Radiator not heating up

One radiator not heating up? There could be several reasons for it. Some of these issues can be resolved on your own, while others may require professional assistance. Read more for details.

-So why is the radiator not heating up?
–Stuck thermostatic radiator valve (TRV) pin.
–Is the radiator turned off
–Radiator full of air (needs to be bled)
–Radiator not heating up air lock in pipework
–Blockage in pipework
–Radiator not heating up due to sludge
–Balancing the radiators is required
–What causes radiators not to heat up in summer
–My radiator doesn’t heat the room
–Radiator not heating up after bleeding

If only one radiator doesn’t heat up when the rest of the heating does work then you can discount several things. It probably isn’t the electrical controls, it probably isn’t the diverter valve and it’s probably not the boiler itself. So what is it?

So why is the radiator not heating up?

The reason a radiator doesn’t heat up when the rest of the system does is simple. The hot water from the boiler isn’t being circulating down the pipe to the radiator then back to the boiler. The most common causes are:

Stuck thermostatic radiator valve (TRV) pin.

Difficulty rating: Easy but potentially messy if it leaks

This tends to happens after the summer, the TRV head is set at a low temperature (eg frost). TRVs are spring loaded so want to be open. As the temperature is high for many months the head constantly pushes down on the valve keeping it closed.

When the weather changes and the radiator is turned on the valve has been closed for so long that it is stuck in the closed position. The spring that opens the valve isn’t strong enough to open it so the radiator stays off.

Radiator not heating up as the pin of a thermostatic radiator valve can get stuck shut.
The pin of a thermostatic radiator valve can get stuck shut.

To solve this issue you must assist the spring in reopening the valve.

Take off the valve head and try to gently pull the pin up. You also need to to gently tap the radiator valve body (attached to the pipe) with a hammer or spanner to release the stuck valve.

Do this with the heating turned on. You will know that the valve is released when the radiator begins to heat up.

Is the radiator turned off

Difficulty rating: Easy

This is a surprisingly common fault. Both valves need to be turned on for the radiator to work. The TRV needs to be turned up and the other valve (the lockshield) needs to be turned on as well.

Take the cap off the lockshield which may require a screwdriver. When you have the cap off turn the valve anti clockwise to open it.

Some radiators have been left partially closed in the past to balance the heating system. It may be that it used to work in that position but a year later it is too closed to work.

Radiator full of air (needs to be bled)

Difficulty rating: Easy

Feel along the bottom of the radiator to see if it’s hot. If the bottom is hot and the rest of the radiator is cold there is a good chance it’s full of air.

Slowly open the bleed vent at the top of the radiator to let the air out.

If you have a sealed system (there is a pressure gauge and filling loop somewhere on the system) you will need to top the pressure back up again.

Radiator not heating up air lock in pipework

Difficulty rating: Easy

This is different to air in the radiator. You can’t bleed a radiator to clear an airlock.

The easiest and most DIY friendly method is to turn off all the other radiators that you can access. This concentrates the force of the pump on one radiator instead of all them. This normally pushes the air into the radiator which clears the airlock.

I listen for a gurgling bubbling noise which tells you the air has moved. The radiator should heat up soon after that.

Blockage in pipework

Difficulty rating: Hard and potentially very messy

If you are confident any airlock has been cleared then that leaves a blockage in the pipe.

This usually happens on smaller diameter pipe (8mm and 10mm) on heavily sludged systems.

Repairing blocked pipework is more involved than a DIY job so I would recommend getting a professional plumber in.

To prove the pipe is blocked (again not really a DIY job and has a very high risk of leaks) turn off the valves on either end of the radiator.

Loosen the valve on one side to take the pressure off then take the valve off from the end of the radiator.

Attach a pipe to the valve and turn it on. If there is a steady flow of water (make sure the pressure stays high enough on the boiler) that side is not blocked.

Now attach a hose to the tail of the radiator and turn on the valve on the other side. This time there should be a steady flow of water from the radiator tail. If the flow slows down or stops the pipe is blocked.

Pipes can be unblocked either mechanically by poking a piece of wire in or by disassmbling any available joints.

You can also use a pressure bottle to pump high pressure water through the pipe to push the blockage away into a radiator.

Both methods have a high risk of causing leaks.

The final technique is to remove the blocked pipe and repipe with new clear pipework.

Radiator not heating up due to sludge

Difficulty rating: Hard and potentially messy

This shows itself as cold spots on the body of the radiator. A thermal image camera shows the difference in temperature across the panel. The radiator doesn’t heat up evenly across the front panel. To clear it remove the radiator and flush it out using a hosepipe into a foul drain.

Radiator not heating up due to blockage with sludge
Radiator not heating up due to blockage with sludge

You can help to remove the sludge by tapping the radiator with a rubber hammer.

This is quite an advanced DIY job so don’t take it on unless you are really confident and experienced.

Balancing the radiators is required

Difficulty rating: Easy but can be very fiddly

Balancing is required when the boiler is ignoring one or more radiators in favour of the closest radiators. The boiler and pump don’t care which radiators the heat is going to all they want is for the hot water to go out then come back.

To test if a radiator needs to be balanced turn off all the other radiators in the house. If the radiator heats up then the pipes aren’t blocked or airlocked.

Balancing is a delicate job involving turning all the radiators off on the lockshield then slowly turning them on again with the minimum amount of flow to heat them up.

Too little flow and the radiator stays cold. Too much and you risk not leaving enough for the rest of the heating system.

Most modern systems in smaller houses don’t need balancing. However most older, bigger houses do.

What causes radiators not to heat up in summer?

This can be caused by the thermostat on the wall being set too low for the warm weather.

Thermostatic radiator valves can be set too low for warm weather. Turn them up to maximum to see if that helps.

Thermostatic radiator valves can be stuck in the off position after a summer of being closed.

The diverter valve in the boiler can be stuck in the hot water position after a summer of not being used. Look at the boiler to see if it is firing without sending the heat out to the radiators. It may look like the boiler is on but not heating the radiators. The heat could be trapped inside the boiler. Is there a demand light on the boiler or thermostat. If there is but no heat in the radiators it could be a fault with the diverter.

Wireless room thermostats can lose their connection with the receiver. Look for a red light flashing on the box near the boiler.

My radiator doesn’t heat the room

Difficulty rating: Easy

The main reasons for a radiator not heating a room are:

  • The radiator is too small.
  • The heating isn’t being left on long enough.
  • Is the radiator clogged with sludge.
  • The central heating flow temperature is too low.

If the radiator has never heated the room or is apparently too small you may need to have a larger radiator fitted. Heat loss calculators are available online. Use a calculator to find the correct size radiator for your room.

To heat a room in cold weather the heating has to run for a reasonable amount of time. Expecting a building to heat up from cold to hot in a few minutes is a bit unreasonable. You need to give it some time to work.

You can very easily increase the temperature of the water going to the radiators by turning it up at the boiler. All boilers have controls to set the heating flow temperature. Its normally an obvious control knob but there can be buttons or menus to navigate. Check your boiler manual for advice.

There is currently a trend for reducing the flow temperature to increase the efficiency of condensing boilers. The down side of this is that radiators may not emit enough heat when the flow temperature is very low. In very cold weather you may have to sacrifice some efficiency to properly (and quickly) heat your house.

Radiator not heating up after bleeding

Difficulty rating: Easy

Have you checked the pressure in the boiler? Bleeding air from the system reduces the pressure which can causes a fault on most modern boilers.

If the pressure is lower than 0.5bar you may need to top it up using the filling loop.


Repairing a radiator can range in difficulty from simple to messy and potentially complicated. However, diagnosing the problem is typically easier than solving it.

You can save a lot of time and money by attempting the easy fixes. If you are inexperienced it may be better to leave the more complicated jobs to professionals.

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