No hot water but heating works — the usual culprits
When the heating works but the taps run cold, the boiler is firing — it just is not sending the heat to the right place. That narrows the fault down to a handful of components.
On a combi it is very often the diverter valve. On a cylinder system it is far more likely to be a stuck motorised valve or a failed cylinder thermostat.
Try this first — free
Checks you can safely do yourself
Check the hot water schedule
Sounds obvious, but a programmer that lost its settings after a power cut is one of the most common callouts we get. Set hot water to constant and wait 20 minutes.
Check the boiler is not in eco or holiday mode
Many combis have a preheat or eco setting that changes hot water behaviour dramatically.
Test more than one tap
If only one tap is cold you have a tap or a blocked aerator problem, not a boiler problem.
Listen for the diverter valve
Run a hot tap and stand by the boiler. A healthy diverter gives a soft mechanical clunk within a few seconds.
Never remove the boiler casing or touch gas components yourself — that's illegal as well as dangerous. Anything behind the front cover is Gas Safe work.
If that didn't fix it
What it usually turns out to be
Diverter valve stuck on heating
The classic combi fault. Cartridge replacement usually solves it.
£220 – £380
Failed plate heat exchanger
Scaled up in hard-water areas. Sometimes flushable, usually replaced.
£280 – £420
Motorised (2/3-port) valve stuck
Cylinder systems only. The actuator head fails far more often than the valve body.
£180 – £320
Cylinder thermostat or immersion failure
Cheap part, quick fix, easily missed.
£140 – £260
Failed PCB
On older boilers, the point where replacement usually wins.
£400+ or replace
Prices inc VAT and include our £99 diagnostic. We confirm the price before any work starts — no surprises on the invoice.
Straight talk
When we'd tell you to replace instead
- A combi over 10 years old needs both a diverter and a plate heat exchanger
- The PCB has failed and the boiler is out of guarantee
- You are on a cylinder system that no longer suits the house and a combi swap solves it permanently
No hot water with young children or elderly residents in the house is a priority booking — say so when you call.
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Frequently asked questions
Why is my heating working but not my hot water?
The boiler is producing heat but the diverter valve, motorised valve or plate heat exchanger is not routing it to the domestic hot water side.
How much is a diverter valve replacement?
Typically £220 to £380 inc VAT depending on the boiler and whether the whole valve or just the cartridge is needed.
Can you fix it same day?
Usually yes. We carry the common diverter cartridges and plate heat exchangers for Worcester, Vaillant, Ideal, Baxi and Viessmann.
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