Why is my radiator cold at the top/bottom?

Why is my radiator cold at the top/bottom?

One of the major ironies of central heating is that we only discover that something's wrong when we most need it. It's rare that we discover problems with our heating systems at the height of summer. In most cases, it's when the temperatures are at their coldest that we discover our central heating isn't operating as efficiently as it should be.

One of the most common problems that we all face at one time or another is that of a radiator not getting hot evenly across the whole surface area. Sometimes it can be hot at the bottom and cold at the top or vice versa. There are a number of possible reasons why a radiator might be heating up unevenly, so read on to discover the different ways you can diagnose and solve the problem.

Air in the system

Issue
Air trapped in your radiator can prevent hot water from circulating properly and stop your radiator from getting hot evenly. Trapped air will often result in your radiator heating up at the bottom but getting cooler the further you get up the radiator. The air trapped at the top stops the hot water from reaching the top of the radiator.

Solution
You need to bleed the radiator to release the trapped air. You can do this by turning off the heating system, using a radiator key or screwdriver to open the radiator's bleed valve (which is usually located at the top of the radiator). Allow the hissing air to escape until a small amount of water starts coming out and then close the bleed valve.

Buildup of sludge or debris

Issue
Over time, internal corrosion within your radiators and heating system can cause a rusty sludge or debris to accumulate at the bottom of your radiator. This can hinder water circulation and can cause your radiator to feel cool to the touch, particularly towards the bottom.

Solution
You can get rid of the sludge and debris by flushing your radiator. This involves draining the problem radiator of all the dirty water, removing the radiator and then flushing it through with clean water from a hosepipe. You can also go a step further and get your radiator professionally 'powerflushed' by a heating engineer.

Thermostatic radiator valve (TRV) issues

Issue
Your thermostatic radiator valve (TRV) could have a stuck pin resulting in the valve not letting hot water into the radiator. TRVs have an internal pin that moves up and down to allow or stop the flow of water into the rad, according to the room temperature. If the pin gets stuck in the closed position, the radiator won't heat up.

Solution
You can check the pin inside your valve by removing the valve head (often the twisty bit with the dial on) to see if it moves up and down when you press it. You can loosen a stuck pin by clasping it with an adjustable pair of grips and gently wiggling it up and down to free it off. If this doesn't work, consider replacing the valve.

Faulty radiator valve

Issue
If your radiator isn't getting hot but you don't have a thermostatic radiator valve, then it might be that your manual valve has developed a fault and isn't letting water into the radiator. If you've tried all the other suggesstions on this page to no avail then this could be the issue.

Solution
Inspect the valve and feel if the pipe leading into the valve is getting hot. This will indicate if hot water is reaching the valve but not passing through it into the radiator. Check that the valve is open. If this doesn't solve the problem, call in a professional to get their opinon on whether the valve needs to be replaced.

Unbalanced heating system

Issue
If you've got radiators in some rooms heating up but in other rooms staying cold, then your heating system might be unbalanced. This means that your boiler is not distributing water evenly to all the radiators around your home. Often when a boiler is downstairs, an unbalanced system might cause the upstairs radiators to stay cold.

Solution
You can balance your radiators yourself by switching off your heating and bleeding all your radiators. You can then go to the radiator nearest the boiler and closing completely your lockshiled valve. Then you need to open it by a quarter of a turn and also open your TRV. Switch on your heating to see if this solves the problem. If not, repeat the process on all radiators. A more comprehensive guide on this is available via the link below.

System pressure issues

Issue
If you've got a pressurised heating system then certain radiators might not be getting hot because the boiler pressure could be too low. Low pressure means the hot water will struggle against gravity and iwon't reach upstairs radiators orradiators that are a long way from the boiler. If you have a combi boiler then your heating system will be pressurised.

Solution
The ideal pressure level is approximately 1 bar when the heating is off and about 1.5 bar when the heating is switched on. If your pressure gauge on your boiler reads lower than this then you need to locate your boiler's filling loop. Open the filling loop to allow water into the system until your boiler pressure reaches 1 - 1.5 bar and then close the filling look. More details are available in the article linked below.

Circulation problems

Issue
Uneven heating could occur due to a faulty hot water pump or other component of your heating system that is stopping hot water from circulating around your system properly. If this is happening then it could explain why you have uneven heating.

Solution
You pump speed could be set too low, which means the water is being pumped around your system properly. The best thing to do is consult with a heating professional to diagnose and address problems with the pump or circulation system as soon as possible.

Blockage in the pipes

Issue
If something has happened to the pipework in your home that is responsible for delivering hot water into your radiators, you could have a blockage somewhere that is causing the problem.

Solution
This is an issue that needs to be looked at as soon as possible by a professional. They will be able to take the necessary steps to assess the situation, locate the blockage and clear it in a safe way. Blocked heating pipes should be left to professionals to avoid massive leaks and water damage.

Ben Clarke

I have over 10 years of experience researching and writing about a broad range of heating-related subjects. I understand the significance of home heating and strive to provide you with high-quality radiator content in an easy-to-understand manner.

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