We all know that heat rises, so having a radiator that’s cooler at the bottom than the top might not trouble you too much. But with radiators, it’s not how they should be, and could well indicate a problem that needs addressing. Here we’ll have a quick look at what might be causing your radiators to be cool at the bottom, and suggest a few solutions.

How radiators work

Hot water leaves your boiler and enters a loop of pipework that ultimately ends up back in the boiler, so the water that has cooled down can be re-heated. Your radiators branch off this loop. Each one takes hot water from the loop, circulates it around the radiators, then lets it leave on the other side, to enter the hot water loop again. Inside the radiator, the hot water is directed through channels that keep it flowing first sideways and upwards, then downwards to the exit piping, so the whole surface is covered with constantly moving hot water.

If the boiler is switched off or the thermostat temperature is reached, the boiler pump will stop, the flow will cease, and the radiators will start to cool down. As soon as the boiler fires up again, they will start to warm up again.

Coldness at the bottom

If your radiators are cold at the bottom but hot at the top, this can only mean that the flow of hot water is somehow being restricted or redirected so it’s not reaching the whole of the radiator. Often with a cold-bottomed radiator, it’s still hot around the water entry and exit points at the bottom, so the restriction is usually in the middle and at the bottom.

The culprit in 99% of such cases is a build-up of sludge and grime. Radiators are made of steel or iron, so iron compounds will gradually be created as the water passes over it. Magnetite and haematite (two forms of iron oxide) are the biggest problems, but there can also be other impurities in the water like limescale that can cause grime to accumulate. Once a small accumulation has started, it’s easy for more material to add to it and it quickly multiplies.

As soon as the sludge has grown enough to block off one of the flow channels inside the radiator, that channel will no longer let hot water through it, and that accounts for the coolness in concentrated areas. If several channels are blocked, the whole of the bottom part of the radiator will not be getting any hot water. Any residual warmness in these parts of the radiator is coming from the radiator’s metal being warmed, and perhaps a small amount of warming from the sludge.

Fixing radiators that are cool at the bottom

There are three ways of solving this problem: chemical, physical cleaning and a power flush.

Adding cleaner

A chemical can be added to the radiator system that will clean up the sludge, just like you’d pour drain cleaner down the plug hole. It’s a one-off job – you add the cleaner to the system, let it warm up and run for an hour or so, then flush it out and replace the water. In particularly severe cases, the cleaner can be left in for up to a week, but once it has done its work, it needs to be removed and replaced with fresh water. Cleaning this way is probably best left to a professional central heating engineer, so we won’t go into detail on how it’s done.

Cleaning a single radiator

Removing and cleaning a radiator isn’t too hard if you’re a relatively accomplished DIYer. If only one radiator is affected, this could be all you need to do.

  • Start by isolating the radiator. If you have a thermostatic radiator valve (TRV), turn this down to zero. At the other end there will be another valve called a lockshield valve, which is probably covered with a plastic cap. Close the valve with a spanner, but note the angle that you have to turn it – it’s probably somewhere between a quarter and half a turn. That’s how much you’ll want to open it later. If you don’t have a TRV, you’ll need to turn off both valves with a spanner.
  • Put an old towel or rags down underneath the connector nuts, and place a container such as a bowl or empty ice cream tub on the towel.
  • With a spanner, turn the radiator nuts slightly. A small amount of water might drip out.
  • Using a bleed key, open up the bleed valve at the top of the radiator. Air will now be let into the radiator, and water will start pouring out near the loosened nuts. You will probably need a bucket to hand, as the water will fill more than one tub.
  • Once the water has stopped flowing, fully undo the valves and lift the radiator off its brackets.
  • Take the radiator outside, attach a water hose to one end and blast water through it for a few minutes until it flows clean. You can try the hose in different openings to make sure.
  • Re-hang the radiator on the brackets, re-attach the pipes at the nuts and turn both valves back to the positions they were at. Water will start re-filling the radiator, so get ready with your bleed key and close the bleed valve as soon as water squirts from it.
  • If you have a pressurised system, you’ll probably need to add more water to the loop to bring it back to pressure. If it’s a conventional feed and expansion setup, it will re-pressurise itself.

Power flush

A power flush is basically the same as the operation above, except you don’t remove the radiator – you force high-pressure water through the system to clean it out. This cleans all the radiators, not just one. Again, it’s a tricky job that needs specialist equipment, so leave it to the professionals.

Prevention of sludge build-up

It’s much better to prevent sludge from building up in the first place. The easiest way to do this is to put inhibitor into the system. It slows down the chemical reactions that cause the iron oxides to form, so you’ll get much more life out of the system. You can also install a filter in the loop, which takes out grime before it gets a chance to settle and accumulate.


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