Ignite Plumbing & Heating
No Hot Water — Causes and What to Do
No hot water with heating working usually means a faulty diverter valve. No hot water AND no heating points to the boiler itself. Immersion-only systems have different causes.
Failed diverter valve (combination boilers)
Combi boilers use a diverter valve to switch between heating and hot water. When it fails (often getting stuck in the heating position), you lose hot water while heating still works. Very common fault on 5–10 year old combis.
Boiler fault or lockout
The boiler may have gone into lockout — a safety shutdown triggered by a fault code. Check the display for an error code and look it up for your boiler model.
Tripped immersion heater thermostat
Immersion heaters have a thermal cutout that trips if the water gets too hot. There's a reset button under the plastic cover on the immersion heater head. Press it and restore power.
Faulty hot water cylinder thermostat
On systems with a separate cylinder, the cylinder stat controls when the immersion or coil heats the water. If it fails, the cylinder won't reach temperature.
Failed motorised valve (S or Y plan systems)
Open-vented systems use motorised valves to direct flow. A failed valve can cut off hot water independently of heating.
Practical checks and next steps
Hot water faults usually involve the boiler, diverter valve, or cylinder — all of which require a qualified engineer. There's one exception: if you have an immersion heater, check the isolator switch and reset button first.
- Check if your heating is also affected or just the hot water.
- Look at your boiler display — note any error or fault code.
- If you have an immersion heater, check the isolator switch is on and press the reset button on the heater head.
- Check the timer/programmer — is hot water scheduled to be on right now?
- Check the cylinder thermostat is set to at least 60°C.
- If none of the above resolves it, call a plumber.
Why do I have heating but no hot water?
On a combination boiler, this almost always means the diverter valve has failed. It's stuck directing flow to the heating circuit and not switching to domestic hot water when you open a tap.
How long does a diverter valve replacement take?
Typically 1–2 hours. It's a straightforward repair and most engineers carry common diverter valves for popular boiler brands.
My boiler shows a fault code — what should I do?
Note the exact code and look it up in your boiler manual or on the manufacturer's website. Some codes (like low pressure) are user-resettable. Others require an engineer.