Skip to the content
Trustpilot logo 0800 61 22 302

Why is my radiator cold at the bottom?

Radiators being warm at the top and cold at the bottom, is likely caused by something stopping the flow of water and preventing the whole radiator from heating.

Often, this is sludge.

Get a Fixed Price Boiler repair for only £125

Why is my radiator hot at the top and cold at the bottom?

A radiator that is cooler at the bottom may present you with the temptation to simply turn up the thermostat. But resist, as this will make little difference to the overall heat of your home and instead only increase your gas bill. However, it is usually the result of something blocking the free flow of water.

The water that flows through your pipes can build up limescale and rust over time. In turn, this eventually forms a black, thick sludge that lies at the bottom of your radiator. This can cause a blockage that prevents the bottom of your radiator from warming. This issue may occur in one or all of your radiators.

If you have convection radiators, radiators that are cold at the bottom may not indicate an issue at all. If your radiator is simply cooler at the bottom (but not entirely cold or with cold patches) it may well be working fine.

How to fix cold radiators

As mentioned earlier, DON'T TURN UP THE HEAT! Reaching for your thermostat will only put increased pressure on your already over-working central heating system.

To fix the issue you will need the assistance of an engineer to carry out either a chemical flush or a powerflush.

Chemical Flush

In the process of a chemical flush:

  • Chemicals are circulated around your heating system, but not at high pressure as with a power flush.
  • These chemicals eradicate the sludge, limescale and dirt that has built up over time to allow water to flow efficiently again.
  • Once that sediment has been broken down, the system gets flushed by the engineer to replace the water the chemicals have circulated in with clean water.
  • A rust inhibitor is also applied to prevent future issues. The chemicals can be left in the system before flush for a minimum of one hour up to as much as a week depending on how bad the build-up is in your system.


In a powerflush:

  • A chemical is circulated at high pressure around the central heating system via a pump. This frees any blockages and build-up of material in order for everything to flow smoothly again.
  • Alongside the pump, a corrosion inhibitor and descaler will be used to prevent future sludge build-up. This process will usually take around an average working day for the engineer to complete (6-10 hours). A power flush not only solves the problem at hand, but it also gets your whole system running at a more efficient level.

Does your heating need some TLC?

Let our experts take care of it with an emergency repair

If your radiators aren’t heating up, you may need to seek help from an engineer.

Our Boiler Plan Gas Safe experts will point out any work that needs to be carried out, they'll also provide a fixed price repair quote.

With our 24 hour emergency repair and 95% first time fix rate, we’ll have your heating up and running again in no time.

Book a Boiler Repair

Take the worry out of repairs with a Boiler Plan Care Plan

With our range of affordable boiler cover, you’ll never have to sit in the cold again. Whatever plan you choose, each comes with an annual service to ensure your system is fully maintained and ready to keep your home reliably warm.

Choose from:

  • Service PlanBoiler service every year to ensure it’s working correctly
  • Gold PackageIncludes: 3 call outs per year, annual service, carbon monoxide test.
  • Platinum PackageIncludes: Unlimited call outs, all parts and labour, repairs to the entire heating system, £500 towards a replacement boiler.
  • Boiler Plan Plus Package - For those with a Boiler still in warranty. Includes: Unlimited call outs, all parts and labour, repairs to the rest of your entire heating system.
Take out a care plan

Is it dangerous if I don’t get rid of the sludge?

Leaving sludge untreated can lead to a complete failure of your boiler and the need for a much more expensive boiler repair. However, if it does come to this, we can make the price of a new boiler less scary with one of our payment plans. Also, ensure you’re choosing a new boiler that works best for your home with our boiler buying guide.

How to stop sludge from appearing in radiators again

  • Have a magnetic filter installed - This attracts rust and sludge to avoid its build-up in radiators.
  • Use a scale reducer - Adding this to your water will reduce limescale, which is very important if you live in a hard water area.
  • Add a central heating inhibitor to the water - Adding this to the system is another way to prevent the build up of limescale and eventual sludge.

Tips for keeping central heating system in good shape

Opt for a boiler service plan to ensure that your system functions well all year round and to avoid any scary and unexpected costs. For as little as £6 a month, you can get an annual boiler service, carbon monoxide test and much more - offering complete peace of mind.

If you just want a one-off boiler service to ensure that the unit is working efficiently, you can book one with a great local gas engineer now.