Why do my radiators heat up when the hot water is on?

Have you noticed your radiators randomly heating up while using the hot water tap? It’s a strange occurrence, but does indicate that something could be wrong with your boiler. Continue reading our helpful guide below to discover why your radiators heat up when the hot water is on.

Problems with the diverter valve

The reason for your radiators heating up at the same time as your taps could be something to do with the diverter valve.

The diverter valve opens and closes to allow, or cut off, hot water that flows through your heating system and radiators. When the hot water is sent to the radiators from the boiler, instead of your taps, the diverter valve will close.

So, ‘why do my radiators heat up when the hot water is on?’, we hear you ask. Well, having a bathtub that also has a showerhead could be the reason for your faulty diverter valve. The diverter valve will direct the flow of hot water to either the showerhead or the taps, depending on which one you’re using.

When the taps and radiators are in use, the valve will sit in a ‘mid’ position. With the valve constantly being opened and closed throughout the day, it can sometimes get stuck in an open position, leaving hot water to flow through to the radiators when you only wanted it in the taps.

If this is the case, do not attempt to take apart your boiler yourself. Instead, you should seek the help of a Gas Safe registered engineer to resolve the problem. If you’ve got Boiler Cover from toasti, a qualified engineer will be able to assist you quickly. Fortunately, a faulty diverter valve is not a huge fix.

Broken check valve

If the diverter valve is not the problem for hot water reaching your radiator at unwanted times, then the issue could potentially be a broken check valve.

Older boilers store hot water at all times and it travels through the radiators by natural convection. When you want to heat up your home with the central heating, the boiler pumps the hot water through to your radiators. Once your home has reached the desired internal temperature, the heat will turn off, causing the pump to stop running.

This is where the check valve comes into play, because that’s what stops the natural convection from warming up your home when the heating is switched off. If the check valve is faulty, the heat will continue to rise through the system, causing your radiators to stay hot.

Once again, this fix will need to be done by a qualified professional. Your Gas Safe engineer will run a diagnostic test on your boiler and fix the issue quite easily. However, if your boiler is more than 10 years old, or is out of warranty, you may be recommended to purchase a new, up-to-date boiler instead.


The most likely reasons for your radiators heating up when you turn on the hot water taps would be a fault in the check valve in older boilers, or the diverter valve in newer ones.

Both control the flow of heat and hot water to the different parts of your heating system. You should, under no circumstances, attempt to fix them yourself. If you’re not a professional then you could cause greater damage to the system, which could threaten the health of the occupants in the household.

Always seek the services of a professional Gas Safe registered engineer to resolve issues with your boiler. Not only could doing a DIY job on it cause a safety issue at home, you could also incur much larger costs in the long run.

Boiler cover and services at toasti

Here at toasti, we are a trusted and experienced team of highly qualified, reliable energy specialists. We provide all our customers with a professional service, covering all your energy services needs. We work throughout the North West and surrounding areas, including South Yorkshire, West Midlands, Cheshire and Wales.

With Boiler Cover from toasti, you can rest assured that, should anything go wrong, we are on hand to get your boiler back up and running again in no time. We understand that a broken boiler can put your life on hold until it’s fixed. That’s why we offer our impressive boiler cover and central heating insurance.

We also provide expert boiler services, as well as a customer-friendly installation service. If you’d like to speak to toasti about your boiler needs, contact us now.