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Why is Your Radiator Cold at the Bottom?

Becky Mckay
By: Becky Mckay
Updated: 6th July 2022

Plumbers or Heating engineers

A radiator which is cold at the bottom but hot at the top will leave your home feeling cold. While you may want to turn up the thermostat, your boiler will be using more fuel to reach a higher temperature leading to higher heating bills. Definitely not what you want.

Note: If you have a convection or convector radiator, there may not be a problem. This is because the hot water in convection heating systems 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.

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Why your radiator is cold at the bottom

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Radiators become cold at the bottom when something is stopping the flow of water which is likely to be sludge.

Let's start by looking at sludge. Over time, the hot water flowing through the metal pipes of the heating system will lead to limescale and rust. These particles accumulate into a thick, black substance called sludge which settles at the bottom of the radiator. Eventually, it can form a blockage and stop the hot water from heating the radiator effectively.

It may be that only one radiator has developed a significant sludge blockage. However, as a central heating system is a circuit other radiators may be cold too.

Never ignore cold radiators

Sludge in a heating system can cause damage and, if left for too long, could lead to a complete boiler breakdown. And you won't want to have to deal with an urgent boiler replacement.

On average, replacing a boiler can cost £2,000 - £4,500. Compare that to the average price of a powerflush at £300 - £400 and it makes sense to act sooner rather than later.

Turning the heating up won't work!

No matter how high you turn the thermostat up when a radiator is cold at the bottom, it simply won't work. This will just put additional strain on the boiler putting it at risk of running into a fault.

How to fix radiators that are cold at the bottom

Radiators that are cold at the bottom need quick attention. You have a couple of options including a powerflush or chemical flush. Both of these must be carried out by a heating engineer.


A powerflush involves a chemical being circulated around your heating system at a high pressure. This process will free any blockages to allow the hot water to flow efficiently and effectively – allowing your radiators to heat up properly.

To perform a powerflush, a heating engineer will connect a pump to the central heating system. This pump will push a chemical through the pipework and radiators at high speed to break down and remove the sludge. A descaler and corrosion inhibitor should also be used to remove limescale and prevent rusting in the future.

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've 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.

A powerflush is well worthwhile if your radiators are cold at the bottom as you'll benefit from:

  • Radiators that heat up quicker
  • Warmer radiators
  • Quieter heating system
  • More energy efficient system
  • Lower heating bills
  • Higher hot water temperatures
  • More reliable (less chance of breakdown)
  • Increased life expectancy of the heating system

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.

Chemical flush

A chemical flush involves chemicals being circulated around the heating system but not at a high pressure like a powerflush. As the chemicals flow through the system, they remove any build-up of dirt, limescale and debris. The chemicals can be left for an hour or as long as a week in some cases.

Once the chemicals have done their job, the system is then flushed to refill it with clean water and a rust inhibitor.

DIY flush – not recommended!

While it isn't recommended and, for the best results, we highly recommend hiring a professional, it's possible to flush the heating system yourself.

Safety is paramount when attempting a DIY flush so turn the heating off and wait for it to cool right down before starting.

Essentially, flushing the heating system yourself involves removing each radiator from the wall one-by-one. You then take them outside to clear out any sludge by running clean water through it.

How to prevent cold radiators

Prevention is far better than the cure. Once the blockage has been cleared from the radiator, it's a good idea to take steps that will prevent it from happening again. This could include:

  • Central heating inhibitor
  • Scale reducer
  • Magnetic filter

Add Central Heating Inhibitor

A central heating inhibitor is a chemical which can be added to your system to prevent the buildup of limescale and sludge. It does this by coating the system to keep the water running through smoothly.

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.

If you live in a hard water area then a scale reducer is a great option as hard water increases the speed at which limescale forms.

Install a Magnetic Filter

A magnetic filter can be fitted within a central heating system to collect any rust, limescale and debris from the circulating water. This will prevent a build-up of sludge and reduce the chance of a blockage. The filter will be emptied during an annual service.

Read more with our guide to Water Treatment in Heating Systems.

Why radiators might be cold at the top

A radiator which is cold at the top but hot at the bottom suggests that air is trapped in the system. You can sort this issue yourself by bleeding your radiators.

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.

Note: If you're the slightest bit unsure about bleeding a radiator then it's always best to contact a qualified heating engineer.
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Find an engineer to fix your radiators

The best way to remove sludge from your heating system is by hiring a professional heating engineer. And you're in the right place!

At Boiler Guide, we have a network of qualified heating engineers ready to quote on your job. Complete our online form and you'll get free quotes from up to 3 engineers in your area. There’s no-obligation to accept these quotes.

Once you have your quotes, you'll be in a position to compare them and be confident that you're getting the most competitive price.


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Becky Mckay

About the author

Becky Mckay

Becky is one of our home heating and renewable energy experts and has a wealth of experience writing about the world of heating.

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