Radiator Cold Spots: Causes and Fixes

When winter hits, it’s the worst time possible for your radiators to get a cold. Cold radiator spots can span from a small section along the bottom of the radiator, right up to the entire top half of the radiator being completely cold. This can add a big chuck to heating bills if you don’t realise that this is the issue, and you’re just scratching your head wondering why the house isn’t as warm as it used to be. 

There’s a few things that might be causing your cold radiator spots, and the good news is that the cheapest fix is free(ish). Depending on the cause though, they do get drastically more expensive.

Let’s start with cold radiator spots at the top.

The first thing to do is double check that it is indeed a cold radiator spot, rather than an area that just hasn’t warmed up yet. Radiators heat up from left to right, bottom to top. This means that if you’ve just turned the heating on, and the radiator is warm at the bottom but cold at the top this might indicate why. If, however, the radiator is still cold at the top after about 20 minutes of the heating being on, then that’s your cold spot. 

Check whether it’s cold all the way across the top. If it is, then congratulations, because it’s the most common issue, and the cheapest to fix. Though it is a bit messy. 

A Cold Radiator Spot at the top of a radiator is usually a buildup of trapped air in the system. Water is 900 times denser than air, so when you have water coming in and leaving the radiator at the bottom, it’s easy for small bubbles in the system to gradually build up into sections of air trapped at the top of the radiators. 

To remove this trapped air, you need to bleed the radiator. It’s worthwhile checking your other radiators for cold spots at the top as well, so get this out of the way first. Identify whether it’s one radiator or several and then let’s get to work on fixing the issue. 

Cold radiator spots - solution - bleeding the radiator

Bleeding your radiator to remove cold radiator spots.

Step 1: Start by turning off the central heating and wait for it to cool all the way down, there’s going to be water coming out of the radiators and you don’t want it to be boiling.

Step 2: Use the radiator key, which you can buy from most DIY shops for often 50p to £3, to turn the bleed valve at the top of the radiator in an anti-clockwise direction. Place the container underneath to catch any water that may come out of the radiator.

Step 3: As you turn, you’ll hear a hissing sound which is the air escaping. Once this sound ends water will trickle out so turn the valve clockwise to lock the valve. Avoid doing this too tightly.

Follow this process for all of the radiators around your property. Once you’ve finished, check the pressure gauge on your boiler (it should be around 1.5) and make sure there’s no sign of leaking. If your boiler’s dropped in pressure as a result of letting air out of the system then you can read our guide on pressurising your boiler here.

This leads us on to cold radiator spots at the bottom of the radiator. The bottom of the radiator is usually the first place to warm up, because the warm water inside fills from the bottom. If this isn’t the case with your radiator and there’s a cold spot at the bottom, then it’s likely you’ve bled a radiator before now. This is because your central heating is a closed system which makes it hard for air to ever get inside. The air trapped at the top of radiators is the result of corrosion and material breakdowns within the system.

The pipes and radiators themselves corrode over time, which is a chemical reaction which breaks material away from the lining inside of them. This reaction causes a sludge of metal and rust to start flowing through the pipes, but it also creates bubbles every time something breaks away from the inside of the system. This is usually what causes the cold spots at the top of radiators and it results in deposits of sludge at the bottom when the heating is off and the water settles in the system. 

The settled water stops pushing the debris through the rest of the pipes and fittings so it sinks wherever it is. This can get stuck to the bottom of the radiator which then stops the next lot of debris which gets trapped on the settled sludge. Over time this builds up to become significant areas of your radiator not being heated as there’s no water flowing behind it. 

Unfortunately, this is where it could get pricey. 


Flushing Radiators to Remove Cold Radiator Spots

If it’s just one cold radiator spot that you’re dealing with, or just one problem radiator, then you may be able to flush it yourself, which is absolutely free minus the tools to do it. This will involve:

  • Shutting off the mains water supply
  • Turning off the boiler
  • Loosen the nut at the bottom right of the radiator, with a bucket or many towels underneath
  • Open the bleed valve to remove the pressure
  • Catch any water and sludge that falls out of the radiator. 
  • Once that side has stopped flowing, empty the bucket and move it to the other side of the radiator
  • Loosen the other side of the radiator to catch any water stuck inside.
  • Once drained, close the bleed valve again. 
  • Remove the radiator from the wall by removing the nuts on both sides where you’ve drained the water from. 
  • Place towels down between the radiator and the closest external door and take it outside. 
  • Hold the radiator over a drain, then put your hose pipe inside the inlet for the radiator, before turning it on full. 
  • Having the radiator upright may dislodge some of the sludge that’s built up. If you have a rubber hammer you can also try giving it a tap in the area you had a cold radiator spot with that from each face to force more of the sludge to budge. 
  • Keep the hose going until the water runs clear and you’re sure a decent amount of sludge has evacuated the radiator. 
  • When you’re done, reconnect the radiator to the wall and reattach the nuts/piping. 
  • While you’re at it, check the boiler pressure while it’s off – it’ll have dropped when you emptied the water out of the radiator. If it’s below 1 Bar, top it back up. 

When you fill the radiator back up, open the bleed valve again to allow the air inside to escape or you’ll end up with airlocks in the system. 

If you’re still having issues with the radiator, it may be that there’s sludge stuck in the pipework instead, or if you have multiple cold spots spanning across a few radiators, then you need a whole system powerflush. 

Powerflushing your central heating

Powerflushing a heating system takes many hours, sometimes days. It involves using a high pressure machine to force chemicals around your system, which then start to break down the sludge, bacterial growth and debris. Think of it as if a firefighter blasted your oven clean with a hose, but a lot less messier. 

Cleaning a central heating system is integral to its health and powerflushing is a popular cleaning method due to the rapid but comprehensive clean it delivers.

The benefits of a clean heating system are well-documented:

  • more even heat distribution throughout the property
  • improved overall energy savings
  • lower household bills
  • increased system reliability
  • extended system life
  • removes cold radiator spots from buildup

So how do you know if a system needs a powerflush? The obvious one here is the cold radiator spots at the bottom of your radiator, but here’s some more symptoms to look out for.

  • Are certain rooms much colder than others?
  • Does it take a long time for the system to warm up?
  • Do radiators need frequent bleeding?
  • Is the boiler noisy?

While it is possible for very technically minded people to perform a Powerflush themselves, we fully recommend you get a professional to do this for you and there can be substantial damage made if it’s done incorrectly, so we won’t be giving instructions on how to perform this. 

Are you entitled to a Free Replacement Boiler?

At Borthwick Heating we work with the UK Government to install free boilers to eligible UK Homeowners. To check if you qualify click the button below.

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