home water radiators

Why Is My Radiator Hot At The Top And Cold At The Bottom?

Sep-2020 / 4 min read

Reading Time: 4 minutes

With the colder months fast approaching, it is always important to make sure that your central heating system is in tip-top shape and ready to go. Be warned though. Often, after a long period of being unused, people will find that their radiators are not heating up properly, with its tops feeling warm, while the bottoms stay cold.

If you have a radiator hot at top cold at bottom, you’ll be glad to know that there could only be a few things wrong with it and that it could easily be remedied. After reading this guide, you would ideally be able to identify what is wrong with your radiator and how exactly you can fix it!

What’s Causing A Cold Radiator Bottom?

First, make sure you are not using a convection or convector radiator. Modern radiators like these work differently because the hot water rises to the top, thus forcing the cold water down to the bottom, where it is reheated in a cycle. Otherwise, you may be dealing with radiator sludge.

Radiator sludge is a mixture of different contaminants like dirt, rust, dirty water, and limescale that settled at the bottom of your radiator. The sludge prevents and blocks hot water from freely flowing around your radiator, thus leaving some areas within your home, as well as the bottom of your radiator, cold. 

The buildup of sludge will make heating your home more difficult than usual. Moreover, this will also increase the cost of your heating bills and possibly cause irreparable or costly damages to the rest of your central heating system, including the boiler.

Hence, you must make sure not to ignore your cold radiator and ensure that it is always clean and free of sludge.

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How Do I Free My Radiator Of Sludge?

There are several ways you can get rid of central heating sludge. Here is a list of the most common ways you may do so:

Manually Draining and Cleaning Your Radiator

If you’re looking to get to the bottom of the issue yourself, then you may manually drain and clean your radiator. To do this, you will have to find the affected radiator before you may begin. (Yes, the problem is probably isolated to one radiator. However, as your central heating system is all connected, a blockage on one may easily affect all the others connected to it, or even the entire radiator system). Here’s a quick step-by-step guide you may follow:

  1. Turn off your central heating system and wait for everything to cool down. You wouldn’t want to accidentally come into contact with scalding hot water from your radiator system.
  2. Prepare some towels or buckets to catch the sludge and dirty radiator water.
  3. Locate a valve on both ends of the radiator, and make sure they are closed. Remember how many times you turned the valve to close it. Loosen the thermostatic valve as well. (The thermostatic radiator valve helps regulate the flow of water into your radiator).
  4. Use a radiator key to turn the bleed valve. Doing this will release dirty radiator water and trapped air from your radiator.
  5. Once you’re sure that almost all the water has been drained, dislodge your radiator and take it outside so you can flush its insides.
  6. Attach a hosepipe to the valves or a lower radiator hose to flush any dirt and debris inside.
  7. Reconnect your radiator into the central heating system and reset the radiator. Do not forget to turn the valves as many times as you did to shut them off.
  8. Because bleeding radiators may affect boiler pressure, you may want to check your boiler’s pressure. If the boiler pressure is low, you may need to repressurise it. You can learn how to do that here.

These steps should be able to kickstart your radiator into working efficiently again.

Make Use Of Heating System Cleaners (Chemical Flush)

A heating system cleaner can quickly help get rid of the debris within your radiator and your system, thus also getting rid of the cold spot or cold patches on your radiator.
You have to introduce the cleaner to your heating system, and leave it in there from at least an hour up to a week, depending on how much sludge or debris the chemicals are up against. Once satisfied, you may start flushing and draining the system of all the broken down debris until the water runs clear.


You may also opt to have your heating system undergo a power flush. By doing this, a high volume of forced water and cleaning agents will be circulating the heating system. The powerful water flow will then forcibly remove all debris and contaminants.
Because this requires a flushing pump, the power flushing of a central heating system should be done by a professional heating engineer.

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What Can I Do To Prevent This From Happening Again?

Once you’ve gotten rid of the sludge and other debris in your radiator system, you may start taking steps to prevent or at least slow down its redevelopment, as well as keep your heating system running smoothly. Here are some ways you may do so:

Install A Magnetic Boiler Filter

A boiler filter is a powerful magnet often installed along your central heating system pipework or somewhere on the boiler. It will collect the sludge, rust, and other heavy metal debris from the circulating water before it can build up in radiators and reduce the efficiency of your heating system.

Make Use Of A Scale Reducer

Similar to a magnetic boiler filter, a scale reducer will protect your central heating system from the damage caused by hard water and limescale. If you live in an area with hard water, a scale reducer is vital to keeping your central heating system efficient.

Add Radiator Inhibitors

You will want to add this liquid chemical into your central heating system. It serves as a protective coating against corrosion within your boiler, pipes, water pump, etc.

Schedule Regular Radiator System Flushing

To maintain the warmth that your heating system provides, make sure to have it regularly checked and flushed. After all, you wouldn’t want to wait until your boiler is on the verge of breaking down again!

What If None Of The Methods Above Work?

If none of the methods above work, you and your gas safe registered engineer may want to discuss the possibility, and the cost, of getting a new boiler or a new radiator. This is especially the case if you are using an old central heating system.

Final Thoughts

Fixing a faulty radiator doesn’t always have to be so complicated! Hesitant about dealing with a defective radiator yourself? Call an expert heating engineer from Plumbingforce today!

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