Why Is My Radiator Cold At Bottom?

Are you wondering, “Why is my radiator cold at the bottom?” You’re not alone — limescale build-up and heating sludge make millions of radiators cold at the bottom all across the UK. And luckily, the fix is neither complex nor costly.

In our guide below, we’ll explain why your radiator is warm at the top and cold at the bottom, and recommend some practical solutions. Read on to learn more!

Radiators Cold at the 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 — the underlying issue is by no means fatal to your boiler. No, this radiator problem is caused by central heating sludge.

What Is Central Heating Sludge?

There are 2 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 a radiator not heating up correctly.

Limescale

Limescale comes from natural minerals in the water.

Those living in hard water areas are affected much more than those in soft-moderate areas, which have fewer minerals in the water.

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

However, even those in soft water areas can be affected; it just takes longer for the issue to manifest itself. 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 corrodes, and this rust breaks off into the water of your central heating system. The more it circulates, the more it breaks down, and eventually, you end up with thick, black, sludge. This sludge settles at the bottom of the radiators, and that’s part of the reason you get a radiator that’s hot at the top and cold at the bottom.

How Heating Sludge Makes Your Radiator 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 pipework, and even settles in the bottom of radiators.

Essentially, it’s creating a blockage.

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

But, as the bottom of the radiator is clogged up, hot water heads straight for the top — there’s no hot water circulating through the bottom of an affected radiator.

How to Fix a Radiator That’s Cold at the Bottom

#1 – Determine Which Radiators Are the Problem

Are all radiators showing symptoms of heating sludge build-up? 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, one radiator has a blockage, and consequently, every radiator after it won’t heat up.

#2 – Add Central Heating Inhibitor

To avoid sludge build-up, all central heating systems should be dosed with a central heating inhibitor. In fact, most boiler manufacturers insist on this as a precondition for their warranty.

The central heating inhibitor will help to break off loose pieces of rust and limescale within your heating system. So, instead of a gooey build-up on the pipes and at the bottoms of radiators, you’ll get tinier sludge particles that are far easier to filter and flush out.

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

#3 – Flush Out The System

Performing a power flush will give you the best chances of removing sludge out of your system.

During a power flush, a special flushing machine will inject a mixture of water an chemicals into your heating system under high pressure. The procedure can last for hours, depending on the size of your heating system. And, in the end, your boiler, pipes, and radiators should free of central heating sludge.

Unless you’re an experienced engineer or plumber, we highly urge you to contact a professional to perform a power flush. It’s a messy job, and you’ll need qualified assistance if something goes wrong.

#4 – Fit a Scale Reducer and a Boiler Filter

Now that you’ve cleared the blockage, you need to prevent it from accumulating again. 

For this, you’ll need 2 devices.

  • A scale reducer to catch limescale as it gets broken down by the heating inhibitor. You can check them out here (Fernox, Magnaclean and Sentinel are the best brands).
  • A boiler filter to remove all other deposits (rust, dirt etc) that break off within your heating system.

The filters will store all this junk, until an engineer empties it out during your next boiler service (make sure you remind them).

A Single 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. What’s more, newer radiators are 50% more efficient than ones that were built around a decade ago. So, you could end up saving money on your heating bills.

A Radiator Is Cold at the Top

If you’re in a situation when your radiator is cold at the top, but warm at the bottom, the likely culprit is trapped air. And unlike heating sludge, air is a simple enough obstruction to remove without assistance. You can read our guide to bleeding radiators to learn more about the problem and the fix.

What’s Next?

Thanks for reading our 5-minute explanation of why a radiator can get hot at the top but 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

  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. 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?

  8. 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.

  9. 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

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

  11. Hi there,
    I have a 3 storey 4 bed house with 16 radiators. I have had an extension built and added a double radiator which does not seem to be getting very warm. It’s the farthest away from tanks. Upstairs get very hot and all other radiators generally work well. Replaced grundfos pump which made the new radiator come on, but still not as hot as others. Do I need a bigger pump?

  12. Hi phil,the house we just purchased was build in 2019 we have move in 8 weeks ago 4 out of 5 radiators downstairs is only look warm i have to 2x mop bucket of smelly water out and still only look warm at the top. Would there be anything else that am not doing to get this sorted.thank you

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