If your heating works but there’s no hot water, the cause depends on your boiler type. On a combi, it’s usually a stuck diverter valve or a faulty flow sensor. With a hot water cylinder, it’s usually the hot water timer, the cylinder thermostat or a stuck zone valve. If it’s the other way round and you have hot water but cold radiators, see our guide to hot water but no heating.
We’re Clever Energy Boilers, Gas Safe registered (no. 554138) and accredited Worcester Bosch installers based in Bingley. We fit combi, system and regular boilers across Yorkshire, the North West and the Midlands, so we know the different ways each one can lose its hot water.
Why is there no hot water when the heating works?
It comes down to how your boiler makes hot water, so start by working out which type you have. If you have a large hot water tank in an airing cupboard, you have a system or regular boiler. If there’s no tank and hot water comes straight from the boiler when you turn on a tap, you have a combi. Our guide to the types of boilers explains the differences.
| Boiler type | Most likely causes of no hot water |
|---|---|
| Combi | Diverter valve, flow sensor, low mains water pressure, low boiler pressure |
| System boiler with cylinder | Hot water timer, cylinder thermostat, zone valve |
| Regular boiler with cylinder and loft tank | Hot water timer, cylinder thermostat, zone valve, empty cold water tank |
Why does my combi boiler have heating but no hot water?
Is the diverter valve stuck?
This is the most common cause on a combi. The diverter valve sends hot water either to the radiators or to your taps. If it sticks in the heating position, the radiators stay hot and the taps run cold. Our guides on a faulty diverter valve and how to free a stuck diverter valve explain what’s going on. Replacing one is a Gas Safe engineer’s job.
Is the flow sensor detecting the tap?
A combi only fires for hot water when its flow sensor detects water moving. If the sensor is faulty or blocked with scale, the boiler simply doesn’t know you’ve turned a tap on. Try opening the hot tap fully. If that gets it going, the sensor or a filter may be partly blocked.
Is the mains water pressure too low?
Combis need decent mains flow to trigger. Check the cold taps are running normally and that your stopcock is fully open. If your water company is working nearby, low pressure may be the cause.
Is the boiler pressure too low?
If the gauge is under 1 bar, the boiler may struggle to supply hot water even if the heating seems to work. Top it up using our guide on how to repressurise a boiler.
Why does my system or regular boiler have heating but no hot water?
Is the hot water timer on?
Programmers often have separate channels for heating and hot water. It’s easy for the hot water channel to be switched off or set to the wrong times after a power cut or clock change. Press the hot water “advance” or “boost” button to test it.
Is the cylinder thermostat set too low?
Most cylinders have a thermostat strapped to the side. If it’s turned right down, the boiler won’t heat the water. Around 60°C is the usual setting.
Is the zone valve stuck?
A motorised zone valve opens to let hot water from the boiler into the cylinder coil. If it sticks closed, the cylinder stays cold while the heating works fine. An engineer can test it and replace the motor head if needed.
Can the immersion heater help in the meantime?
Yes. If your cylinder has an immersion heater, switch it on for hot water while you wait for a repair. Our guide to the immersion heater explains how to use it.
Why does my hot water start hot then go cold?
That’s a different problem from having no hot water at all. It’s usually caused by low flow, a faulty thermistor or a diverter valve that’s wearing out. Our post on combi boiler hot water going cold covers it.
What if my Worcester, Vaillant or Baxi has no hot water?
The causes are the same across brands, but each shows its own fault codes. Note the code on the display before you call anyone.
Worcester boiler: no hot water but heating works
On a Worcester Bosch combi, a sticking diverter valve or a hot water flow sensor clogged with scale are the usual causes, especially in hard water areas.
Vaillant boiler: no hot water but heating works
Vaillant ecoTEC combis show the same pattern. The diverter valve is the first thing an engineer will check, followed by the hot water flow sensor.
Baxi boiler: no hot water but heating works
On Baxi combis, it’s again usually the diverter valve or the hot water flow sensor. Older Baxis that have been in a hard water area for years are especially prone to it.
If your boiler is a Worcester Bosch, we’re accredited Worcester Bosch installers. We also fit Vaillant, Baxi and Ideal boilers.
When should I call an engineer, and when should I replace the boiler?
Diverter valves, flow sensors and zone valves all need a Gas Safe engineer. On a combi over 8 years old, though, a failing diverter is often the first of several worn parts, and replacing the boiler can work out cheaper than a string of repairs.
We offer boiler replacement at a fixed price, and long, market-leading warranty plans are available through Clever Energy too. If you’re on a cylinder system and fed up with running out of hot water, we can also talk you through switching to a combi.
No hot water FAQs
Why do I have heating but no hot water on my combi?
In most cases it’s the diverter valve sticking in the heating position. A flow sensor fault or low mains pressure are the next most likely causes.
Can low boiler pressure cause no hot water?
Yes. Below about 1 bar, many boilers lock out or won’t fire properly for hot water.
How long does a diverter valve last?
There’s no fixed lifespan, but they often start to wear on boilers over 8 years old, and hard water and sludge can bring that forward.
Is no hot water an emergency?
It’s not dangerous, but it is inconvenient. If you have an immersion heater, use it until the boiler is sorted.
