Why are my radiators hot when the heating is off

Heating off but radiators hot

No one wants to be too hot in summer, or to have to resort to flinging open the windows just because the radiators won’t turn off.

Radiators have two common but completely different ways of getting the hot water that makes them hot. Depending on what type of heating system you have, the radiator can stay hot for different reasons.

What causes your radiators to be hot when the heating is off?

It normally boils down (because it’s too hot) to a:

• Broken check valve (in the case of older boilers)
• Broken diverter/zone valve (which may cause your radiators to fire up when your hot water is on) or;
• Broken anti-gravity valve (which may cause your radiators to turn on when your thermostat is off)

Are your radiators coming on when your hot water is on?

This is a good sign that a stuck diverter/zone valve is the culprit. When it gets stuck, the valve is possibly in the ‘mid’ position or the zone valve isn’t closing correctly and hot water will keep on being sent to your radiators – even when the thermostat says that your heating is supposed to be switched off.

What’s a diverter valve?

A diverter valve can either be within a boiler or separately, usually in the vicinity of the hot water cylinder. Once the boiler is on, the diverter valves’ job is to divert water to either the radiators or to a hot water cylinder/hot water tap, depending on what you’ve chosen on the boiler’s interface.

How does a diverter valve work?

Your diverter valve opens and closes to determine where the water goes. For example, when you want heat in your radiators, the tap diverter valve will close so that the hot water is sent from the boiler to the radiators instead of sending it to the taps. If you think about a bathtub that has a shower hose coming from the normal taps – it’s similar to the valve used there – water flows to either the showerhead or the tap.

What is a zone valve and how does it work?

A zone valve is an electrically operated valve that allows the flow of heating water to a specific area of the property. The zone valve is controlled by a thermostat and is usually used on larger heating systems. Zone valves allow greater flexibility and allow the heating system to be broken down into a specific number of zones.

For example a mid position diverter valve will allow heat and hot water so water to all radiators and the hot water cylinder. With the use of zone valves water could be controlled to the hot water cylinder (zone 1), downstairs radiators (zone 2), upstairs radiators (zone 3) and the conservatory (zone 4). Each zone is capable of working independently from one another for extra efficiency and comfort.

How diverter/Zone valves get stuck

Let’s say it’s the middle of Winter, and you need hot water flowing to both your hot water system for the kids’ bath and your radiators on for when they’re getting into their PJs, your diverter valve will go to a ‘mid’ position allowing a flow to both heating and hot water.

If you have 2 zone valves each valve will open when a demand for heat or hot water is required. The valves are continually opening and closing as required during the day and night, so sometimes they can get stuck, become damaged, or get blocked with debris.

What to do if your diverter/zone valve is stuck

If you suspect you have a stuck diverter/zone valve, call us to take a look. Whether it’s us or another engineer, if the diverter valve is inside the boiler you should call out a qualified Gas Safe registered engineer to take a look at the valve and check the boiler. HomeServe offers a repairs service with a local tradesperson in your area, so take a look and get in touch if you need our support.

If the diverter valve or Zone valve is external to the boiler then a good quality Plumbing and Heating Engineer should be able to get you up and running quite quickly.

The engineer will test your system to check if it’s the diverter valve that’s the problem, and either clean or replace it. If your boiler is still under warranty you may be able to get it replaced for free.

A broken check valve

If you’ve got an older boiler, the reason could also be a broken check valve. A check valve is a small component inside the boiler that’s activated when you switch off the boiler to switch off the natural convection process of an older boiler. This stops it from heating up your home. When the boiler and its water are still hot, but your thermostat has stopped calling for heat, it’s the check valve that stops the heat.

Check valves aren’t common in new boilers so you may be able to rule this out depending on the age of your boiler.

What happens when the check valve fails

If the check valve becomes damaged or fails, the natural convection process is unregulated and just continues. This is what causes your radiators to feel hot even though your boiler is saying the heat is “off.”

What causes a check valve to fail?

The most common reasons for a check valve to fail is either getting clogged up by debris, or a spring breaks inside the boiler.

What to do if your check valve is faulty or damaged

Speak to us – a HomeServe engineer can arrive to help fix or replace your check valve.

Radiators coming on when thermostat is off

If the anti gravity valve is broken or blocked and is not doing its job of stopping the natural convection process, it means heat will rise through your central heating system. Your radiators will be on, even though your thermostat is off.

How a anti-gravity valve works in detail

As mentioned above, these older boilers work through natural convection and the water stays hot at all times, unlike more modern systems where a pump is needed as the water is allowed to cool.

When you switch your boiler on in this type of system, a pump starts off the process of drawing water from the boiler to the radiators, and natural convection does the rest.

Now, even if you paid no attention to your science teacher back in the day, you probably know that heat rises because it’s less dense. This is natural convection at work – so the hot water from the boiler rises and travels around the pipes by natural convection to the radiators.

When the heat is turned off, or your rooms reach the temperature you want (indicated by your thermostat) – the pump stops.

When the pump stops, the anti gravity valve will close and stops the natural convection from happening. When the pump turns back on, the force overcomes the resistance of this spring loaded anti gravity valve and it opens.

What to do if your anti-gravity valve is faulty or damaged

Speak to us – a HomeServe engineer can arrive to help fix or replace your check valve.

We’re on our way

If you suspect you have either a faulty diverter valve or anti gravity valve, get in touch with us for a repair and we can send out one of our qualified Gas Safe Registered engineers. If you do have an older boiler, it’s well worth considering investing in a much more efficient condensing boiler – check out our boiler cover and heating breakdown insurance comparison.

FAQs

Why are my radiators not turning off?

This is usually a problem with older boilers and it’s usually due to one of two issues; a faulty diverter valve or a broken check valve inside the boiler.

How do you stop a radiator from heating up?

If you have either a faulty diverter valve or a broken check valve, you’ll need to call a Gas Safe Registered engineer to either clean or replace the valve.

Why won’t my central heating turn off?

If your boiler interface and thermostat are telling you your boiler is off, but your radiators are still hot, it’s probably due to a broken diverter valve or check valve inside the boiler. You’ll need to call out a Gas Safe registered engineer.