No hot water? Here's why — and how we fix it fast
No hot water usually points to one of four faults inside a combi boiler. On a system boiler with a cylinder it's a different set of possibilities. Here's how to narrow it down before you call.
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The likely causes, in order
- Diverter valve stuck — The most common combi fault — heating works fine but the boiler won't switch to hot water. Usually a diverter valve cartridge or motor replacement.
- Diaphragm failure (older combis) — Rubber diaphragm perishes over time, especially in pre-2015 Vaillant and Worcester units. Straightforward part swap.
- PCB fault — Electronics not commanding the hot water side. Diagnostic tools needed to rule this in or out.
- Low system pressure — If pressure has dropped below 0.5 bar the boiler will lock out. Topping up the filling loop restores service — but a repeated drop means a leak somewhere.
What you can safely try first
- Check the boiler pressure gauge — if it's below 1 bar, top up via the filling loop
- Check the boiler display for a fault code — write it down before you call
- Try running just a hot tap for 30 seconds — if the boiler fires briefly then cuts out, it's likely a diverter valve
Request a help · urgent quote in your area
Real engineer reply — usually within the hour during working times. Fixed written quote, no commission-driven sales, 12-month workmanship guarantee.
- Gas Safe registered
- Same-day cover where possible
- Fixed written quotes
- Manufacturer warranties up to 12 yrs
Frequently asked questions
Should I switch it off overnight?▾
Switch it off if there's any smell of gas or visible water leak. Otherwise leave it on standby — most fault codes clear briefly after a reset.
Is it worth repairing an old boiler with no hot water?▾
If the boiler is over 12 years old and the repair is more than £400, we usually recommend replacement — we'll give an honest cost comparison.
Related pages you might want
Still stuck? Get a real engineer on it
Call us if the DIY steps don't clear it — same-day response where possible, fixed price before any tools come out.