On this page
Quick answer
If you only get hot water when the central heating is switched on, the part that should open the hot water circuit on its own is not working. On system and regular boilers with a hot water cylinder, this is usually a failed hot water zone valve, a three-port valve stuck in the heating position, a cylinder thermostat fault or a wiring error. On combis, it usually points to the diverter valve or a flow sensor. It is a common fault and normally a repair rather than a replacement.
Key takeaways
- The problem means the boiler is not being told to fire for hot water alone, or the water cannot reach the cylinder.
- Check the programmer: is the hot water channel set to on or timed?
- On cylinder systems, the hot water zone valve and cylinder thermostat are the usual suspects.
- On combis, the diverter valve or flow sensor is more likely.
- Incorrect wiring after a controls change can cause the same symptom.
First, check the programmer
Many programmers have separate channels for heating and hot water. If the hot water channel is off, or set to a timed period that does not match when you need water, it will seem as though hot water only comes on with heating. Set the hot water channel to “constant” or “on” briefly as a test.
System and regular boilers
Hot water zone valve. A two-port zone valve opens to let boiler water into the cylinder coil. Its microswitch also tells the boiler to fire. If the motor or microswitch fails, the boiler does not fire for hot water alone. When the heating is on, the heating zone valve fires the boiler and, depending on plumbing, some heat reaches the cylinder.
Three-port valve. A mid-position valve should move to “hot water only” when there is no heating demand. If it is stuck, you get heating or both, but not hot water alone. See what a zone valve is.
Cylinder thermostat. The cylinder stat tells the valve when the water needs heating. If it has failed open, or is set too low, the hot water circuit never calls.
Wiring. After a new programmer or smart thermostat is fitted, wiring errors can link hot water operation to the heating channel.
Combi boilers
A combi does not have a hot water channel in the same way. If hot water only works when the heating is on, suspect:
- The diverter valve, which may be sticking in the heating position. See diverter valve problems.
- The flow sensor, which tells the boiler a tap is open. If it fails, the boiler may not respond to a tap alone.
Also read no hot water but heating works for related symptoms.
Is the immersion a stopgap?
On cylinder systems with an immersion heater, you can use it for hot water while you wait for a repair, but it is more expensive to run than gas. Ask one of our gas engineers if you are unsure how yours is wired.
Book with Gasmaster
Hot water tied to the heating? Gasmaster Plumbing & Heating, Gas Safe registered (220932), fixes zone valves, controls and boilers across Stoke-on-Trent, Newcastle-under-Lyme and Biddulph. See our boiler repair service.
Frequently asked questions
Is this a boiler fault or a controls fault?
On cylinder systems it is usually a controls or valve fault outside the boiler. On combis it is more often inside the boiler.
Can I get hot water without heating in the meantime?
On a cylinder system you can use the immersion heater if you have one. On a combi, you may have to run the heating briefly.
Is it expensive to fix?
Zone valve heads and cylinder thermostats are relatively modest repairs. Diverter valves cost more. See boiler repair costs.
Sources
- Energy Saving Trust, heating controls guidance (energysavingtrust.org.uk)
- Manufacturer wiring guides for S-plan and Y-plan systems
- Gas Safe Register (gassaferegister.co.uk)
Related services
Abdul runs Gasmaster Plumbing & Heating and has worked on boilers, heating and gas appliances across Stoke-on-Trent since 2005. He is Gas Safe registered (reg. 220932), and these articles come from his day to day work in homes and businesses across Staffordshire, Cheshire and Derbyshire.
