Radiator cold at top: 2 common causes and how to fix5 min read

When a radiator is cold at the top but hot at the bottom there’s usually a simple explanation and fix for the issue. Sadly many homeowners spend money on plumbers where this isn’t often necessary.

Radiators cold at the top but hot at the bottom are normally caused by a build-up of air bubbles. Air rises to the top of the radiator and prevents hot water from distributing evenly. Bleeding the radiator will fix the issue. Another cause is a build-up of sludge.

Within this article, we’ll help you diagnose the two most common causes of radiators that stay cold on the top, and how you can easily fix them at home.

fix a radiator that is cold at the top

Radiators are heated by the circulation of hot water generated from the boiler. If the boiler and pump are functioning correctly then radiators should heat in a uniform way and heat loss should be consistent throughout the system.

Trapped air

If the problem is specific to one radiator and this radiator stays cold at the top but the bottom heats up, the most likely cause is trapped air. Read this article if all your downstairs radiators have stopped working.

Hot water stays at the bottom of the radiator and can’t reach the top. This is because water is 900 times denser than air whereas the air bubbles rise to the top.

Air bubbles can be released from the radiator by bleeding it. Bleeding the radiator involves releasing trapped air from the bleed valve by using a bleeding key. This should be performed whilst the heating is off and the radiators are cold.

Turn the bleeding valve anti-clockwise until air starts to escape and until water escapes. Then close the valve.

Bleeding a radiator

If bleeding the radiator does not fix the issue, the second most likely cause is a build-up of sludge in the radiator.


Sludge is formed when rust and dirt in radiators mix with water. It forms a thick build-up that prevents hot water from reaching parts of the radiator. Usually, sludge is found at the bottom of old radiators near pipes, but it can form anywhere within radiators creating cold patches.

When sludge is left untreated, central heating bills will rise and further damage such as holes in pipes can occur. Sludge can also lead to radiators requiring more frequent bleeding.


Sludge can either be treated by a plumber with a power flush, or radiators can be individually removed and cleaned by homeowners.

bleeding a radiator

Whilst trapped air can be a cause of radiators not heating through to the top, homeowners should also regularly bleed radiators to maintain an efficient HVAC system. Regular bleeding will reduce central heating bills.

Radiators that stay cold at the top are the most common symptom of trapped air. In severe cases, the entire radiator will stay cold when the central heating is turned on. Trapped air prevents hot water from reaching the entire radiator and should be released via the bleeding valve.

how does air become trapped in radiators?

Air can become trapped in radiators due to a variety of reasons ranging from leaks to the heating system itself.

Most newer central heating systems are sealed systems and lack a ‘feed and expansion’ tank. Older systems are open-vented systems that are open to the atmosphere.

When new water enters the system from the expansion tank, air can also enter the system causing air bubbles in radiators. Older HVAC systems are therefore more likely to experience air buildup than newer ones.

Other ways in which air becomes trapped in radiators include:

  • Tiny air bubbles enter the system from the movement of the water pump
  • Initial installation or maintenance. When radiators are initially filled with water a small amount of air may remain in the system
  • Leaks within the system. This is more likely if the boiler is frequently re-pressurized
  • Rust or a build-up of sludge can cause an accumulation of hydrogen
  • The heating and cooling of water cause elements within the central heating system to expand and contract. This can allow tiny air bubbles into the HVAC.

The good news is that regardless of the cause, trapped air is easily treatable at home by bleeding the radiator.

how to bleed a radiator

Bleeding a radiator is quick and easy, and only requires two items: a radiator bleeding key (or a flat-head screwdriver) and an old towel.

  1. Turn off your central heating and wait until your radiators are cool
  2. Locate the bleeding valve on the top of the radiator and open the valve (turn anti-clockwise) using your bleeding key or screwdriver
  3. Close valve once water starts dripping out
  4. Check boiler pressure and readjust if necessary. Releasing air from the system can reduce water pressure

Once you’ve bled the radiator, try turning the central heating back on. With any luck, you will have fixed the issue of trapped air and your radiator should heat all the way through.

Note: if you notice dirty water whilst bleeding the radiators, you should read until the end of this article and treat your radiators for sludge build-up.

If you still have cold radiators on top, the second most likely cause is a build-up of sludge in the top half of the radiator. This however is uncommon as sludge normally affects the lower half.

Treating a sludge build-up (prevent further damage)

A sludged-up system is not viewable from the naked eye, but over time it will continue to build up and clog up the system. This can cause further damage to the system and should be treated with urgency.

If left untreated, sludge can block radiator valves and pipes, and even cause corrosion to the boiler itself leading to the need for a replacement.

Signs of a central heating system suffering from a sludge/magnetite build-up:

  • Radiators have patchy areas of hot and cold spots
  • Radiators need to be bled more regularly than yearly
  • Radiators don’t get up to full heat despite the boiler being on and thermostatic valves turned to max
  • Dirty water is produced whilst bleeding the radiators

The best way to test your system for a build-up of sludge is to bleed the radiator and look for dirty water. Whilst sludge is more commonly formed on the bottom half of radiators, it can form anywhere so could be causing radiators to remain cool at the top.

methods for removing sludge

There are three main methods for removing sludge from a central heating system: A power flush, manually cleaning radiators, or replacing radiators.

A power flush

A power flush will thoroughly clean a dirty system and requires a power flushing machine. A machine is connected to your system which pushes a powerful, low-pressure flow of liquid through your pipes and radiators.

Powerflushing takes several hours but this can be much longer if you have a large house or if there is a significant build-up of sludge.

Powerflushing can be done at home or by a professional plumber. If you choose to power flush your radiators at home, there are many options for renting power flush machines which cost around £50 per day.

A plumber normally charges between £300 and £800 for the service depending on where you’re based.

One potential risk to a power flush is damaging the system and causing leaks, and this risk is higher for older pipes.

manually cleaning radiators

The cheapest way to remove sludge from your radiator is to manually clean the sludge from the radiator yourself. This is a relatively time-consuming task as the radiator needs to be removed from the wall, cleaned with a garden hose, and then refitted again.

  1. Turn off the central heating and allow the system to cool
  2. Turn off valves on radiators (thermostatic and lockshield)
  3. Drain the radiator ensuring a bucket is placed beneath the thermostatic radiator valve to collect water
  4. Remove radiator
  5. Tilt the radiator to remove excess water and sludge (in the garden!)
  6. Place the hose pipe in the pipe inlet and turn it on. Keep running until the water runs clear
  7. Refit the radiator

Whilst this is the cheapest option, you may struggle to remove the most stubborn sludge due to the restrictions of a garden hose.

Replacing the problem radiator

If your problem is limited to one radiator it may be worth considering replacing the radiator with a new one. This would also come with efficiency improvements.

A single-panel radiator costs between £100 and £200 and can be fitted easily and quickly.

Check out this video on what it takes to replace a radiator:

Replacing a radiator

Final thoughts

The most likely cause of a radiator remaining cool at the top is trapped air, which can easily be dealt with by bleeding the radiator. This should be attempted first and demonstrates good HVAC maintenance.

A sludge build-up can also cause cold patches on radiators, and although slightly more time-consuming, it isn’t too tricky to fix.

other articles you may be interested in

How To Repair A Leaky Radiator Valve Quickly & Easily

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Hi, I'm Ed, and I run BuildFanatic! I enjoy providing the best possible information on a range of home improvement topics.

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