If your radiators are warm but the taps are running cold, the boiler is working but something is stopping it from heating your water. The cause depends on what type of system you have. With a combi boiler, it’s often a faulty diverter valve. With a hot water cylinder, it’s often a timer setting, a thermostat or a valve. Here’s how to narrow it down.
First, which system do you have?
A combi boiler heats water instantly as you turn on a tap. There’s no hot water tank. A system or regular boiler heats water and stores it in a cylinder, usually in an airing cupboard. Knowing which you have makes it much easier to find the problem.
Quick checks you can do yourself
- Check the controls. Make sure hot water hasn’t been switched off or set to the wrong times on the boiler or programmer. Clocks often go wrong after a power cut or when the clocks change.
- Check the hot water temperature setting on the boiler or cylinder thermostat. It may have been turned down by accident.
- Look for a fault code on the boiler display and look it up in your manual.
- Check the boiler pressure gauge. Low pressure can affect hot water on some boilers.
- Try more than one tap. If only one tap is cold, the problem is likely that tap or its pipework, not the boiler.
Common causes with a combi boiler
A faulty diverter valve
This is the most common cause we see. The diverter valve sends hot water either to your radiators or to your taps. If it gets stuck in the heating position, your radiators stay warm but the taps run cold. It’s a repair for a Gas Safe registered engineer.
A faulty flow sensor
The boiler needs to sense water flowing when you turn on a tap. If the sensor fails or water pressure into the house is very low, the boiler doesn’t know to start heating.
Limescale
In hard water areas, scale can build up inside the boiler’s heat exchanger and stop it heating water properly. You might notice hot water that goes lukewarm, or a kettle-like noise from the boiler.
Common causes with a hot water cylinder
- A failed motorised valve. This valve opens to send hot water from the boiler to the cylinder. If it sticks closed, the cylinder never heats up.
- A faulty cylinder thermostat that tells the boiler the water is already hot.
- A broken immersion heater, if you rely on one as a backup.
- A problem with an unvented cylinder, which should only be worked on by an engineer with the right qualifications.
Can I stop it happening again?
Many hot water faults come from parts wearing out over time, but a few things help. A yearly boiler service lets our engineers spot a sticking valve or worn sensor before it fails completely. In hard water areas, a scale reducer fitted on the cold water supply to a combi boiler can protect the heat exchanger. And if you have a cylinder, keeping the stored water hot enough helps it work properly and stay safe.
When to call an engineer
If the controls and settings all look right, it’s time to call us. Most of these faults involve parts inside the boiler or valves on the system, which need a Gas Safe registered engineer. Our team handles boiler repairs and hot water cylinder servicing across Birmingham and Solihull.
MS HEATING AND PLUMBING has been fixing boilers and hot water systems for over 28 years. Our engineers are Gas Safe Registered, and we’re a Which? Trusted Trader.
