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How to fix cold radiators – A BestHeating guide

How to fix a radiator that doesn’t get hot

Everybody knows that the main function of any radiator is for it to heat a space to a nice, warm and inviting temperature – but what if one or more of your radiators just refuses to heat up, what do you do then?

Woman holding hot drink

During the colder months, everybody wants their home to be warm and cosy and for it to stay that way for the duration of winter, so knowing how to fix any little issues with your heating will mean that your home can stay nice and warm without having to fork out for an expensive engineer.

There are a few different factors that can cause your radiators to become cold and not heat up properly, so we thought we’d put together this handy list of tips to help you solve your cold radiator problems – that way, you can save a little extra money to spend on the finer things in life and only call out the experts when there’s a real heating emergency.

This is the BestHeating guide to fixing cold radiators.

Why is my radiator cold at the top?

If you find that your radiator is heating up nicely at the bottom and yet the top of it remains cold, the chances are you have air trapped in the system.

Because of this air, the hot water at the bottom of your radiator will be unable to reach the top of the appliance until the blockage is removed.

a graphic that shows a radiator that is cold at the top and warm at the bottom

This is a pretty simple problem to fix and involves you bleeding the radiator to expel any air that has become trapped inside.

Check out The BestHeating Blog guide on bleeding radiators and simply follow the instructions to get your radiator working properly again – it’s really easy and you don’t need to be a DIY expert to do it correctly.

How does air get trapped in my radiator?

There are a few reasons to find air trapped in your central heating system and a lot of the time it will find its way into one of your radiators and result in a cold spot.

It may even be the case that you hear the air banging around your pipework from time-to-time and it can often be tough to diagnose just how it got there and why.

The main causes of air finding its way into your central heating system, are –

  • If you have your central heating pump installed ABOVE your supply tank.
  • If your system has an open tank in the loft used for immersion heaters.
  • A build-up of hydrogen in the heating system – this can be caused by rust or a sludge build-up.
  • Small pin-hole leaks in the system – this will often mean you are regularly topping up your boiler pressure.

If you are unsure if any of the above apply to your central heating system, we advise that you contact a qualified heating engineer before proceeding.

Why is my radiator cold at the bottom?

A radiator that is cold at the bottom (instead of the top) is a more difficult issue to resolve than just releasing a little trapped air, but what causes your radiator to become cold at the bottom in the first place?

Cold radiator diagram showing effect of sludge

This problem is usually caused by a build-up of radiator sludge at the bottom of the appliance that prevents the hot water from reaching every column of the radiator.

Don’t worry, I can hear you now: “Radiator sludge! What’s that then?”

Well, radiator sludge is a mix of a few different things that are often found floating around the water in your central heating system.

This could include (but isn’t limited to) dirt and rust from the internal corrosion of the system – things like iron oxide and hydrogen build-up – both of which can begin to interfere with the flow of water through the system and settle at the bottom of your radiator, causing those cold radiator problems.

a picture of fingers holding a pipe that is filled with sludge and sediment

Radiators with a heavy build-up of radiator sludge and other contaminants can make heating your room very difficult and dramatically increase your heating bills!

In some more extreme cases, a system with a big sludge build-up – that is left unchecked – can even cause severe damage to the rest of your central heating, so it’s important to fix it as soon as you can.

Check out our expert guide on how to fix radiators that are cold at the bottom.

It can take years for sludge to build up in a heating system and for it to begin causing problems – so if you have a newer system, it could be something else causing the problem.

How do I fix a radiator that is cold at the bottom?

In the unfortunate event of sludge being the cause of your cold radiator or radiators, there are a few things that you can do to fix the issue.

Firstly, though, it’s important to determine which radiators are causing the issue and being affected by radiator scale.

Hand touching top of radiator to check warmth

Do you find that all the radiators are showing symptoms of heating sludge?

Or are the radiators downstairs working perfectly fine, while those upstairs are not?

It could be that one radiator is causing a blockage, that then has a knock-on effect for all the others in the system – so be sure to check if this is the case or not.

A simple way to do this is by working your way from your boiler to the first radiator in the system. If this radiator is heating up fine, you know the problem lies elsewhere, so move on.

If you find that the first radiator is cold at the bottom – before you have checked any others – there’s a good chance that this problem radiator is preventing the others from heating up correctly – so fix this radiator first.

So, how do you fix this cold radiator yourself?

Remember: Clearing sludge from a single radiator is only a stop-gap solution for an individual appliance. There could still be sludge in your system that could do more damage in the longer term – so if you’re unsure, contact a professional.

How to manually remove radiator sludge

If you’re comfortable with basic DIY, removing sludge from one radiator is a pretty simple (though sometimes messy) job.

So how do you do it?

You will need a couple of things to get you started –

  • A couple of old sheets and a towel
  • A radiator bleed key
  • TWO adjustable wrenches or spanners
  • A bucket or a washing up bowl
  • A hosepipe and a water supply

Step 1

Switch your central heating off and allow it to cool down for a while – the last thing you need is a trip to A&E because you’ve scalded yourself with hot, sludge-ridden radiator water.

Step 2

two men lifting a radiator from its mounting on a wall

Once you’ve let the system cool down, you’ll need to lay your sheet or towel on the floor under your radiator and carefully remove it from the wall – you may need a friend to help you. We have a guide to doing that here.

Step 3

Pour the contents of the radiator into your bucket or washing up bowl – this liquid is likely to be black and dirty thanks to the sludge build-up you’re trying to get rid of.

A man pouring away the contents of a radiator into a washing up bowl

Remember: The sludge will still be in the radiator, so be very careful as you’re removing it from the wall and try not to get any of the mess on your flooring!.

Step 4

Once you have the radiator off the wall, take it outside – and using a hosepipe – flush out the inside of the radiator from both sides.

a radiator being flushed with a garden hose to clean out the inside of the radiator

Please note: This should be effective enough to remove the sludge from a single radiator, but if you have issues all over the house it’s probably best to have a professional undertake a complete power-flush of your central heating system.

Step 5

Once you have flushed the radiator out with your garden hose, return it to the wall and reconnect it and refill the heating system with the filling loop at your boiler.

It may be an ‘external’ filling loop, like this –

External filling loop

Or, it may be an ‘internal’ filling loop, like this –

internal boiler filling loop

If you are unsure of which type of filling loop you have, don’t go playing around with your boiler – Contact a trained and professional heating engineer!

Do I need a flow diverter?

Believe it or not, but the problem with your cold radiator could be something as simple as having a ‘flow’ diverter installed.

Some – but not all – radiators require flow diverters to ensure that the water flows in the right direction through the appliance.

A flow diverter and radiator valve being placed inside a radiator

A pretty simple piece of kit, a flow diverter is designed to make sure all the parts of your radiator that should get warm, actually DO get warm.

It could be the simple solution that you require to get your radiator kicking out some heat again – and believe it or not, engineers often forget to install them when replacing radiators, so be sure to get your plumber or heating engineer to put one inside your new radiator (particularly if it is a tall, vertical style).

Things to remember

If – after you’ve been through most of the tips above – you are still not sure what is going on with your radiators, the best thing you can do is call a professional heating engineer and have them come and power flush your system.

If you are in any doubt whatsoever over what is causing the problem, DO NOT try to fix it yourself because misdiagnosis of radiator problems can be costly.

So, if you need some advice on how best to fix your cold radiator, call our expert team at BestHeating.com – or drop us an email at info@bestheating.com.

Failing that, you can always search the BestHeating Blog and find tips on balancing radiatorsbleedingreplacing and positioning your radiators too – so don’t be shy, take a look at our expert heating advice, tips and know how so you can stay safe and get some happy heating!

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