How to Fix a Cold Radiator [2023] Re February 6, 2023

How to Fix a Cold Radiator [2023]

Cold radiators

How do I fix my cold radiator?

Nobody likes a cold radiator unless it’s the 3 days a year in the U.K where it’s hot enough to have your heating off. So if it’s not one of those days and your radiator is cold, what’s going on? You expect maximum performance all year round from your rads.

Could it be that you need a boiler replacement? Unless all the radiators are cold then it’s not that. If it’s just one radiator giving you problems then there are a few simple explanations as to why it’s not heating fully. Sometimes radiators will get cold at the top and hot at the bottom but it’s much more common to have issues with a radiator that’s cold at the bottom and hot at the top.

In this article, we’ll explain exactly why your radiator is cold, and how to get it back to full toasty warmness again.

Why is one radiator cold when the heating is on?

This either means there’s air in the system or there’s a stuck valve within that radiator. 

The thermostatic radiator valve (TRV), controls the flow of hot water to the radiator. You may find that from time to time the TRV control can get stuck, which means that it won’t allow the hot water in when the heating is on.

This is very common when the heating hasn’t been turned on for a few months and is also common in old radiators.

How to check if the TRV valve is stuck?

To check if the valve is stuck, you can remove the rotatable head on the TRV so you can take a look at the raised pin underneath.

Once you’ve done that, you should be able to push the pin down with your finger. Once you lift your finger, the pin should rise back up again.

If the pin didn’t rise up or isn’t moving easily, then this is more likely the problem. 

How to safely fix a stuck TRV pin?

You can try to fix the pin yourself, just spray some WD40 onto the pin, then use a mole grip to gently pull the pin up and down a few times until you can move it in and out with ease. Be careful not to apply excessive pressure because it may damage the pin.

If the pin is completely stuck or you’re not sure about doing this, we’d advise you to call out a heating engineer.

On the other hand, if you’ve you’ve checked the TRV and it seems to be fine, there could be some air trapped in the system, which is causing one radiator to stay cold, that’s why we’d advise you to bleed your radiator.

If both solutions didn’t work, it’s time to call out a heating engineer.

Why are my downstairs radiators cold?

If the downstairs radiators stay cold while the upstairs ones are hot, then you’ve got a balancing issue.

How to rebalance your radiators?

You can try to go to the radiators upstairs and close the lockshield (the smaller valve at the opposite end to the thermostat) then open it a quarter turn. This shouldn’t reduce the effectiveness of the upstairs radiators but it will allow more heat to flow to the downstairs radiators. 

If this didn’t solve the problem, you’ll need to give the heating engineer a call. 

Why is my radiator cold at the top?

If your radiator is hot at the bottom and cold at the top, then it’s more likely to be down to air pockets. Air pockets can stop the water from distributing evenly around your radiator, but the good news is it’s easy to sort out. All you’ll need to do is bleed air out of your radiator!

How to bleed my radiator?

You’ve heard of people ‘bleeding’ radiators and thought it was some weird cult, it isn’t. Bleeding a radiator is a way of releasing air from the system. It’s easily done by using a radiator bleed key (available from any hardware type place or from your neighbour with tools), then inserting it into the valve in the top right of the radiator and turning it anti-clockwise (AKA Lefty Loosey).

If there is air trapped in the radiator, when you turn the bleed key you will hear the hiss of air escaping, don’t open the valve too much because water will come out once all the air has escaped, and you don’t want to get it on your nice clothes. Once you reach the little bit of water at the end, tighten the valve clockwise (righty tighty) and scoop up those last few drops with a cloth.

Why is my radiator cold at the bottom?

If your radiator is cold at the bottom and hot at the top, then the most likely cause is ‘sludge‘. You may be wondering how sludge has made its way into your heating system, well don’t panic, it’s a fairly common occurrence.

Sludge is the affectionate name given by heating engineers to the build-up of metallic elements mixing with dirty system water. It’s formed from the inside of your heating system (pipes and radiators) rusting over time, as water is pumped through your system it gathers up the rust and settles at the bottom of radiators. 

This gathering of sludge is what stops your radiator from getting hot as the water can’t evenly distribute itself around your radiator.

Another contributor to sludge in your system is limescale, particularly if you live in a hard water area. The minerals present in hard water can solidify and become limescale. Limescale mixes with the rust and sludgy water to create an ungodly mixture sent straight from the depths of hell to ruin your heating system.

But don’t panic just yet, we can help you banish this hellish sludge back to whence it came! Begone foul creature, this is not your home.

How do I remove sludge from a radiator?

1. Chemical Flush

A chemical flush is exactly what it sounds like, flushing your system with chemicals that banish nasty sludge from your system. It works by breaking up the sludge so it can be washed out of your system with a final flush of freshwater.

After this process has been completed, a chemical inhibitor will be added to your system to prevent future rusting.

Cost: £100-£200 depending on the area you live, and how expensive your house looks.

WarmZilla performs a chemical flush free of charge with every boiler installation. A chemical flush is included to ensure that the new boiler is installed into a system free from sludge so the performance of your new boiler is not affected.

It may not be worth putting money into an older boiler when you could put the money towards a new boiler from WarmZilla. If your boiler is over ten years old then it may be time to retire your old boiler and get the system flushed when the new boiler is installed.

WarmZilla boiler prices start from £1489 inc. VAT and there are boiler finance options available if you’d prefer to pay monthly. This price is based on an Ideal boiler with a 5-year guarantee, system filter, chemical flush, smart thermostat, and installation.

2. Powerflush

A powerflush is a chemical flush on steroids. A heating engineer will connect a ‘powerflushing’ machine to your system which will pump water and chemicals through your system at a very high pressure to dislodge the sludge in your radiator.

A powerflush is much more effective at dislodging sludge in your system but it’s not our first choice because of a couple of reasons. The main one is the cost. Powerflushing will cost at least £400 and sometimes as much as £600. This is a substantial amount toward the cost of a new boiler.

Secondly, a powerflush is quite a vigorous process and the water is pushed through your system at a fast rate to dislodge the sludge but it can also have a detrimental effect on your system by shaking pipes loose and causing leaks. You already had the sludge problem, you don’t want to add to them with a leak. If you have a newer system then it’s unlikely that you would need a powerflush anyway.

WarmZilla doesn’t carry out powerflushes, if you think you might need one then you can find a trusted heating engineer from the Gas Safe register to quote on it for you. If you somehow manage to get hold of a powerflushing machine and think you might have a go yourself, we really wouldn’t recommend it. There are lots of things you’re good at and powerflushing isn’t one of them, leave this one to the experts.

We’ve looked at the differences between a powerflush, chemical flush and magnacleanse in a previous article if you’d like to know more.

3. Inhibitor

Ok, if you’re at the stage where you’ve diagnosed a cold radiator as being caused by sludge then it’s unlikely that adding an inhibitor will solve the issue for you but we’ve added it in because it’s cheap and worth a try.

You can pick up a leading brand inhibitor (Fernox, Magnacleanse, Sentinel) from Amazon for as little as £15, so it’s worth a shot, but an inhibitor is more of a preventative measure than a cure. It works by coating the inside of your central heating system to stop it from rusting and contributing sludge.

How to protect my radiators from sludge?

We’ve now fixed the issue, give yourself a big pat on the back, you’ve done well. But even though we now know how to fix it, we would prefer not to have the issue again. So what can we do? Here’s what…

1. Use a System Filter

A system (or magnetic) filter is fitted to the return pipe on your boiler and works by attracting all of the sludge BEFORE it enters your boiler. It does this using powerful magnets, a simple solution to an annoying problem.

During your annual boiler service (if you don’t you should), your engineer will empty your system filter as well as perform other crucial safety and performance checks. Regular servicing can help you stop issues building up and can fix small problems before they become big problems.

WarmZilla installs a system filter with every new boiler we install. We want to give your new boiler the best scenario so it can last at least 15 years, that’s why we flush your system, add inhibitor and then fit a system filter. Getting a filter installed would normally cost you upwards of £100.

2. Add Scale Reducer

Scale reducer is short for limescale reducer. It works by breaking down the minerals that cause limescale so that the build up of sludge in your system is reduced.

We highly recommend using a scale reducer especially if you live in a hard water area.

3. Add Inhibitor

As part of an ongoing boiler maintenance plan we recommend adding inhibitor to your system every year to keep the inside of your central heating system healthy.

4. Replace Old Radiators

If your radiators are quite old then it’s only natural that they would be corroded inside and contributing to system sludge. If your radiators are getting on a bit, you may choose to replace them with something more aesthetically pleasing (radiator styles are much more varied than they were twenty years ago).

As well as not being rusty, modern radiators are also more energy-efficient and heat up faster than traditional radiators.


Whilst having a radiator with a cold patch at the bottom is not the end of the world, it also isn’t ideal. Yes, you can turn up the heating and reach the same temperature but your boiler is working harder to reach the same temperature which puts more of a strain on it and shortens the lifespan. 

It will also increase your heating bill as more energy is required to do the same amount of work, this is also bad for the environment.

Leaving sludge to roam free in your system is also a bad move for your boiler as the sludge can get into your boiler and cause issues, including needing a boiler replacement.

It’s more cost effective to deal with a sludged up radiator than just leaving it be and hoping it will just disperse in an orderly fashion. If your boiler is nearing the end of its life then you can get a boiler quote from WarmZilla by clicking the link below and we’ll make sure we flush your system before it goes in and fit a system filter to make sure nothing nasty gets into your new boiler.

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