Why do radiators go cold when hot water is on?

Author: Marcia Zboncak  |  Last update: Thursday, March 20, 2025

One cold radiator usually indicates that either there is air in the system or there is a stuck valve within that radiator. The thermostatic radiator valve (TRV), like the one pictured below, controls the flow of hot water to the radiator.

Why do my radiators go cold when I run a bath?

Yes, the system will serve hot water requirements in preference to room heating requirements. Consequently, during long periods of high demand for hot water the radiators may temporarily appear to cool down.

Why does my heating go off when I run hot water?

If your heating is on and you run your hot water, the hot water that normally flows to your radiators is diverted to the plate heat exchanger to heat the water for your taps. so your radiators won't heat if your hot tap is running. When you close the tap it is then diverted back to your central heating automatically. I.

What does it mean when your radiator is cold at the bottom and hot at the top?

Radiators being colder at the bottom than the top is often caused by sludge build-up. As with most things, it's much better to prevent the problem rather than trying to solve it retrospectively. We strongly recommend looking after your radiators by trying to prevent sludge from building up in the first place.

Why is hot water working but not radiators?

If you are getting hot water to your taps but not to your radiators, this could be an indication that the diverter valve fault is stuck. This is not something that you should try to fix yourself. You need to get in touch with an experienced engineer who is Gas Safe registered.

Combi Boiler - Hot Water Running HOT and COLD

Why is my radiator cold but I have hot water?

Air pockets in the radiator: Air can often become trapped in the central heating system and obstruct the flow of water, leaving the radiator cold. There are several ways to fix this, but the most basic is to bleed the radiator. Depending on the type of system, you can also inspect vents, air filters, or air ducts.

What are the symptoms of a faulty diverter valve?

Symptoms of a faulty diverter valve often include inconsistent hot water flow and radiators not heating properly. Homeowners may notice that radiators remain cold even when the boiler is on. Inconsistent water temperatures are common, with water switching from hot to lukewarm or cold unexpectedly.

Why is my radiator not getting fully hot?

If your whole radiator is cold it could be an issue with the thermostatic valves. First, check to see if the valve on your radiator is turned to the on position, turn it to the highest temperature if required. If your radiators still don't warm up, it could be that your thermostatic valves are stuck.

How often do you need to bleed radiators?

You should bleed your radiators at least once a year, typically before winter arrives in order to ensure the efficiency of your heating system. Regularly checking for trapped air also helps to make sure that your system operates effectively and reduces energy consumption.

Why does my hot water have to be on for my heating to work?

The valve most likely to cause this to happen is the diverter valve as this switches the output and flow of your boiler system between hot water and heating.

How to free a stuck diverter valve?

Well, that brass knob, known as the diverter valve, that connects pipes in your combi boiler might be the culprit. But you won't be able to free a stuck diverter by yourself; a qualified heating engineer will need to be called in to save the day.

Can I heat hot water and radiators at the same time?

A combi, or combination, boiler is a type of boiler that combines a water heating system and central heating unit into a single body. This acts as a unified way to handle heating and hot water without the need for a separate tank.

What to do when your radiators are cold?

Check your boiler or heat pump

If your radiators are cold when you're expecting them to be warm, you should check that your boiler or heat pump is working properly. If you have a boiler, you should check that it's on the right setting and working properly.

Why do my radiators come on when the hot water is on?

This means that hot water from the boiler might keep running through your radiators and heating them up even though the heating is turned off. If you have noticed that the radiators are getting hot when the hot water is on, this probably means that the diverter valve is stuck in the mid position.

Why does my heating go off when I have a shower?

Hi the answer to your question is the hot water always takes priority over heating and so as long as you're in the shower and running hot water any where in the house the heating will shut off until done.

How do I know if my radiator is air locked?

Trapped air can manifest in a number of different ways:
  1. Poor or loss of heating performance from the radiators in general (airlock in the pump)
  2. Cold spots on an otherwise functioning radiator (trapped air in the radiator)
  3. Tapping or banging sounds from your heating system (trapped air in the system)

What happens if you bleed a radiator with the heating on?

If the heating is left on, you not only risk scalding yourself, you might end up with a flood as the water gushes out. It's also not an efficient way to bleed radiators. If the water is pumping when you let out the air, it could be counterproductive and actually cause the system to draw in air instead.

Will an airlock eventually clear itself?

Will an Airlock Eventually Clear Itself? Sometimes, small airlocks may clear themselves as water circulation continues with one or more radiators. However, larger airlocks often require manual intervention, like bleeding the radiators or using a hose to release the trapped air.

Why are my radiators only getting luke warm?

If your radiator feels lukewarm…

chances are there is nothing wrong and your radiator has simply reached the desired temperature. To be on the safe side, set a higher temperature for a short space of time to determine whether the radiator starts to heat up.

Why is my radiator not working with hot water?

Radiators might not be getting hot due to trapped air requiring bleeding, a malfunctioning boiler or central heating pump, thermostatic radiator valves that might be stuck or set too low, or the central heating system having sludge or rust which causes blockages.

Why is my radiator full of water but cold?

If your radiator is hot at the top but cold at the bottom, there may be a build-up of scale, rust, or sludge which is obstructing the flow of water. As with the middle of the radiator, if you have an open-vent system which is unpressurised and tank fed, you'll be able to use a sludge remover to flush your radiator.

How do I know if my diverter valve is stuck?

Signs a diverter valve is stuck

If your diverter valve is sticking, you'll soon know about it. Whether that's standing under a cold shower because you've got no hot water or facing a chilly evening because your radiators won't switch on. Other things to look out for include: No hot water from the taps.

How much does it cost for a plumber to replace a diverter valve?

$100 – $350 shower cartridge replacement cost

Plumbers charge $100 to $300 in labor alone to replace a shower mixing valve or diverter valve.

How do I know if I need a new diverter valve?

The most common symptoms for a faulty diverter valve are:
  1. I have hot showers but my radiators stay cold. A diverter valve may be faulty if the hot water taps still work while the radiators do not turn on.
  2. I can only get lukewarm water from my taps. ...
  3. My hot water only turns on when central heating is on.

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