Quick answer
Heating working while hot water fails usually points to something specific to the hot water side rather than the whole boiler: a diverter valve that is not switching on a combi, a failed hot water thermistor, or a cylinder-related fault such as a stuck zone valve or a failed immersion on a stored system. Check your controls, the boiler display and the hot water setting first — the rest is engineer work.
This is a good fault to have, oddly enough. If the radiators are getting hot, the boiler is firing, the gas supply is fine and the pump is running — so the problem is narrowed down before an engineer even arrives.
Safe checks you can make
- 1Check the hot water is actually called for — on a system with a programmer, heating and hot water are separate channels.
- 2Check the hot water temperature dial on the boiler has not been turned down.
- 3Look at the boiler display for a fault code and note it down.
- 4Run a hot tap for a full minute — combis need flow before they fire for hot water.
- 5If you have a cylinder, feel it: warm cylinder but cold taps is a different fault from a stone-cold cylinder.
The usual causes on a combi
- Diverter valve stuck in the heating position — the classic cause of this exact symptom
- Failed hot water thermistor, so the boiler does not know to fire for hot water
- Blocked or scaled plate heat exchanger, common in hard water areas
- Low flow through the tap because of a partially blocked filter or isolation valve
The usual causes on a cylinder system
- Hot water zone valve seized or its motor failed
- Cylinder thermostat failed
- Programmer set to heating only
- On an indirect cylinder, a failed coil — less common, but it happens
Hot water gone but heating fine?
It's usually a single component rather than a whole boiler. We'll diagnose it and tell you the cost before we start.
Book a boiler repairWhen to call a professional
Safe homeowner checks stop at pressure gauges, radiator valves, controls and visible pipework. Anything involving gas, combustion, the boiler casing, the flue or internal components is work for a Gas Safe registered engineer. If you smell gas, follow the appropriate emergency gas safety guidance and do not attempt to work on the appliance.
Call 07881 963508The Betaflo recommendation
What it means when the radiators are hot but the taps are cold, and which of the likely causes you can check yourself. If you'd like a second opinion before spending money, speak to us first — we'd rather tell you your boiler is fine than sell you something you don't need.
Frequently asked questions
Is this an emergency?
It's not a gas safety emergency, but it is a same-week job for most households. If you smell gas, treat that as an emergency and follow gas safety guidance.
Could it just be the tap?
Worth checking one other hot tap. If only one outlet is cold, the fault is in that tap or its supply rather than the boiler.
Will a repair be worth it?
On a boiler under roughly ten years old with no repair history, usually yes. We'll give you both figures so you can compare.
About the author
Betaflo Heating UK
Betaflo Heating UK is founded and run by Miles Davis and Sam Boyle, Gas Safe registered heating professionals installing, servicing and repairing boilers across Tamworth, Birmingham and the surrounding areas. Our guides are written from the work we do in customers' homes every week. More about Betaflo.
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