Why is My Radiator Cold at the Bottom?

Heating radiator

Radiators are low-maintenance household appliances. Generally, they don't require rigorous upkeep, but they do need a little care and attention from time to time to make sure that they’re working their best. When we neglect them completely, it’s common for them to take longer to heat up and get less hot than usual, with patches that are cold to the touch (even when they’ve been switched on for a long time).

It's easy to fall into the trap of believing that because hot air rises, it is okay to have cold radiator bottoms. While this is true for hot air in general, it's not how radiators work. When the heating is turned on, hot water exits your boiler and enters a network of pipes that connect to your radiators. The hot water circulates around your radiators, where it flows along channels that run sideways and up, then down and out through the exit piping. This process ensures that the whole surface of the radiator is constantly in touch with hot water.

A cool area at the bottom of your radiator can indicate that something is wrong with your heating system, and should be looked at straight away.

First, run these checks:

  • All your radiator valves are fully open
  • Your boiler pressure is at the correct level
  • The central heating and thermostat are turned on properly
  • Your boiler isn’t displaying any error messages

If the above are all okay, then a cold radiator at the bottom usually indicates a blockage that is restricting the flow of hot water. In most cases, this restriction comes from a build-up of sludge and grime created by iron oxide compounds, limescale and other materials. These materials layer up over time to the point that no hot water can get through. If you do not fix the sludge issue, the problem can spread to your boiler and cause some serious damage

Resolving cold radiators

It is possible to resolve cold radiators on your own as a DIY project, but if you are ever in doubt or unsure of your DIY abilities we recommend that you seek professional help from a plumber. There are three main ways to fix cold radiators: by manual flush, by power flush, or by chemical flush. A manual flush (also known as a bleed) can be carried out without professional help. It is the ideal solution for when a single radiator is affected.

A power flush uses specialist equipment to surge pressurised water through the system. It can get very messy and requires professional tools, so always work with a plumber for power flushing. It's the best option when several of your radiators are cold at the bottom.

With chemical flushing, a cleaning agent is run through your heating system to dissolve and sweep away the sludge and grime deposits. You should seek the help of a plumber or heating engineer for chemical flushing, which can take between an hour and a week to work its magic.

If you have installed a brand new radiator and it's cold at the bottom, run the checks we mentioned above. If everything seems okay with the boiler and valves, you might be experiencing a blockage in a different radiator or somewhere elsewhere in the system. In this case, you should call a plumber to investigate exactly what is going on.

How to prevent reoccurrence

As with most things in life, the prevention of sludge and grime is better than having to treat it later on down the line. You can add inhibitors into your heating system, which slow down the chemical reactions that cause the iron oxides to form and limit the sludgy deposits. You can also stop grime from settling by installing a filter.


Even though you can't see the build-up of sludge and grime in your radiators, it doesn't mean it's not happening. The last thing you want is poorly performing heating, especially when the winter months come and a warm home is vital to feeling happy and comfortable. If you aren't experiencing cold radiator bottoms yet, you should still perform a manual flush to make sure your system is ready for winter.

If you know that some of your radiators have cold spots, first run the simple checks listed above to troubleshoot the problem. If they are all okay and not an issue, then now is the time to invest in flushing so that you can use your heating when you need it, safe in the knowledge that it is working as efficiently and effectively as possible. For stronger, more effective flushing such as power flushing or chemical flushing, enlist the help of a professional who has the correct equipment, tools and chemicals to do the job properly (and less messily).

If you have any questions or need some extra advice, our radiator experts are always on hand to speak to — get in touch today.

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