Combi boiler — diverter valve
The most common cause. The diverter valve switches flow between heating and hot water — when it sticks in the heating position, taps run cold. Replacement is a 1–2 hour job by a Gas Safe engineer.
Combi boiler — DHW flow sensor
A small sensor detects when a tap is opened and tells the boiler to fire for hot water. When it fails, the boiler doesn't respond to hot water demand. Usually a same-day fix.
Combi boiler — plate heat exchanger scaled up
Common in hard-water KT/TW postcodes. Limescale in the DHW plate exchanger restricts flow and heat transfer. Symptoms: hot water becomes lukewarm and flow drops over months. Descale or replace.
System boiler — cylinder immersion / motorised valve
If you have a hot water cylinder and the immersion isn't heating (or the motorised valve controlling heat from the boiler has failed), taps run cold while the boiler still services the heating circuit.
Quick checks first
Are hot taps opened fully? Is the DHW temperature dial on the boiler turned down? Is the pre-heat function (if fitted) enabled? These free fixes rule out a real fault.
Hot water gone but heating fine? Book a same-day repair across KT/TW.
Call 07932 898534Frequently asked questions
How much does a diverter valve replacement cost?
Typically £180–£350 including parts, labour and VAT, depending on boiler model and access.
Can I use my heating whilst waiting for a hot water repair?
Yes — heating continues to work normally. Boil a kettle for washing up in the meantime.
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