Why is One Radiator Cold When Heating is On?

Single radiator in empty room

There is nothing worse than popping your heating on before you leave the house and expecting to return to a warm, cosy scene – only to come back to a freezing cold room because one of your radiators has failed to heat up. Central heating systems should heat through all radiators when turned on, as if one or more aren’t heating sufficiently, the others have to work harder to maintain temperature and the overall performance of the heating is impaired. If you have this issue and are wondering why one radiator is cold when the heating is on, even if others are working perfectly, read on, and hopefully we can help diagnose – and resolve – the issue!

Why is one radiator cold?

There are several reasons why one radiator may be cold when your heating is turned on, even if the others all seem to be working fine.

The first – and perhaps most obvious – cause is that your radiator has been turned off. This is more easily missed in the case of electric radiators, as they’re often not all running through the same system and can be individually turned on, turned off, or adjusted. However, radiators working through gas central heating can be easily turned off or down with the use of a valve, so this should be checked as your first port of call… and turned on or up if it’s off! These valves do sometimes get stuck, or can loosen, and if you have a toddler or small child you may find it’s changed more often than you’d like.

If the radiator isn’t entirely cold, but has cold patches or is giving off less heat than you’d like or expect, then it may have air trapped inside it that has passed through the system.

If the radiator is cold throughout, or feels warm or hot in the adjoining pipes but cold through its body, the cause of the fault is likely a build-up of ‘radiator sludge’ - debris and dirt sitting in the bottom blocking the hot water from passing through in the way it needs to for proper heating.

If you live in a house that spans more than one storey, and only the radiators downstairs are cold, this is likely down to a balancing issue. This is simply where the hot water in your central heating system rises, and so heats the radiators on upper storeys of the house first without quite making it back down.

If none of these situations answer your question of why one radiator is cold, then it’s time to call in a professional heating engineer to seek expert help.

How to fix a cold radiator

How to fix a cold radiator depends on what’s wrong with it. Using the above, take the following actions depending on what the issue appears to be.

If a radiator is cold or has cold patches as a result of trapped air, the excess air in the system needs to be ‘bled’ out. To do this, simply use the bleed key provided by your radiator manufacturer to undo the bleed valve. This valve is usually on the top corner (or thereabouts) of a radiator. Use a cloth to loosen the valve and remove it to allow the air to flow out. There may be some fluid leaking as this happens – this is normal. Simply replace the valve when complete!

If the radiator has a build-up of debris inside it, it needs to be internally cleaned, or ‘flushed’. This can be done by a specialist plumber or heating engineer but can also be done at home with a bit of hard work. To do this, remove the radiator from the wall and take it outside to a grassy spot. Remove all valves and flush through clean water from a hosepipe until the water leaving the radiator at the opposite end runs clear. Replace all valves and pop back on the wall, and test for better performance.

A balancing issue which sees only downstairs radiators run cold while those on upper storeys heat thoroughly can be fixed without professional help. Simply close the lockshield (the small valve at the opposite end to the TRV or thermostat) and then open it just a quarter of a turn on each radiator upstairs. This encourages more heat to flow to the radiators downstairs and can help balance the system overall.

It can be frustrating and stressful to find your radiators aren’t working as you’d expect them to, but fear not – the Great Rads team are here to help! We won’t necessarily be immediately able to answer why one radiator is cold out of your system of many, but we’re always happy to troubleshoot and offer advice where we can.


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