Radiator cold at the bottom

Radiator Cold at the Bottom Hot at the Top: How to Fix

Last updated on January 23rd, 2024

When you have a radiator cold at the bottom but hot at the top, it’s not the end of the world as you are still getting some heat in your room.

It will not be giving out as much heat as it could be but it will only get worse if left alone. A radiator not heating up at all is a lot worse.

I have been a full-time heating engineer for over a decade and have fixed my fair share of cold radiators.

Here I’ll go through what to do based on my on-the-job experiences over the years.

Why is a Radiator Cold at the Bottom?

A radiator being cold at the bottom but hot at the top is almost always because of sludge building up in the bottom from a dirty central heating system.

Sludge removed from radiators
Sludge removed from radiators

Iron oxide builds up in central heating systems over time and collects in the bottom of radiators and in parts in the boiler which will eventually cause problems in both.

Sludge collecting in boiler parts is the number one cause of boiler breakdowns but thanks to central heating magnetic filters it’s getting a lot better.

How to Fix a Cold Radiator at the Bottom

Sometimes it’s better to replace the radiator if it’s really old and rusty or not in good condition, or sometimes it’s full of solid sludge that can’t be removed.

rusting leaky radiator
Rusting leaky radiator

Most of the time you can remove the sludge to fix a cold radiator at the bottom.

Here’s how:

  • Flush the radiator
  • Flush the heating system

How to Flush a Radiator

If you only have one radiator that’s cold at the bottom, you can remove the radiator and flush it.

Here’s how to flush a radiator:

  1. Drain the heating system or single radiator
  2. Remove the radiator
  3. Take the radiator outside
  4. Connect a hose to the radiator
  5. Turn the water on and flush the sludge out
  6. Refit radiator
  7. Refill the radiator or system

How to Flush the Heating System

Typically, when one radiator is cold at the bottom and full of sludge, it means other radiators will have sludge in so flushing the heating system is often the best solution.

Drain down hose on drain off valve

Here’s how to flush a heating system:

  1. Add a chemical cleaner to the heating system
  2. Turn the heating on
  3. Turn the radiator temperature to the maximum at the boiler
  4. Leave it on for a couple of hours
  5. Turn every radiator off apart from the one that’s cold at the bottom by closing one valve on every radiator
  6. Tap the radiator with a rubber mallet repeatedly all the way along the bottom to loosen the sludge
  7. Repeat on any other problematic cold radiators
  8. Open all the other radiator valves and wait for the radiators to get hot
  9. Drain the system completely
  10. Fit a magnetic system filter
  11. Add some inhibitor
  12. Refill the system with fresh water

New Radiator Cold at the Bottom Not Sludge

A new radiator that’s cold at the bottom will almost certainly not be from sludge.

The most likely cause of a new radiator being cold at the bottom with no sludge in there is either an airlock or it’s piped up incorrectly. This could be undersized pipes or pipes on the wrong side.

Pipes on the Wrong Side

Some radiators have to have the water flow going in one specific side and coming out of the other, especially on column radiators. It will usually have stickers on saying inlet and outlet, or flow and return.

Manual radiator valve
Inlet sticker on column radiator

To fix this problem, the radiator pipes will need to be switched around or you can flip the radiator around on some double-panel column radiators.

Undersized Pipes

When a new radiator isn’t getting heated properly, it could be caused by undersized pipes and commonly when a radiator has been added to a system with microbore (8mm,10mm) pipes.

If you add a new radiator to your heating system, you must connect the pipes for the new radiator back to the 15mm or 22mm pipes. The old small pipes must be connected to bigger pipes somewhere.

You can add a new radiator to a 15mm pipe system from the nearest pipes but if you connect to old 10mm pipes the new radiator will most likely not get enough heat.

This can cause it to stay cold at the bottom or top and can cause cold spots everywhere in the radiator.

Magnetic Central Heating Filters

Central heating magnetic filters are great for keeping your system clean after flushing it. They are meant to be fitted on all new boiler installations after flushing the system.

Magnaclean Pro 2 Fitted
Magnetic filter fitted

Once your system is cleaned and the sludge removed, a magnetic filter will collect the iron oxide and sludge that builds up and passes through it. This filter can then be isolated from the system and cleaned out.

This stops sludge from building up in radiators and boilers and extends the life of your boiler.

Helps to Flush the System

Fitting a magnetic filter after flushing the system will collect the sludge that’s left in there as you probably won’t get it all out after flushing it.

You can check and clean the magnetic filter every week or so after flushing it to get as much sludge out of the system as possible.

how to clean a Magnaclean filter


A radiator that’s hot at the top but cold at the bottom will most likely be caused by sludge in a dirty central heating system.

Flushing your system or removing a radiator can be very difficult, especially if you haven’t done anything like this before but if you flush your system, make sure you fit a magnetic filter!

Feel free to ask me any questions in the comment section below and I’ll try my best to help.

Please share this post if you found it helpful.


  • Steven Reid

    I am a full-time plumber and Gas Safe registered engineer. I incorporated Housewarm Ltd. in 2011 to provide heating and plumbing services to homes in Newcastle upon Tyne, England. I now blog about what I've learned over the years to help DIYers and plumbers.

    View all posts
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