Have you switched your hot water on only to find that your radiators are getting warm too? Or perhaps your heating only seems to work when the hot water is switched on?
This can be confusing, particularly if you have separate controls for your central heating and hot water. While a little warmth in the system can sometimes be normal depending on your boiler and heating setup, radiators becoming properly hot when you have only selected hot water can indicate a problem with the controls or a component within the heating system.
The cause can also depend on whether you have a combi boiler, system boiler or conventional boiler with a hot water cylinder.
At Kyle R C Perry Heating & Plumbing, we regularly help homeowners across Essex with heating and boiler problems. In this guide, we explain why your radiators might be coming on with the hot water, what you can safely check yourself and when it is time to call a heating engineer.
Why Do My Radiators Come On When I Put the Hot Water On?
If you have selected hot water only but your radiators are heating up, the system may not be correctly separating the demand for domestic hot water from the demand for central heating.
The exact cause depends on the type of heating system you have.
For example, a combi boiler uses a diverter valve to direct heated water between the central heating system and your taps. A system or conventional boiler with a hot water cylinder will usually use motorised valves and heating controls to manage the flow of hot water around the system.
If one of these components is sticking, failing or receiving the wrong signal from the controls, you may find that the radiators heat when they should not.
Top Tip
If your radiators become properly hot when you have selected hot water only, don’t simply turn the radiator valves down. This could be a sign of an issue with your heating controls, diverter valve or motorised valve.
Watch a Video of Our Engineer Kyle Fixing the Issue
Need Help With Your Heating?
If your radiators are coming on when the hot water is switched on, or your heating only works when the hot water is running, let us take a look. We can help identify the cause and advise you on the right solution.
Get in Touch for Help & AdviceWhat Could Be Causing Your Radiators to Heat With the Hot Water?
There are several possible causes, and identifying the right one usually requires looking at how your heating system is controlled.
1. A Faulty Diverter Valve
If you have a combi boiler, a faulty diverter valve could be responsible.
A diverter valve directs heated water to the appropriate part of the system. When hot water is required, it should direct heat towards your taps and showers rather than continuing to heat the radiators.
If the valve becomes stuck or starts to malfunction, hot water can be directed to the wrong part of the system.
One possible sign is that your radiators become warm or hot when you are running a hot tap, even though the central heating is switched off.
Vaillant explains that a diverter valve can become stuck and cause hot water to pass through to the radiators when it should be directed elsewhere.
A diverter valve is a boiler component, so it is not something you should attempt to repair yourself. A qualified heating engineer can diagnose the fault and determine whether the valve needs cleaning, freeing or replacing.
2. A Problem With a Motorised Valve
If you have a system or conventional boiler with a hot water cylinder, your heating system may use motorised valves to control the flow of heated water.
These valves help direct hot water to the heating system or hot water cylinder depending on what your controls are asking the boiler to do.
If a valve becomes stuck, fails to close properly or develops an electrical or mechanical fault, the system may not respond correctly when you select hot water only.
This could result in your radiators warming up when they should remain off.
3. A Fault With the Heating Controls
Sometimes the boiler itself is not the problem.
Your programmer, thermostat, smart heating control or wiring could be sending an incorrect signal to the heating system.
For example, your central heating may still be programmed to come on at certain times, or a thermostat could be calling for heat without you realising it.
Modern heating systems can have several different controls working together, so it is worth checking the basics before assuming a boiler component has failed.
Worcester Bosch explains that boiler controls manage when heating is required and can include programmers, room thermostats and programmable thermostats.
4. The Heating Has Accidentally Been Left On
It sounds obvious, but it is worth checking your controls before calling an engineer.
Depending on your system, you may have separate settings for:
If your programmer is set to run both heating and hot water, your radiators heating up is not a fault. The controls are simply doing what they have been programmed to do.
| Setting or Control | What It Controls |
|---|---|
| Central heating | Controls the heating system and sends heat to your radiators. |
| Hot water | Controls the heating of domestic hot water, depending on your type of boiler and heating system. |
| Heating and hot water | Allows both heating and hot water to operate according to the system controls and settings. |
| Timer or programmer | Controls when your heating and, where applicable, hot water are scheduled to operate. |
| Room thermostat | Measures the room temperature and can call for heat when the temperature falls below the selected setting. |
| Thermostatic radiator valve (TRV) | Helps control the temperature of an individual room by regulating the flow through the radiator. |
Radiators Heating When Hot Water Is On? Here’s What to Do
If your radiators are coming on when you only want hot water, don’t panic.
Start by checking your programmer, thermostat and heating schedule. If the central heating is definitely switched off but the radiators still become hot, the system may need professional attention.
Likewise, if your heating only works when the hot water is switched on, don’t keep changing the controls and hoping the problem disappears.
The right solution depends on your boiler and heating system, so getting an accurate diagnosis is important.
A Quick Guide
| What You’re Experiencing | Possible Cause | What To Do |
|---|---|---|
| Radiators heat up when hot water is switched on | A valve or heating control issue could be allowing heat into the central heating circuit. | Check your heating controls first. If the problem continues, arrange a professional inspection. |
| Heating only works when the hot water is switched on | Possible problems with controls, wiring or a motorised valve. | Have the heating system and controls checked by a qualified heating engineer. |
| Radiators heat when you run the hot tap on a combi boiler | A diverter valve problem could be allowing heat into the heating circuit. | Have the boiler and diverter valve checked by a qualified heating engineer. |
| Heating comes on at unexpected times | The programmer, thermostat or smart heating controls may be calling for heat. | Check your heating schedule, thermostat and smart control settings. |
| One radiator becomes warm while the heating is off | A radiator valve or wider heating system issue may be responsible. | If it continues to happen, have the radiator and heating system checked. |
| Heating and hot water work normally | The issue may simply be related to the way the heating controls have been programmed. | Check your programmer, thermostat or smart heating schedule. |
How Do I Stop My Radiator From Turning On With Hot Water?
The first thing to do is check your heating controls.
Make sure the central heating is switched off while the hot water is switched on. If you have a programmable thermostat or smart heating controls, check that there is not a heating schedule currently active.
You can also check the thermostat. If the room temperature is below the temperature set on the thermostat, the heating may be receiving a demand to come on.
If your controls are definitely set to hot water only and the radiators still become hot, this is more likely to indicate a fault with the heating system.
Things You Can Safely Check
Before calling a heating engineer, you can check:
- Is the central heating definitely switched off?
- Is there a heating programme currently running?
- Is the room thermostat calling for heat?
- Are your smart heating controls showing a heating demand?
- Does the problem happen every time you turn the hot water on?
- Do the radiators become slightly warm or properly hot?
- Do you have a combi boiler or a hot water cylinder?
These details can help your heating engineer understand what is happening.
You should not remove the boiler casing or attempt to repair internal boiler components yourself.
Still Having Problems?
If your heating and hot water aren’t working as they should, don’t keep adjusting the controls and hoping for the best. Get in touch with Kyle R C Perry Heating & Plumbing for help and advice.
Contact UsFAQs About Radiators and Hot Water
Why do my radiators come on when I put the hot water on?
If your radiators become hot when you have selected hot water only, there could be an issue with a diverter valve, motorised valve, heating controls or wiring. The exact cause depends on your boiler and heating system.
How do I stop my radiator from turning on with hot water?
First, check that your central heating is switched off and that your thermostat or heating schedule is not calling for heat. If the radiators still become hot, the system may have a valve or controls fault that needs to be diagnosed by a heating engineer.
Why does my heating only come on when the hot water is on?
This can be caused by a problem with the heating controls, motorised valve, wiring or another component within the system. If the heating will not work independently of the hot water, it is worth having the system checked.
Should my radiators heat up when the hot water is on?
It depends on the type of heating system and how it is operating, but your radiators should not normally become properly hot when you have selected hot water only. If they do, particularly on a system where heating and hot water are controlled separately, there may be a fault.
Can a faulty diverter valve make radiators come on?
Yes. On a combi boiler, a faulty or sticking diverter valve can cause heated water to be directed towards the radiators when it should be supplying hot water to the taps. A qualified heating engineer can check whether the diverter valve is responsible.
Can a motorised valve cause heating problems?
Yes. Motorised valves control the flow of heated water around some central heating and hot water systems. If a valve becomes stuck or develops a fault, the heating and hot water may not respond correctly to the controls.
Can I fix a faulty diverter valve myself?
No. A diverter valve is part of the boiler and should be diagnosed and repaired by a suitably qualified heating engineer. Do not remove the boiler casing or attempt to repair internal boiler components yourself.
Why is my radiator warm but not fully hot when the hot water is on?
A small amount of warmth does not necessarily mean the same fault as a radiator becoming fully hot. The behaviour of the system depends on the boiler type, controls and how heat is being circulated. If the radiator becomes consistently hot when heating is switched off, it is worth getting the system checked.