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Why Your Radiators Are Cold At The Bottom [And The Fix]

In the next 5 minutes, I’ll explain why your radiators are cold at the bottom, even when they’re hot at the top.

And, I’ll tell you what you can do to fix the problem.

The Reason Radiators Get Cold At The Bottom

If your radiators are cold at the bottom, but at least a few are warm at the top, that’s a good sign.

This radiator problem is caused by central heating sludge.

What Is Central Heating Sludge?

There are two things that make up central heating sludge.

  • Limescale
  • Dirty heating water

Without filters, heating sludge can build up quickly.

But, typically it will take years (assuming filters are not fitted to your heating system) for sludge to cause severe problems, like radiators not heating up correctly.


Limescale comes from natural minerals in water.

Those living in hard water areas are going to be affected much more than those in soft-moderate areas – there are less minerals in water in soft-moderate areas.

Check this map. If you live in a hard water area, there’s a good chance limescale has become a problem.

But, even those in soft water areas can be affected.

But in these cases, the build-up takes much longer. Eventually though, you’ll find the deposits are just too strong, and radiators will only stay half heated.

Dirty Heating Water

The second component of heating sludge, is dirty heating water.

Over years, the interior of radiators, towel rails and pipework rusts.

This breaks off into the water of your central heating system.

The more it circulates, the more it breaks down.

Eventually, you’ll end up with thick, black, dirty sludge.

This settles at the bottom of radiators, causing them to be Luke-warm or cold at the bottom.

How Heating Sludge Causes Radiators To Be Cold At The Bottom

A combination of limescale and dirty water is now circulating around your heating system.

It finds its way into radiator valves, microbore pipe work, and even settles in the bottom of radiators.

Essentially, it’s creating a small blockage.

Radiators will naturally get slightly hotter at the top as heat rises.

But, as the bottom of the radiator is potentially blocked, it heads straight for the top; there is no hot water sitting or circulating through the bottom of the radiator.

How To Fix Radiators That Only Get Hot At The Top

#1 – Determine Which Radiators Are The Problem

Are all radiators showing symptoms of heating sludge? Or, do your radiators work downstairs, but not upstairs?

The first step is to determine which radiators are the problem.

In some cases, it’s a singular radiator that won’t heat up. In other cases, it’s a singular radiator that causes a blockage so that and every radiator after it won’t heat up.

#2 – Add Central Heating Inhibitor

All central heating systems should be dosed with central heating inhibitor. 

In fact, most boiler manufacturers insist on this, otherwise their warranty is void.

Central heating inhibitor will help to break off loose pieces of rust and limescale within your heating system.

You can check out the different inhibitors here (Fernox, Sentinel and Magnaclean are all well known brands).

#3 – Flush Out The System

You don’t need a full on power flush to get rid of central heating sludge. A hot flush with a hose is usually enough to get rid of the majority of heating sludge.

#4 – Fit A Scale Reducer, And Boiler Filter

Now you’ve cleared the blockage, you need to prevent it happening again. 

A scale reducer will catch limescale, that’s broken down by the heating inhibitor.

You can check them out here (Fernox, Magnaclean and Sentinel are the best brands).

The boiler filter will catch all other deposits (rust, dirt etc) that break off within your heating system.

All this “junk” will be stored in the filters. With your next boiler service, you can ask your heating engineer to empty the filters.

One Radiator Won’t Heat Up, Or Is Cold At The Bottom

Not all radiators can be saved. If you’ve got an old radiator and you live in a hard water area, there’s a chance that the heating sludge build up is just too stubborn.

Even a power flush won’t get rid of it.

In this case, find the radiator that won’t heat up, and replace it.

Economy radiators are fairly-cheap. And, newer radiators offer up to 50% improved efficiency on ones that were built around a decade ago. So, you could end up saving money on your heating bills.

What’s Next?

Thanks for reading our 5 minute explanation of why radiators get hot at the top, but are cold at the bottom.

Still got problems with radiators not heating up? Leave a comment below and we’ll get back to you as soon as we can.

Leave a Comment:

Phil says 17th January 2018

I have all radiators hot at top but cold at bottom.
It is a new system so sludge would not be a problem
What else could it be please

    heatingforce says 19th January 2018

    Hi Phil,

    So, all news rads & boiler?

    We the radiators balanced and bled?


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