Radiator Cold at Bottom, Warm/Hot at Top | Here’s How to Fix It

Radiator Cold at Bottom, Warm/Hot at Top

If your radiator is cold at the bottom but warm/hot at the top, your problem is central heating sludge. Read our guide to find out the best next steps.

Why is my radiator cold at the bottom but warm/hot at top?

There’s good news and bad news. Firstly, if your radiator is cold at the bottom only, we can assume it is managing to warm up at the top?

If this is the case, great, it’s quick to troubleshoot: Your problem is central heating sludge.

The bad news is, the length you need to go to in order to sort your sludge could vary, depending on how stubborn the sludge is, how old your system is, how well it’s been looked after and how many of your radiators are cold at the bottom?

In this article, we will cover:

  • What is central heating sludge?
  • Why do you have central heating sludge?
  • Why does it make your radiator cold at the bottom only?
  • How do you fix a radiator that is cold at the bottom: simpler fixes
  • Stronger fixes for stubborn sludge
  • Preventatives for the future

If however, your entire radiator is cold – this isn’t the article for you. Check out our blog for common central heating troubleshooting and especially this one.

What is central heating sludge?

The main causes for central heating sludge are limescale and dirty water.

Black Sludge

Simply put, sludge is a build-up of product in our central heating system. Naturally, chemical reactions are happening all the time in our heating system. These occur, because the water we use isn’t completely pure, it has minerals in it.

The radiators we have in our homes are made out of metals that eventually begin to react, like steel. The more rust that is produced, the more internal parts of your heating system (radiators, pipes etc) start to breakaway and move round and round your heating system, building up into a thick, black sludge.

Black sludge is also known as Magnetite but it is just a posh word for rust.

Limescale builds up in a similar way due to impurities in the water. It is usually worse in areas with harder water, because there are more impurities in it.

Brown Sludge

Got brown sludge? This is usually something a little different. It is less common and can only occur if there is a foreign body in the system which produces bacteria. This usually comes in via fresh water (a leak), poor ventilation or header tanks which haven’t got a properly fitting lid.

Why do you have central heating sludge?

Sludge will occur in all central heating systems over time, but for it to cause problems as severe as this, i.e. your radiator being cold at the bottom and not heating up properly, it means your system has gone some time without being treated/flushed and there are no central heating magnetic filters fitted in your system or your filters are not working properly.

What makes your radiator cold at the bottom?

As we discussed earlier, the black sludge, in particular, is essentially a build-up of rusted tiny bits of your interior pipes that is moving round and round your system over a long period of time.

Just like the name suggests, sludge isn’t particularly light. As it becomes heavier than water, your system struggles to keep it moving through the system and eventually the sludge will fall to the bottom of your radiator and stay put.

With the sludge settling there, your radiators will struggle to heat up, because there is little to no hot water in the bottom of the radiator.

It isn’t just at the bottom of your radiator though. These same bits of sludge could be blocking valves or pipework elsewhere in your system, making your entire system less efficient.

Fixing old or new radiator cold at bottom problem

Step one

Locate the radiators that aren’t working properly.

Things to consider: is it just one radiator that is cold at the bottom? Are all your radiators cold at the bottom? Is it all upstairs? All downstairs? Could one radiator be stopping the rest from working and causing a blockage?

Step two

Have you been regularly putting central heating inhibitor in your system? It’s not always up to you to do this. If you pay a trusted engineer to do an annual service, they should be checking inhibitor levels.

If you don’t have a service, it’s worth getting in touch with PHS to start a regular preventative maintenance routine. Otherwise, a recommended brand would be Magnaclean – these work like a magnet, pulling off loose pieces of rust within your system … and… once this is done, you can do step 3, which is to flush it all out.

Step three                   

Flushing out the system can be done by a professional or by yourself.

To do it yourself, you will need to drain your central heating system and remove the affected radiator. Take the radiator outside / somewhere you don’t mind getting wet.

Put a normal garden hose into each side of the radiator and let it run for some time. While this can cause a bit of mess in your garden it can work a charm for radiators that have been hit by a long term build-up.

More stubborn sludge, or sludge that’s got in places that can’t be reached as easily will require something called a power flush. It can cost anywhere between £400-800 depending on the size of your system.

Step four

Sometimes not all radiators are worth saving. If the radiators are very old, or if you have one in particular that is causing an issue – it may be time to replace that radiator.

It isn’t a particularly costly fix and could save you money due to the fact your older ones are probably not very efficient anyway, and if you think the radiator is very old or you live in a very hard water area, the chances are the sludge isn’t going to be pushed out even with a power flush.

We’d just recommend a new radiator. If it’s the whole house that’s struggling, we would try step 2 and 3 first though, unless you’ve got the budget for all new radiators.

Can I prevent this from happening again?

Absolutely. At PHS we maintain than preventative maintenance of your heating system is one of the best investments you can make.

The best route to take to stop blockages occurring is to arrange regular / annual boiler services – central heating service plan.

However, there are also some added extras that can be installed to prevent sludge build up as well.

  • Scale reducers – a scale reducer will catch any bits of limescale that are going around in your heating system. When the inhibitor breaks down the limescale, the reducer can then catch it and prevent it ending up in the bottom of a radiator.
  • Boiler filter – this works in a similar way, but it catches all other mineral deposits, such as rust or debris. The filter can then hold all of these deposits until your next boiler service where your heating engineer can empty them for you.
  • Draining your system – did you know that it is recommended that you drain your central heating system every 5 years? You should use specialist chemicals to do this and you should follow the instructions closely.

Still suffering from radiator cold at bottom?

If you’re still having difficulties with cold radiators, or you’re not sure if you’ve got the right problem, give PHS a call.

We can provide support in London, Hertfordshire, Kent, Essex and Suffolk, getting an engineer out to you within 24 hours to locate your issue and get your home warmed up quickly.

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