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Why Is My Radiator Cold At Bottom?

In our guide, we’ll try to answer the question Why is my radiator cold at bottom?, even when they’re hot at the top.

And, we’ll also explain what you can do to fix the problem with your radiators.

Radiator Cold At Bottom (Reasons)

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

Limescale comes from natural minerals in the 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 the 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 the 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 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.

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  1. 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
    Regards
    Phil

    1. Hi Phil,

      So, all news rads & boiler?

      We the radiators balanced and bled?

      Thanks

  2. ive replaced 3 new ones in my house and there still cooler at the bottom nearer the return valve please help ive tryed every thing even flushing it out count less times no luck

  3. We have just replaced two radiators as they were cold as they were cold at the bottom. The new radiators are exactly the same so no sludge build up. We have balanced upstairs radiators and put inhibitor in the system what could this be? We have a valiant combi boiler. thanks

  4. I had a rad in bedroom that was replaced following a power flush stays cold at the bottom the filter has been cleaned after service – the boiler bangs occasionaly how do I get that air out? its not sludge

  5. I have a radiator in the hall that is only getting warm the radiator in the living room warm at thre top cold at the bottom, pressure on boiler keeps dropping so plumber has fitted an expansion tank on wall next to combi boiler looks awful, but it hasn’t helped so he said we need a new radiator in living room in that case won’t I need a new one in hall ??? So I’ve asked not to bother with radiator for the moment as I have my doubts. Can u help please ??

  6. I’ve just moved to a place where most of the radiators are not working (there’s one where not even the pipes get hot, another one where both the entry pipe and exit one get hot but the radiator is completely cold and a third one that’s recently started working but before that not even the pipe got hot!) It seems that shutting down all radiators and then turning them one by one helped (I don’t know if putting the one that worked at max previously helped?).
    Anyway, the radiator that started working is only hot at the top. The central heating system is not in my flat but it’s communal for the whole building, what can I do in that case??

  7. How do you hot flush with hose

  8. I had my radiators flushes around 8 months ago and had new valves fit to them all. Heating was working fine yesterday. Today one of them is heating up quite hot at the top, but cold and the bottom, the other downstairs is a little warm at the top and cold at the bottom. All upstairs radiators are cold. Could it need flushing again after only 8 months?

  9. Hot water to taps too hot to keep hand under(oil fired heating of a condensing boiler) tried turning temp down at boiler ,still too hot. Any ideas.

  10. I have flushed my radiator no sludge put it back on still hot at the top and cold at the bottom please help me with ideas

  11. I have flushed my radiator and still hot at top and cold at bottom please help

  12. ive just fitted a new radiator and it won’t get hot at the bottom. there’s no sludge and system has been flushed

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