No Hot Water But Heating Works — What's Wrong?
When the radiators are hot but the tap runs cold, the fault is almost always in the hot-water side of a combi — usually the diverter valve or the plate heat exchanger.
Why heating and hot water can fail separately
On a combi boiler the same heat exchanger heats both your radiators and your hot water, but a component called the diverter valve directs the heat to whichever is needed. If the heating works but the hot water doesn't, the boiler can still fire and circulate — it's just sending the heat the wrong way, or a separate hot-water sensor or heat exchanger has failed.
The 4 most common causes
On combi boilers across Glasgow, these four account for the vast majority of 'heating works, no hot water' call-outs:
- A worn or stuck diverter valve — the most common single cause, especially on boilers over 7–8 years old.
- A blocked plate heat exchanger scaled up by hard water, which chokes the flow of hot water.
- A failed domestic hot-water (DHW) temperature sensor (e.g. Worcester Bosch CD).
- A failed flow sensor that can't detect the tap is open, so the boiler never fires for hot water.
Quick checks you can do first
Before booking an engineer, try these — they rule out the simple causes:
- Run the hot tap slowly — a trickle can be too low a flow to trigger the boiler. Try opening it fully.
- Check the boiler display for a fault code — a DHW sensor fault will often show a code.
- Confirm the heating still works; if both have stopped, the fault is upstream (gas, ignition, pressure), not the diverter.
- If you live in a hard-water area and the hot water has been getting gradually slower, suspect a scaled plate heat exchanger.
How it's fixed
A stuck diverter valve is replaced or, on some boilers, the motorised actuator cartridge alone can be swapped — a much cheaper repair. A scaled plate heat exchanger is usually descaled or replaced. A faulty DHW sensor is a straightforward swap. All of these are Gas Safe work; none are homeowner repairs because they involve the sealed water-side of the boiler.
Why it matters to fix it promptly
Running a combi with a partially blocked plate heat exchanger forces the boiler to work harder and run hotter, which can shorten its life and trigger overheat faults. And a failing diverter valve will only get worse — what starts as occasional cold water becomes permanent. Booking the repair early usually means a cheaper, faster fix.
Frequently asked questions
Why do I have heating but no hot water from my combi boiler?
It's almost always the diverter valve, a blocked plate heat exchanger, or a failed hot-water sensor. The boiler can still heat radiators because the fault is on the hot-water side only.
How much does it cost to fix a diverter valve?
Replacing a diverter valve (or its actuator cartridge) is one of the most common combi repairs. Central Heating Scotland quotes the repair cost before starting work — call 0141-374-0987 for a same-day engineer across Glasgow.
Can hard water cause no hot water from a boiler?
Yes. In hard-water areas scale builds up inside the plate heat exchanger and gradually chokes the hot-water flow until the tap runs cold or only warm. The plate is usually descaled or replaced.
Is no hot water an emergency?
It's not a gas safety emergency, but it's a genuine inconvenience most households want fixed fast. We treat it as a priority call-out and usually have a Gas Safe engineer out the same day across Glasgow.