Why are my radiators not as hot as they should be?

It's rare that we discover problems with our heating systems at the height of summer. In most cases, it's when the temperatures are at their coldest that we discover our central heating isn't operating as efficiently as it should be.

One of the most common problems that we all face at one time or another is that of a radiator not getting hot evenly across the whole surface area. Sometimes it can be hot at the bottom and cold at the top or vice versa.

This article will explain the most likely reasons why this happens and what you can do to fix it.

• Air in your radiator
• Sludge in your radiator

Air in your radiator

If you've got a radiator that is hot at the bottom but cold at the top, the most probable explanation is that you have air trapped at the top of the radiator.

The good news is that this is a simple issue to fix as all you need to do is to 'bleed' the air out of the radiator.

When bleeding a radiator, it's a good idea to switch your central heating system off, just to avoid any problems such as boiling hot water spilling out of your radiator onto you.

Using a radiator key (or a flat head screwdriver if your rad has that option), gently loosen the bleed valve on the top or side of the radiator.

As you slacken the bleed valve, air will start to escape with a large hissing sound. Once the air stops hissing, you should start to get a little bit of water spurting out.

It's at this point, you can be sure that the air is out of your system and you can tighten up the bleed valve before turning on the central heating system again to check the radiator is getting hot fully.

One of the most common reasons for air getting trapped in your radiators is due to a lack of chemical inhibitor in your heating system. If you do need to bleed air out of your radiators, it's a good opportunity to top up your inhibitor levels as well.

The video below has Jimmy the plumber explain in detail how to bleed a radiator and how to stop air building up in your system again.

Sludge in your radiator

If you have a radiator that is getting hot at the top but cold at the bottom, it isn't a problem with air being trapped in that rad.

This is much more likely to be caused by so called 'radiator sludge' accumulating at the bottom of the radiator and preventing it from being heated by the hot water in your system.

Radiator sludge is actually lots of little bits of rusted metal that has flaked off from the inside of your system and come to rest in the bottom of your radiator.

When water comes into contact with metal, there is a chemical reaction that can cause the metal to corrode. Again, in order to stop this process of corrosion from occurring, it's important that your chemical inhibitor levels are topped up.

The best way to remove sludge from the bottom of a radiator is to give it a power flush. However, this is something that is beyond the abilities of a casual DIYer and normally requires calling in a professional plumber.

An alternative is to remove the radiator from it's pipework, take it outside and flush it out using a hosepipe.

When your radiator is cold at the bottom, it’s likely that there’s a debris blockage that’s restricting the flow of hot water to the whole radiator. Usually, the culprit of this particular problem is sludge, debris, rust and other bits of dirt. Over time, the metal that the pipes are made of can begin to disintegrate because they’re constantly subjected to oxygen and water. This combination causes the metal to rust and as this rust breaks off, it is carried around your radiators and pipework. If you live in a hard-water area, limescale can also build up in the system. All of these things together can cause a blockage. A small blockage can very quickly turn into a large one.

Generally, a radiator that is warm at the top but cold at the bottom suggests that the hot water is able to flow into the radiator but is getting stuck somewhere in the middle.

If it seems to be just one radiator that’s causing a problem, you can remove it and clean it thoroughly. Do this using a garden hose to remove as much dirt as you can, then refit it to the wall. If you think the problem stretches further than just one radiator, you can have the whole system power flushed. A power flush uses pressure to push water and other cleaning chemicals through the system, removing all the dirt and flushing it down an outside drain. This process is worth doing every now and then to ensure the system is running as efficiently as possible.

A radiator not heating up is a fairly common problem faced by homeowners — but it is a nuisance nonetheless, particularly if it is impacting more than one radiator in your home. Usually, there is a simple explanation and putting the problem right should not be too difficult.

So if you have found your radiators are cold and are puzzling over the cause we are here to help, with a list of the most frequent causes and their solutions. Some radiator problems will need to be fixed by a professional, while others are simple to put right on a DIY basis.

Why Are My Radiators Cold?

There are several probable reasons why your radiators are not not getting hot, including:

• Debris in the pipework or filter
• Circulation pump not working or has air in it
• Air in the system
• Thermostatic radiator valve (TRV) is stuck
• Lockshield has been fully opened or closed
• Boiler or heat pump not working properly
• Central heating has turned off
• Fuel source has run out

Here, we take a look at the possible reasons individually to help you identify which one is causing your cold radiators and explain what you can do to get them toasty again.

1. The Radiator is Blocked

"If the system has not been serviced then there could be a build up of debris in the pipework or the filter," explains David Hilton, Director at Heat and Energy Ltd (opens in new tab). The simple solution is to get the system serviced by a professional.

Blocked radiators can be caused by a build up of limescale, sludge, rust or debris flowing through the system. This restricts the flow of water meaning blockages that start of fairly small can quite quickly become big blockages. 

Blocked radiators are usually cold at the bottom and warm at the top.  

If the problem is only affecting one radiator, it can be possible to remove it and flush it out with a garden hose, before replacing it. However, if you have several radiators not heating up then it may be necessary to have them power flushed by a professional.

A power flush uses pressure to shoot water and chemicals through the system, removing dirt and flushing it away down an outside drain. 

2. The Circulation Pump is Not Working

If a few, or all, of your radiators are not heating up, the circulation pump could be to blame.

Circulating pumps move hot water around a heating system, taking it from the boiler through the pipes to your radiators and hot water cylinder in some cases — before taking it back to the boiler. 

Sometimes, blockages or airlocks in the system will occur. Good indicators that this may have happened, other than radiators not heating up, include a water leak from the pump, unusual noises coming from the pump, casing that is hot to touch and no hot water. 

You will need to call in a plumber to fix the problem.

3. Air in Heating System

A very common cause of radiators not heating up is an airlock in the central heating system.

"There could be air in the system which would mean that the radiator may only heat a little bit at the bottom, or not at all if there is a lot of air," explains David Hilton. "You will need to bleed the radiator to get the air out. 

"As you let air out of the system you need to allow more water in. A gravity fed central heating circuit should fill automatically but a high pressure system will have a valve (or two) to let the water in. Don’t leave this valve open. Pressurise the system a bit and then bleed the radiator. It is usually best if two people do this but if in doubt call in a professional."

4. A Thermostatic Radiator Valve is Stuck

If it is just one radiator not heating up, then a stuck thermostatic radiator valve could be the culprit.

A thermostatic radiator valve (TRV) controls the flow of hot water to a radiator and occasionally these can get stuck, meaning they can't open to allow the hot water in.

"The TRV pushes a pin in and out and sometimes this pin corrodes and will not move," says David Hilton. "By unclipping the TRV you may be able to free it up, but be careful as it may be badly corroded and pop out or leak."

Try to free the pin using pliers and grease but don't use excessive force as you might damage the pin. If you can't free the pin then we would advise you call a professional.

5. The Lockshield is in Wrong Position

"On the opposite side of the radiator to the TRV is a lockshield," says David Hilton. "This looks like a small valve with a cap on it and it is used to balance radiators. What can often happen is that this is closed when a radiator is moved (for example, to paint behind a radiator) but then fully opened again when the radiator is replaced, rather than returning it to its previous position —  which is often only a quarter turn open. 

"This means that the boiler water will now circulate more through the closed radiators and hardly ever get heat to furthest away radiators."

A good signal that this is your problem will be if the radiators upstairs (or those nearest to the boiler) are getting hot but the downstairs radiators are cold. The system will need to be rebalanced.

6. The Boiler or Heat Pump is Not Working

An obvious reason for radiators not working is a faulty boiler or heat pump. Check your boiler is on the right setting and appears to be working  — a fault code will usually alert you to a problem. You may find you can put it right easily, but in some cases a Gas Safe registered heating engineer will need to be called out. 

If you use a heat pump to heat your radiators, check it is working as normal and that it hasn't become clogged with any debris. 

In some instances, you may need a new boiler. For a free boiler quote, visit boilerguide.co.uk.

7. The Water Pressure Has Dropped

If you have a combi boiler, or a high-pressure central heating system, the problem could lie with a drop in pressure, causing the boiler to cut out.

Take a look at the pressure dial — it should be in the range specified in the instruction manual. If not, you will need to top-up the water (check your manual for instructions) to repressurise the boiler.  

7. The Central Heating is Off

Yes it sounds obvious, but before you do any of the above, you really should check that your central heating hasn't been switched off or set to summer mode, or to heat the water only, by accident — you really don't want to go calling out a plumber only to discover this was the cause. 

8. Running on Empty

Another embarrassing cause of radiators not heating up (a bit like running out of petrol). 

If you use LPG or heating oil to power your heating system, could it be that you have forgotten to check your fuel levels and run out?  

One issue here is that running out oil can result in debris in the tank entering the heating system. Your system might need power flushing by a professional as a result. (So the bottom line is, regularly check the bottom line on your oil tank to ensure you don't run out!)

Why is my radiator not getting very hot?

How do I get more heat out of my radiators?

Why are some of my radiators hotter than others?

How hot should radiators be to touch?