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 heating bills are on the rise? What could the problem be? Has your central heating system failed you?

A cold radiator is always unpleasant. Luckily, we know the culprit behind your cold-bottomed 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.

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 since heat rises up. So, if this is the case, you don’t need to worry. However, if one or all of your radiators have cold spots at the bottom, then there is an issue.

If the bottom of your radiator is cold, 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 you’ve had new boilers and radiators fitted, sludge is most likely not the culprit. If they are still cold at the bottom and hot at the top, 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 manually flush the heater to get rid of it. First, though, you need to figure out which cold radiator 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 you have a pressurised system, 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 radiators to remove sludge 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:

Time needed: 1 hour.

  1. Turn off your heating system.

    Wait for it to cool down before proceeding.

  2. Lay down sheets or an old towel.

    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.

    Find the bleed valve and let the trapped air out. Then, drain your radiator and 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, your radiators 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 corrosion 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 boiler filter will also benefit your radiators. A magnetic filter captures all those dirt and rust particles and stops them from roaming around your central heating system. Having this installed on a new boiler can prevent you from dealing with cold patches caused by sludge in the first place.

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 completely 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 one of your heaters is cold, but the other radiators work fine, then the issue is most likely contained to the cold one. Check if the valves on both sides are open.
  • Only downstairs radiators are cold.
    Another problem may be that only your downstairs heaters are cold, while the upstairs radiators are heating up, or vice versa. If this is the case, the fault could be in your central heating pump.

Do you have no heating, but the hot water is running? There might be another issue, such as a stuck diverter valve. If your radiators are only cold at the top, then they might simply need a bleed to release the air.

In the interest of safety, always trust a Gas Safe engineer to handle heating and 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, we have good news! You can just let a professional plumber handle it for you!

The expert plumbers at Fantastic Services can perform a powerflush of central heating systems and save you from that pesky sludge. They can clean out your system and add central heating 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 carrying all the specialist equipment they need to complete the job and will be sure to leave your property spotless afterwards. Rest assured, your radiators and boiler are in safe hands.

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  • 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, causing cold patches below.
  • To fix this issue, you will need to flush your cold radiators. This can be done manually or chemically.
  • If the sludge is stubborn, your entire heating system may need a powerflush. This should only be done by a professional heating engineer.
  • A magnetic boiler filter can help trap the particles that may damage your boiler and heating system.
  • If you are in any way uncertain about what to do, call a Gas Safe registered heating engineer to fix radiators.


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

Image source: shutterstock / Vasyl90, Dean Drobot

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