Why is Your Radiator Cold at the Bottom?

Why is Your Radiator Cold at the Bottom?

Plumbers or Heating engineers

A radiator which is cold at the bottom but hot at the top will not be heating your home effectively. This means you’ll either be feeling cold or you’ll be turning the thermostat up to try and compensate for the lack in heat. Turning the heating up means your boiler will be using more fuel to reach a higher temperature leading to higher heating bills, which is definitely not what you want.


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Note: If you have a convection or convector radiator, there may not be a problem. This is because in convection heating systems the hot water rises to the top naturally forcing cooler water back down to the bottom where it is reheated in a cycle. And, to a certain extent, all radiators should be cooler at the bottom than at the top, but they shouldn’t have cold patches.

Why Your Radiator is Cold at the Bottom, But Hot at the Top

Radiators become cold at the bottom when there is something stopping the flow of water. When the radiator is cold at the bottom this usually means that there is a build-up of something called central heating sludge which has settled at the bottom.

It’s natural for a system made up of hot water and metal that particles of limescale, rust and dirty water will form. Over time, these particles accumulate into a thick, black substance called sludge which settles at the bottom of the radiator and stops the hot water from heating this area of the radiator.

It may be that only one radiator has developed a significant sludge blockage, but a central heating system is a circuit so if this is the case it will be blocking hot water from reaching radiators which come after it.

How to Get Rid of Blockages in Radiators

You will need to contact a professional heating engineer who will perform a service called a Powerflush.

The engineer will connect a pump to your central heating system which will push a chemical through the pipework and radiators at high speed to break down and remove the sludge. They should also use a descaler to remove limescale and corrosion inhibitor to help prevent future rust from forming. Depending on how extreme the blockage is, the engineer may also need to use tools on the outside of the radiators to dislodge pieces. When they are finished they will dispose of the contaminated water and, having measured the temperature of your radiators before they started work, will show you the improvement after the Powerflush.

A Powerflush could last between 6 – 10 hours in total. If you think your central heating system could benefit from a Powerflush, contact us today and we’ll find you free, no-obligation quotes from up to 3 Gas Safe registered engineers in your area.


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Sometimes, flushing the radiator is not enough to fix the problem and you may need to get a new radiator altogether. This can be the case is hard water areas where limescale has created a particularly firm blockage.

A Powerflush is usually only needed every 5 – 6 years as long as you’re looking after your heating system with an annual service. If your radiator is relatively new but not heating up fully, then you should contact the supplier or installer.

Why Cold Radiators Shouldn’t Be Ignored

A radiator with sludge at the bottom can’t heat up fully, so it’s likely your home will be colder and you’ll need to turn the heating up to reach comfortable levels of heat. This will result in more expensive heating bills.

In addition, sludge in a heating system can cause damage to the boiler and, if left for too long, could lead to a boiler breakdown. Replacing a boiler can cost £2,000 – £4,500 on average; when you compare that to the average cost of a Powerflush at £300 – £400, it makes a lot of sense to act sooner rather than later.

What if a Radiator is Cold at the Top?

A radiator which is cold at the top but hot at the bottom may suggest there is trapped air in the system. You can sort this issue yourself by bleeding your radiators, but if you’re unsure it’s always best to contact a qualified heating engineer.

To bleed the radiators you’ll need a radiator key and a dry cloth. You’ll need to turn the heating off, allow it to cool and release the open the bleed valve by turning the key. There will be a hissing noise as air escapes; when water begins to appear, you can tighten the valve again and hot water should be able to rise to the top of the radiator.

Find out more in How to Bleed a Radiator and Why It’s Essential.

How to Prevent Radiator Blockages

Once the blockage is cleared you should take some precautions to stop it from happening again any time soon.

Add Central Heating Inhibitor

Central heating inhibitor is a chemical liquid which should be added into a central heating system to add a protective coating against corrosion and the build-up of sludge and limescale.

Add a Scale Reducer

Scale reducer is a very good idea for homes in hard water areas, as limescale is much more likely to form a blockage in a heating system.

Install a Magnetic Filter

A magnetic filter can be fitted in a central heating system to collect any rust, sludge or debris from the circulating water before it has chance to build up and reduce the efficiency of the heating system. The filter is emptied during an annual service.


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