Why is my radiator cold? (Troubleshooting Guide)

Warm to cold radiator

Do your radiators struggle to get hot? Have you got one radiator that’s always stone cold no matter what you try?

Take a look at VHL’s guide to figuring out why your radiators are cold and how you can fix it.


All of the radiators are cold

If all of your boilers are cold then the first thing to do is to check that the central heating is switched on. It sounds silly, but sometimes the most obvious things are easily missed.

If it is on but the radiators aren’t heating up, take a look at our list of quick checks to see if it’s a simple problem which you can fix yourself. If you can’t get to the bottom of the issue, or if the issue is more complicated, you will need to call out a professional engineer.


Just one radiator is cold

If only one of the radiators in the house is cold then it’s a good bet that the problem is isolated to that radiator rather than the central heating system as a whole. This means that you should be able to fix the problem yourself.


The radiator is cold all over

If it is cold all over, it is worth checking that the valves on both sides of the radiators are open. If they are not open, then water cannot get into and out of the radiator, so must be opened up. If they are open but the radiator remains cold then they may be blocked with limescale and you may need to remove the radiator and flush it out.


The radiator is cold at the top but warm at the bottom

In this instance, it’s highly likely that there is trapped air in your radiator which needs to be bled. Instead of being completely full of water, an air bubble is preventing hot water from reaching the top of the radiator. This air needs to be let out so that water can fill up the whole space and heat the entire radiator.

Bleeding a radiator is a very simple task and will only take a few minutes to complete. To do so, you will need a radiator key, which can be purchased very cheaply from DIY and hardware stores. You should only bleed the radiator when the heating is off to avoid burning yourself.


The radiator is warm at the top but cold at the bottom

These symptoms usually mean that there is a build-up of sludge settled in the bottom of the radiator. This is a bigger job which you may not wish to do yourself, as it requires removing the radiator from the wall and rinsing it through, which can be a messy and time-consuming job.

You can also purchase sludge remover which can be run into the system to clear everything out. This usually then involves draining the entire central heating system and filling it back up.

If your central heating system is blocked with sludge and slow to heat up, you may need a power flush to fully clear everything out. This will improve your heating’s efficiency, saving you money both on heating and on repairs.


Radiators are cold downstairs but not upstairs (or vice versa)

If just your upstairs or downstairs radiators are struggling to get warm then this could be a fault with your central pump, meaning that there is not enough power to force the water around the whole system.

Pumps can sometimes get stuck, so giving the pump a gentle tap with a hammer can often free it up. If the pump is hot (it’s fine if it is warm) or making a rattling, grating or knocking sound then it’s likely on its way out and needs to be replaced.

Another cause for loss of heat on just one storey is depletion of water in the feed and expansion cistern. This is usually located in the loft and should be full enough to float the ballcock while leaving room for the water to expand.

Refill the tank by moving the float arm valve downwards (similar to in a toilet cistern). If this does not refill the tank then there could be a problem with the float valve. If refilling the tank solves the radiator problem, it is still worth getting a professional to find out the cause behind the drop in water level.



VHL tool kit

If you’re having problems with your radiators that you can’t get to the bottom of then simply get in touch with VHL. We’re experts in all aspects of heating and plumbing, with 24 years’ experience and reliable, qualified engineers.


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