Hot Water But No Heating: What's Wrong With Your Boiler?

Why This Specific Symptom Matters

Hot water working but heating not working is a distinctive fault pattern — and a useful one, because it tells you the boiler itself is functioning. The heat exchanger is operating, the burner is firing, gas is reaching the appliance. The problem is somewhere in the system that directs hot water to the radiators rather than (or as well as) the domestic hot water circuit.

Understanding the three most common causes of this symptom can save you a call-out fee — some are fixable without an engineer at all.

Check the Timer and Programmer First

Before assuming a fault, check your heating controls. The most common cause of heating not working while hot water functions is simply a programmer or timer that has lost its settings — after a power cut, a clock change, or a battery replacement. If your programmer is set to heat hot water only, or if the heating schedule has been cleared, the boiler will correctly produce hot water and correctly not heat the radiators.

Check that the heating is set to "on" or scheduled to run at the current time. Try switching it to "continuous" or "all day" temporarily to see if the radiators respond. If they heat up when the programmer is overridden, the fault is in the timer or thermostat settings — not in the boiler or pipework.

Also check the room thermostat. If it's set below the current room temperature, the heating won't fire even when the programmer says it should. Turn it up to its maximum setting temporarily to test.

Diverter Valve Fault (Combi Boilers)

On a combi boiler, a diverter valve directs heat from the heat exchanger to either the central heating circuit or the domestic hot water circuit — never both simultaneously. Hot water is prioritised: when you open a hot tap, the diverter switches to DHW and diverts heat away from the radiators.

If the diverter valve becomes stuck in the domestic hot water position — a common failure mode, particularly in older combi boilers — you get hot water at the tap but the radiators never receive heat, regardless of what the programmer says. The valve is physically unable to switch over.

A diverter valve fault is confirmed by an engineer using a combination of system testing and visual inspection. Replacement is a standard repair — typically 1–2 hours' labour plus the cost of the valve. It's one of the most common boiler repairs carried out on Worcester Bosch, Vaillant, and Baxi combi boilers over 7 years old.

Motorised Zone Valve Fault (System Boilers)

System boilers use motorised zone valves to direct heated water to different circuits — typically one valve for the heating zone and one for the hot water cylinder. If the motorised valve serving the heating zone fails in the closed position, hot water will continue to be heated normally (its valve is open) while the central heating circuit receives no flow.

Zone valve failure is audible in some cases — a clicking or buzzing sound from the airing cupboard or wherever the valves are located when the heating is called for. An engineer can test zone valve operation electrically and mechanically; replacement is straightforward and relatively inexpensive.

Pump Failure

If the circulating pump fails, water doesn't flow through the radiator circuit — even though the boiler may fire and produce heat for the hot water cylinder. If your hot water comes from a cylinder that relies on the boiler's heat exchanger rather than the pump circuit, you'll have hot water but cold radiators. Listen near the boiler when heating is called for — a completely silent pump (when you'd normally hear a low hum) may indicate pump failure.

When to Call

If checking the timer, programmer, and room thermostat settings doesn't resolve the issue, the fault requires an engineer — whether it's a diverter valve, zone valve, or pump. These are all parts inside a sealed system and are not DIY repairs. Our central heating team diagnoses and repairs all these faults across Peterborough. Book online or call 02039514510. See also our guide to diagnosing central heating problems for a broader fault-finding framework.

Frequently Asked Questions

Could my thermostat be the reason heating isn't working?

Yes — a faulty room thermostat that reads the room temperature as higher than it actually is will prevent the heating from firing, even with the programmer correctly set. If you suspect the thermostat, replace the batteries first (a common fix), then test by turning the setpoint to its maximum. If the heating fires at maximum but not at normal settings, the thermostat needs calibrating or replacing.

How much does a diverter valve replacement cost?

A diverter valve replacement on a standard combi boiler typically costs £150–£300 including parts and labour, depending on boiler make and model. Some older boilers have diverter valves that are no longer available new, requiring a refurbished part or a boiler replacement decision. An engineer can advise on parts availability after diagnosis.

Will the heating work if I turn off the hot water?

On a combi boiler with a stuck diverter valve, turning off the DHW (if the boiler controls allow it) may release the valve back to the central heating position temporarily — giving you a clue that the diverter is the issue. This is a diagnostic test rather than a solution; the valve needs replacing.

Is this fault urgent?

Hot water but no heating is inconvenient but not usually an emergency — provided the property is not at risk of frozen pipes and there are no vulnerable occupants. In cold weather with young children or elderly residents, treat it as a priority repair. Our emergency team is available for urgent heating call-outs across Peterborough.