Why does my radiator feel cold when my central heating is on?

cold radiator fixes

Why do radiators always decide to misbehave on the coldest of days and usually in the room you want to keep warm? Before calling out a heating engineer with a little patience and investigative work you may be able to sort the problem yourself. Firstly, you will need to identify the source of the problem.

A Central heating system is a very simple structure. A boiler (fuelled by gas or oil) is the most important part of a central heating system. Imagine a boiler as a big fire with a continued supply of fuel. When a boiler is switched on, the gas or oil enters a sealed combustion chamber and via an electric ignition system the fuel is set alight. A heater exchanger connected to a cold water pipe is then infused with fine jets of heat (around 60°C/140°F) which in effect heats the water. This heated water is then moved along the pipework system (usually hidden in newer homes) by an electrical pump. Within your home there is a continuous flow of hot water within your pipework when a central heating system is on.


What radiators are cold?

Are all radiators cold or just one? If all radiators are cold, firstly check that your boiler and central heating system are switched on. Do you have a room thermostat? Check that your room thermostat is set at an appropriate setting. Room thermostats are a temperature control unit, it senses the air temperature within a property. When the air temperature falls below the thermostat setting the thermostat will signal to switch the central heating system on. Likewise if the room temperature does not fall below the thermostat setting the heating will not be signalled to switch on. If you have a digital thermostat which is run on batteries, check that it doesn’t need replacement batteries.

If everything is switched on and working as it should, it’s likely the problem could be with your boiler, pipework or even plumbing, in which we would recommend calling an engineer.

Is only one radiator cold?

If only one radiator is cold and you have checked room thermostats, you may be able to rectify this issue yourself.

Only parts of the radiator are warm

If part of your radiator is warm, it is likely you are experiencing a common problem. If the radiator is cold at the top and warm at the bottom it could be a case of having a trapped air bubble. When a central heating system is signalled to switch on, it sends hot water to radiators. The bottom of the radiator will be full of hot water (as it should) but the hot water is unable to rise to the top of the radiator due to the air keeping it low. By simply bleeding the radiator to expel the trapped air the problem will be rectified. British Gas offer simple steps to bleeding your radiator.

One way to avoid air in your radiators and the need for regular bleeding would be to install an autovent valve. Autovent valves are a brilliant design, an autovent valve replaces the existing bleed valve on a radiator and can be installed within minutes. An autovent valve will automatically bleed air and improve efficiency of radiators, heating systems and underfloor heating. With an autovent valve there is no need to manually bleed a radiator as it drains air from a central heating system as the hot water passes through.

If the top half of the radiator is warm but the bottom half cold it usually means a sludge problem. Sludge will stop a radiator heating at the bottom as the heated water will sit above the sludge. Sludge can be removed by removing the radiator off the wall and rinsing it through (in the garden with a hose). This is a messy job as both sludge and water will gush out of the radiator when it is removed off the wall.

A power flush service is available but expect to pay from £200 upwards for this service.

One last investigation

If everything is switched on and working as it should and you are unable to find the troublesome radiator. Turn off all radiators in your home. In order turn on one radiator at a time (in a one-by-one process). If the radiator heats (as it should) it’s not the troublesome one. You may find that several radiators need bleeding during this exercise.

Please don’t attempt to touch any part of your boiler or central heating system if you’re unsure or not confident. Do not use a gas appliance if you think it’s unsafe.

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