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How to fix a radiator that's cold at the bottom or top

Here we outline some of the most common problems that could be the cause of a cold, ill-functioning radiator and how to fix the issue.

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Simply answer a few multiple choice questions about your existing boiler and we’ll give you a fixed price for a new boiler including all parts and installation[28]

Occasionally you may find that there are patches in your radiators that aren't heating up. This is normal even when your boiler and radiators are in good operating condition, and can usually be resolved yourself. The solution will vary depending on whether the radiator is cold at the top, in the middle or at the bottom.

Cold patch at the top of the radiator:

If your radiator is cold at the top and hot at the bottom, there is most likely trapped air in the system. This can be resolved by simply bleeding the radiator to release the trapped air. Follow our step-by-step guide on how to bleed a radiator here.

Cold patch in the middle of the radiator:

If your radiator is cold in the middle, there could be a buildup of rust and sludge trapped inside the system. This is easily fixed by cleaning out the radiator and removing all unwanted substances that are obstructing the bottom length of the radiator. You'll need to remove the radiator and flush it out to clean it. In some cases, your radiator will need replacing which should be done by a registered gas engineer.

If you have an open-vent system which is un-pressurized and tank fed, you will be able to use a sludge remover that can be bought from most DIY stores. To flush the system out, you can either add the liquid to the feed and expansion tank, or alternatively you can attempt to empty and refill the system after a few days.

If you have a pressurized system, you'll need a Gas Safe engineer to help you.

Cold patch at the bottom of the radiator:

Radiators that are cold at the bottom and hot at the top can be caused by a build-up of scale, rust and sludge over a period of time. A mixture of these materials can cause a blockage and obstruct the flow of water. To rectify this, you'll need to flush your radiator. This should lead to the release of the debris, meaning the flow of water is restored.

As with the middle of your radiator, if you have an open-vent system which is un-pressurized and tank fed, you will be able to use a sludge remover to flush your radiator. However, if you have a pressurized system, you'll need a Gas Safe engineer to help you.

If you experience persistent cold patches in your radiators, contact us on 0203 805 4581 for advice on what to do next and if necessary we'll arrange for one of our Gas Safe engineers to visit you.

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[28] Exclusions apply. See

[79] Online discount is available to domestic gas and LPG customers who receive an online quote for all boiler brands excluding Ideal Logic HO18 & HO24 & like-for-like system swaps on the Ideal Logic HE24 & HE30, Ideal Logic HO15, & HO12 boilers. Online discounted price is only available for the duration of the 14-day quote validity which starts from the day the quote was given. Offer is not available to Help-Link UK Ltd or HomeServe employees.

Online discount can be applied as follows: All boiler manufacturer types on like-for-like system swaps excluding the following boilers: Ideal Logic HE24 & HE30, Ideal Logic HO15, HO12, HO18 & HO24, are eligible for a £50 discount and then 5% discount off the final quoted price. Conventional to combi boiler swaps also qualify for an additional £100 discount after 5% initial discount is applied. This offer cannot be used in conjunction with any other offers. Help-Link UK Ltd have the right to withdraw or change the offer at any time. Online discount is automatically applied when the customer gets a quote using the online quote system at and as long as the customer meets all the above criteria.