cylinder thermostat, a small dial strapped to the cylinder — if it's failed or been turned down, the boiler is never asked to heat the water. These are outside the boiler and often the whole story.
Next: the motorised zone valve controlling flow to the cylinder coil. When its motor or microswitch fails, the heating circuit still works and the cylinder never charges. Engineers diagnose it in minutes and the part is modest. Also worth mentioning: a cylinder heating slowly rather than not at all can mean a scaled or failing coil, which is the bigger conversation.
What you can safely check
Run through this before booking: hot water schedule on and set to a sensible time; every hot tap tried (a single failed tap is a tap problem, not a system one); cylinder thermostat present and set around 60°C; no holiday or summer mode left engaged; boiler pressure in the green band; and whether the boiler audibly fires when you open a hot tap (combis) or when hot water is scheduled (cylinders).
Report those findings when you book — particularly whether the boiler fires — and the engineer arrives with the right shortlist. What not to do: open the boiler case, adjust anything gas-related, or run an immersion heater indefinitely as a workaround without checking your electricity costs; immersions are useful stopgaps and expensive habits.
| System | Most likely | Also possible |
|---|---|---|
| Combi | Stuck diverter valve | Scaled plate exchanger, flow sensor |
| Cylinder + system boiler | Motorised valve failure | Cylinder stat, programmer settings |
| Cylinder + heat-only | Cylinder stat or valve | Pump, programmer, coil scaling |
| Any | Controls/schedule error | Boiler pressure lockout |
What the engineer will actually do
Expect a competent visit to run: confirm the reported symptoms, check the system-side basics (pressure, circulation, controls), read the boiler's fault history where the model stores it, name the cause and the price before repairing, then test properly under load. If parts aren't on the van, a straight firm says so and prices the return visit up front. What you shouldn't see: diagnosis by replacement, or a price that materialises after the work.
Frequently asked questions
Why does my heating work but not the hot water?
Because the hot water side has its own components. On combis it's usually the diverter valve or plate heat exchanger; on cylinder systems, a motorised valve, cylinder thermostat or schedule setting.
Is a stuck diverter valve expensive?
Typically £180–£300 all-in including parts and labour — a routine repair on combis and a common one, especially after summers of light hot-water-only use.
Can I use an immersion heater meanwhile?
If your cylinder has one, yes — it's a useful stopgap, though electric heating of a full cylinder isn't cheap. Combis have no immersion option, which is why combi hot water faults feel more urgent.
Is this an emergency?
Usually urgent rather than emergency — you have heating, which matters most in cold weather. With a baby, elderly or medically vulnerable residents in the house, say so when booking and it moves up the rota.
Fixing causes, not symptoms
The theme running through our guides holds here too. A repair that addresses why the fault happened — the pressure that keeps dropping, the sludge that keeps returning, the condensate pipe that freezes every January — is the one you don't repeat next winter. When you book through our network, say you want the cause addressed rather than the symptom silenced; it occasionally costs a little more on the day and reliably costs less across the years you'll own the system.
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