Why is my radiator cold - common problems and how to fix them

It’s lovely when you wake up on a cold, crisp morning in a warm and cosy house, but if your radiators aren’t heating up as efficiently as they should be, you may wake up feeling a little bit chilly. There are a number of reasons why this can happen, from debris or air in the system to faulty valves. Below, you can find three of the most common cold radiator problems and how to fix them. 

When your radiator is cold at the bottom, it’s likely that there’s a debris blockage that’s restricting the flow of hot water to the whole radiator. Usually, the culprit of this particular problem is sludge, debris, rust and other bits of dirt. Over time, the metal that the pipes are made of can begin to disintegrate because they’re constantly subjected to oxygen and water. This combination causes the metal to rust and as this rust breaks off, it is carried around your radiators and pipework. If you live in a hard-water area, limescale can also build up in the system. All of these things together can cause a blockage. A small blockage can very quickly turn into a large one.

Generally, a radiator that is warm at the top but cold at the bottom suggests that the hot water is able to flow into the radiator but is getting stuck somewhere in the middle.

If it seems to be just one radiator that’s causing a problem, you can remove it and clean it thoroughly. Do this using a garden hose to remove as much dirt as you can, then refit it to the wall. If you think the problem stretches further than just one radiator, you can have the whole system power flushed. A power flush uses pressure to push water and other cleaning chemicals through the system, removing all the dirt and flushing it down an outside drain. This process is worth doing every now and then to ensure the system is running as efficiently as possible.

One cold radiator usually indicates that either there is air in the system or there is a stuck valve within that radiator.

The thermostatic radiator valve (TRV), like the one pictured below, controls the flow of hot water to the radiator. From time to time, it can seize and get stuck, meaning that it cannot open to allow the hot water in when the heating is on. This is particularly the case in an old radiator.

To check if the valve is stuck, you can remove the rotatable head on the TRV to reveal a raised pin beneath it. You should be able to depress the pin with your finger. When you release the pressure, the pin should rise back up again. If the pin is already depressed or doesn’t move very easily, then this is more than likely your problem.

You can try to free the pin yourself using some pliers and grease until you can move it in and out with ease. However, you shouldn’t apply excessive force as this may damage the pin. If you are not sure about doing this or are not able to get the pin moving, then it is advised that you seek help from a professional.

If you’ve checked the TRV and it seems to be fine, there could be some air in the system that’s causing one radiator to remain cold. Air in the system tends to gather at higher points and prevents the correct distribution of heating water. Luckily, this particular problem is usually quite simple and straightforward to resolve with the following steps:

1. Fully open all radiator thermostats and run the heating at full temperature for 10 to 15 minutes with all radiators on

2. Turn the circulation pump off and wait until radiators are cool (around half an hour to an hour)
3. Bleed the radiator nearest the boiler using a bleed key or screwdriver until water comes out
4. Repeat this for each radiator
5. Turn on the boiler and check the water pressure.

If you are unsure about bleeding the system or encounter difficulties at any point, it is advisable to contact a professional at the earliest instance.

Cold radiators downstairs can demonstrate a balancing issue. Hot water tends to rise upwards in a system, so you’ll know you have a balancing problem if your downstairs radiators are cold and the upstairs ones are hot.

If this is your issue, go to the radiators upstairs and close the lockshield (the smaller valve at the opposite end to the thermostat) then open it a quarter turn. This should not cause any reduction in the effectiveness of the upstairs radiators, but will encourage more heat to flow to those downstairs.

If this doesn’t make enough of a difference, you may need to balance the entire system. To do this, you can follow our radiator balancing guide. 

If you’ve tried to do any number of the above, but you’re still experiencing cold radiators, here are some other things you can check. 

If your radiators are cold when you’re expecting them to be warm, you should check that your boiler or heat pump is working properly.

If you have a boiler, you should check that it’s on the right setting and working properly. For a combi boiler, check that it is set for both hot water and heating, is not in summer mode and that there is no fault code displaying. While some fault codes can be rectified by the user, others may require the help of a Gas Safe registered heating engineer. Always follow the user instructions and do not attempt to do anything yourself if you are unsure.

If you have a heat pump, you should check that it’s working as it should and that the outdoor unit isn’t clogged with leaves and other debris. You should also make sure that it’s not making any strange or unusual noises. 

If your boiler or heat pump appear to be working correctly, the next thing to check is your water pressure. This is shown on a small display known as a manometer, usually located directly where the water supply is connected to your boiler.

For a residential gas boiler, the correct water pressure is usually around 1.0 bar, often indicated on the gauge by coloured markings. If the pressure is lower than this, we recommend following our guide to increase the boiler pressure.