Home Improvement

Why Is My Radiator Cold at the Bottom?

We all want to feel warm and cosy during the harsh winter months, right? But what if one day you wake up and find your radiator is cold at the bottom and your bills are on the rise? What could the problem be? Has your central heating system failed you?

Luckily, we know the culprit behind your cold-bottomed heaters – radiator sludge. In this blog post, we will go over why the bottom of your radiator is cold, what sludge is, and how to deal with it effectively.

Table of Contents:

So, if this sounds like you:

  • Your radiator is hot at the top and cold at the bottom;
  • You’re struggling to fix it or are looking for the reason;
  • You want to know what to do if your radiator is cold all over,

Then read on! This post is for you.

Why is your radiator cold at the bottom?

All radiators will be slightly cooler at the bottom than they are at the top, so if this is the case, you don’t need to worry. However, if one or all of your radiators are only hot at the top and completely cold at the bottom, then there is an issue.

If your radiator is cold at the bottom, that indicates there is a build-up of sludge inside. This mostly happens to older radiators, as it takes some time for it to form. Sludge is the result of internal corrosion and an accumulation of dirt inside the heating system.

The dirt particles mix with the rust and water, forming a sludge that settles at the bottom of your radiator. This obstructs the flow, forcing the hot water to go straight to the top, leaving the bottom of your heater cool.

A build-up of sludge can cause your radiators to be less efficient when heating your home, increasing your energy bills. It may even damage your central heating system if the corroded particles are allowed to roam around it for too long.

If your radiators have been newly fitted, sludge is most likely not the culprit. If they are still cold at the bottom, your system might not have been balanced properly.

How to flush radiators

If you have found the bottom of your radiator is cold because of sludge, you will need to flush the heater to get rid of it. First, though, you need to figure out which one is the source of your troubles.

Are all of your radiators cold at the bottom, or is it just the one? If sludge has settled inside the first heater in your system, it may be causing issues for the rest of them, as well.

Keep in mind that if your system is pressurised, you will need a Gas Safe registered engineer to carry out the cleaning for you.

However, if you have an unpressurised, open-vent system, you can try and flush the radiator yourself. It’s a good idea to have a helping hand there, as heaters can be heavy.

So, here is how to flush radiators that are cold at the bottom:

  1. Turn off your heating system.

    Wait for it to cool down before proceeding.

  2. Lay down old sheets or towels.

    Prepare a bucket or a washing up bowl to catch the radiator water.

  3. Turn off the valves on each side of the radiator.

    To learn how to safely shut them off, check out our helpful post on changing radiator valves.

  4. Bleed and drain your radiator.

    Collect the dirty water in the bucket you have prepared.

  5. Take the radiator off the wall.

    Move it somewhere where you can hose it down, for example, your bathroom or garden.

  6. Using a hose, run water through both sides of the radiator.

    Rinse it out until the water runs clear. Let the heater dry.

  7. Reconnect the radiator.

    Place the radiator back on the wall, connect it to the heating system, and you’re done!

Sometimes, the grime will be too stubborn to remove. If this is the case, you will need a chemical flush or a powerflush. This should only be done by professionals, so make sure to contact a Gas Safe registered engineer.

Once a technician flushes the radiator, they can add inhibitor to your system to prevent sludge from building up again. Just keep in mind that central heating inhibitor needs to be changed occasionally.

Installing a filter will also benefit your radiators. It captures all those dirt and rust particles and stops them from roaming around your central heating system.

What if your radiator is cold all over?

There are several things that may be wrong here, depending on which radiators are cold.

  • All radiators are cold.
    Are all the radiators in your home cool? If this is the case, the first thing you need to do is check if the central heating is on and the boiler is calling for heat. If everything is on, there might be an issue with your boiler or thermostat controls.
  • Only one radiator is cold.
    If only one of your radiators is cold, then the issue is most likely contained to it. Check if the valves on both sides are open.
  • Only downstairs radiators are cold.
    Another problem may be that only your downstairs radiators are cold, while the top floor ones are heating up, or vice versa. If this is the case, the fault could be in your central heating pump.

If you have no heating, but the hot water is running, there might be another issue, such as a stuck diverter valve.

In the interest of safety, always trust a Gas Safe engineer to handle boiler repairs.

Not sure how to deal with a faulty radiator?

If your radiator is cold at the bottom and you don’t know what to do, let the professionals handle it for you! The expert plumbers at Fantastic Services can perform a powerflush of your central heating system and save you from that pesky sludge. They can clean out your system and add inhibitor to it to prevent any build-up in the future.

The certified and insured technicians can deal with anything your boiler may throw at you! They come equipped with everything they need to complete the job and will be sure to leave your property spotless afterwards. Rest assured, your radiators are in safe hands.

Need a powerflush?

Find a professional plumber to take care of your radiators!

Add a valid postcode e.g. SE1 2TH
  • We’re certified:


  • If your radiators are cold at the bottom, it’s most likely caused by the build-up of sludge.
  • Radiator sludge is a mix of water, dirt and rust and is the result of internal corrosion.
  • If sludge has built up inside your radiator, it obstructs the flow of hot water and forces it to the top.
  • To fix this issue, you will need to flush your radiator. This can be done manually or chemically.
  • If the sludge is stubborn, you may need a powerflush. This should only be done by a professional.
  • If you are in any way uncertain about what to do, call a Gas Safe registered engineer to handle the repairs.


Did you find this article useful? Do you have any additional questions? Let us know in the comments below!

Image source: shutterstock / Vasyl90, Dean Drobot

5 1 vote
Article Rating
Notify of
Inline Feedbacks
View all comments
Would love your thoughts, please comment.x