Radiator Not Heating Up? 6 Causes and Easy Fixes

One or two radiators in a system can be cold for a few reasons.  Try to find and fix the fault yourself by following our guide below, before calling someone in to help.   Some causes are easy to fix. Soon you’ll have a warm home again.

Why is my radiator not heating up?

The most common cause of a cold radiator is trapped air in the central heating system. Other likely causes include sludge and debris blocking the flow of water to the radiator, a radiator leak or a faulty thermostatic radiator valve. Another obvious cause can be from incorrect heating settings.

Radiator not heating up?

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Causes and fixes for a radiator not getting hot

1Trapped air in the radiator 

Put a radiator key in the bleed valve and see if any air comes out.  If only water comes out then there’s no air in the radiator.  Make sure the heating is off when you do this as the water expands when its hot and so may come out quicker than if the system was cold, giving you the wrong impression.

image of trapped air in a radiator being released

Lots of air in central heating system

Having air in a central heating system is normal because of the way a combi boiler works, even if there is no leak. You shouldn’t need to bleed the radiators too often though. Topping up the system with fresh water can also let excess air in, which is why you need to bleed the radiators affected straight after.

2.  Sludge at the bottom of the radiator 

If it’s hot at the top this means there’s no air inside.  If you feel it cold at the bottom though, there is a good chance of sludge being in the radiator in which case it should be isolated and removed from the system and taken outside to be cleaned. 

3.  The lockshield valve is not fully on

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lockshield valve

 The lockshield valve adjusts the hot water flow going into the radiator and may help get the radiator working better. It is on the other side of the radiator opposite the adjustable valve.  The system should have been balanced when installed so check if you can turn the lockshield valve on about half a turn more clockwise with a pair of pliers.   If you can’t turn it clockwise anymore it means it’s on full. 

If you get air in the radiators frequently and find yourself having to bleed one or two, you can purchase a self bleeding automatic radiator bleed valve like this one which will do the job for you. 

4.   Thermostatic valve (TRV) is faulty


If you find one radiator cold, turn off all the other radiators in the house by turning their thermostatic valves TRV) to setting 0 then turn the heating system on.  That radiator should get really hot as it’s the only one on the system turned on.  If it still doesn’t heat up fully, the TRV on that radiator could be faulty. 

Sometimes the pin inside it gets stuck down or pops up slightly which means only a tiny amount of heat is going into the radiator.  This way you can verify it’s only that radiator that is faulty rather than the whole system.

Leaking thermostatic radiator valve

If the trv is leaking water down from the top of the valve head, your only option is to replace the trv completely. If the leak is from one of the compression nuts, this can be easily fixed by tightening the nut. Try turning the valves off at both ends, drain the radiator and apply some white PTFE tape (plumbers tape) around the joint.

5.  Central heating system isn’t balanced properly

If you have had any alterations done such as moving a radiator or adding another one to the system will likely cause an imbalance in the heating.  Your heating system may need balancing again so all the radiators heat up evenly. 

This is done by closing all the valves down then slowly turning them on in small amounts with each radiator working your way to the furthest one in the house away from the boiler which will be turned on full.  

So the nearest one to the boiler will be open a quarter turn, then the next one along will be a half-turn etc. 

image of central heating radiators in a line on the wall

6. A radiator leak

radiator leak

A leak from a radiator will be a sign of internal corrosion inside it.  Although this can be fixed temporarily with leak sealants, it usually means the radiator will need to be replaced sooner rather than later. 

Double check it is the radiator itself that’s leaking rather than the joints, nuts or pipework coming into it; you may get lucky and only need to tighten them.

Radiator leaking from the top

A radiator leaking from the top points to the bleed valve or the blanking cap. The radiator doesn’t need to be replaced in this case. With the blanking cap, apply plenty of PTFE tape around the thread before screwing it back in. It is best to replace the bleed valve if it leaking as you need to use it more often if using it to bleed the radiator

All radiators not heating up?

If multiple radiators in your home are cold, check for trapped air in the system as this is a clear sign. Then bleed your radiators one at a time, starting with the one furthest away from the boiler, finishing off with the radiator closest to it. You will need to repressurise the boiler once the radiators have been bled.


Here are the main reasons why your radiator is not heating up:

  1. Trapped air in the radiator.
  2. Lockshield valve is turned off.
  3. Thermostatic radiator valve pin is stuck.
  4. Sludge in at the bottom of the radiator.
  5. Air lock in the system pipework.

So as you can see, there are many reasons why a radiator can be cold and most of the time there are simple causes and fixes.  But, after trying these without success or if you don’t feel confident attempting any of them, please call someone who is competent at fixing the problem.

If you need a local plumber, go to our recommended page and pick from the list.

*The information in this article should be used for general guidance only and not as financial or health advice.  Full details are on the link in the footer to our disclaimer page.  Always discuss your requirements with a competent and suitably qualified professional before undertaking any work.

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