fixmyplumbing

Simple explanations for plumbing problems

Radiator hot at top cold at bottom or cold patches

Why is my radiator cold at the bottom?

A radiator that is cold at the bottom is suffering from poor circulation. This can be because of blocked or undersized pipework, an unbalanced central heating system, blockages in the radiator valves or faulty/badly adjusted radiator valve.

Why does my radiator have cold patches?

Cold patches indicate a build of sludge and dirt that has accumulated in the channels inside a radiator.

The best way to diagnose this is through the use of a thermal camera. They are simply attached to your smart phone and show an image of the radiator in the IR spectrum.

A radiator that is hot at the top and cold at bottom is generally full of sludge. Thermal camera picture of a blocked radiator. credit: https://www.test-meter.co.uk/
Thermal camera picture of a blocked radiator. credit: https://www.test-meter.co.uk/
A radiator that is hot at the top and cold at bottom is generally full of sludge. Thermal camera picture of a blocked radiator. credit: https://www.test-meter.co.uk/
Thermal camera picture of a blocked radiator. credit: https://www.test-meter.co.uk/

Blockages are obvious in the IR picture which make diagnosis very simple.

What should I do about my cold radiator?

Before a radiator can heat up hot water has to circulate from the boiler, through the pipework, through the radiator and back to the boiler.

Is the boiler working?

Are other radiators coming on? If not blame the boiler or controls.

Check the valves are open

Before you go into complicated fault finding make sure both the valves are open. I’ve been to dozens of radiators that “have never worked” which have just been turned off for a long time.

Force the hot water through

Cold radiators can sometimes just be a matter of air stuck in pipework or the radiator just not getting enough flow from the boiler. The most simple way to resolve this is to turn all the other radiators off.

By turning off the other radiators the force of the pump is concentrated on the remaining (open) radiators. This is generally enough to push any trapped air bubbles out of the pipework.

Wait for 5-10 minutes if the radiator comes on as hot as the others the central heating system needs to be balanced.  See the separate page for balancing heating systems.

Check the TRV pin or head.

Thermostatic radiator valves have a pin that can get stuck in the brass lower part of the valve.

Myson trv2 pins are notorious for sticking in the closed position
Myson trv2 pins are notorious for sticking in the closed position

Take off the head of the valve by unscrewing the silver metal ring in the centre. Try to push the pin with something hard. If it springs up and down it is free. If it is stuck at the bottom it is most likely jammed shut.

The pin can be released by repeatedly banging the valve with a spanner or hammer and pushing the pin down.

Try to avoid pulling on the pin as it can become detached from the valve which makes the valve leak.

Myson trv2 pins are notorious for sticking in the closed position
Myson trv2 pins are notorious for sticking in the closed position

If the radiator suddenly starts to work when the plastic head is removed from the TRV (particularly this type) the head can be broken.

In the centre of the white plastic head is a plastic peg. The peg moves in and out depending on the temperature of the room and the setting of the valve.

When the head breaks the plastic peg is constantly out which pushes the pin down on the metal body causing the valve to be shut.

You can find a replacement on this link.

The head of this type of valve is expensive but it may save you draining the system which is also expensive.

Myson trv2 pins are notorious for sticking in the closed position
Myson trv2 pins are notorious for sticking in the closed position

**Warning** Each type of valve is different and some heads are not interchangeable with others. Do your own research before you buy a replacement TRV head.

If the radiator still isn’t coming on it may be dirt and sludge in the system.  Drain off some water from the central heating system.If its black you may need to have it power flushed.

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