Hot water only works when the heating is on? Here's why, and what to do
Don't open anything up. The parts that usually cause this are wired to the mains. The checks below only involve settings, looking and listening. The repair itself is a job for a heating engineer.
You switch the hot water on, and nothing happens. But as soon as the central heating comes on, the water gets hot. This is a very common fault in homes with a hot water tank (also called a cylinder), usually in an airing cupboard. The boiler is almost always fine. One of the controls that tells it to heat your water has stopped working.
Got a combi boiler with no hot water tank? This guide isn't for you. On a combi, hot water problems have different causes, usually a part inside the boiler. Call a Gas Safe registered engineer.
- Check the hot water settings on your programmer or timer.
- Check the thermostat on your hot water tank.
- Switch on hot water only, then look and listen.
- Use the immersion heater for hot water in the meantime, if you have one.
- Call a heating engineer and tell them what you found.
Each step has important details below. Tap a step to jump to it.
Why it happens
In a home with a hot water tank, the boiler heats two things: the radiators, and a coil of pipe inside your tank that heats your water. Motorised valves, usually in the airing cupboard near the tank, decide where the hot water from the boiler goes. They also tell the boiler when to fire up.
When the heating asks for heat, its valve opens and the boiler fires. If the hot water side has a fault, asking for hot water alone does nothing. The boiler never gets the message.
So why does the water get hot when the heating is on? Because the boiler is running anyway, and some of that heat finds its way into the tank. It's a clue that the boiler works and the problem is in the hot water controls.
The usual causes are:
- A faulty valve motor or the tiny switch inside it. These wear out over time and are the most common culprit.
- A stuck valve. This often happens after a power cut, or after the valve hasn't moved for a long time.
- A faulty tank thermostat (the dial on the side of the tank).
- A programmer fault or wrong setting.
Step by step
1Check the programmer or timer
- Make sure the hot water channel is set to "On", "Constant" or "Timed". It's easy to change the heating settings and knock the hot water ones by accident.
- Check the programmer's clock and day are right. A power cut can reset them, which means the hot water comes on at the wrong times.
- Try the hot water "Boost" or "+1 hour" button if there is one.
2Check the thermostat on your hot water tank
Most tanks have a small dial, usually strapped to the side about a third of the way up. If it's been turned down low, the boiler won't heat the water. For health reasons (to stop bacteria growing), stored hot water should be heated to about 60°C, so make sure it's set to around that.
3Switch on hot water only, then look and listen
Turn the heating off and the hot water on. Then:
- Does the boiler fire up? You can usually hear it, or see a flame symbol on its display.
- Listen to the valves near the tank. You'll often hear a quiet hum or whirr when one opens. Some have a little lever or window that moves.
- Feel the pipe going into the tank's coil, carefully, after 10 minutes. Is it getting warm?
Note down what happens. It helps the engineer find the fault faster, which can mean a shorter, cheaper visit.
4Get hot water in the meantime
Many hot water tanks also have an electric immersion heater, with a switch on the wall nearby. Switching it on will heat your water without the boiler. It costs more to run than gas, so use it as a stopgap and turn it off when you have enough.
Or simply run the heating for a while. As you've found, the water gets hot that way too.
5Call a heating engineer
Replacing a valve motor, tank thermostat or programmer is a routine job for a heating engineer, usually done in one visit. Often only the motor head needs changing, not the whole valve. Tell them:
- "I only get hot water when the heating is on."
- What you found in step 3: whether the boiler fired, whether you heard the valve, and whether the pipe got warm.
- Whether it started after anything, like a power cut, a new thermostat or programmer, or other work.
The boiler itself is on a gas supply, so if anyone needs to work on the boiler, they must be Gas Safe registered. You can check at gassaferegister.co.uk.
Questions people ask
Is it dangerous to keep using the heating to get hot water?
No, it's safe. It just wastes some energy heating radiators when you only wanted hot water. Get it fixed when you can.
My radiators come on when I only have hot water on. Is that the same problem?
It's the opposite way round, but it's caused by the same valves. A heating engineer can usually fix both in the same visit.
Did my new smart thermostat cause this?
It can, if it was wired up differently from the old controls. If the problem started right after a new thermostat or programmer was fitted, ask the installer to come back and check the wiring first.