Diagnose central heating problems

There are a number of common central heating problems that might cause your radiators to be cold. It may sound obvious, but first check that you have turned your heating on and the room thermostat is calling for heat.

When your central heating stops working it seems quite natural to panic, but stay calm and perform some simple checks before calling in the cavalry. You may also find our tips on common boiler problems useful too.

Remember, troubleshooting central heating problems can be tricky, so if in doubt call on the skills of a professional.

No heating?

Check the thermostat

Check that the thermostat hasn't been set too low. The boiler will only work if the heating as been set higher than room temperature.

Check the timer

Most timers will have 4 settings, Off, On continuously, on all day (sometimes called once) or timed this may be 2, 3 or more times a day. Make sure that the heating is coming on when you want it to and that you haven't accidentally set it for 4am instead of 4pm.

Check the pilot light

Check that the pilot light in your gas boiler hasn't gone out. If it has, there are usually instructions on how to re-light it on the front of the boiler or inside the boiler door.
If these measures don't solve the problem then call in a GasSafe Register engineer.

Cold radiators upstairs

Cold radiators located in the upstairs section of your home usually mean that the feed and expansion cistern is empty.

  • The cistern is usually found in the loft, the small tank.

  • If dry then the cistern will need to be refilled.

  • To refill the cistern simply tap the ball valve, the water should start to fill the cistern and stop when there is just enough water to float the ball valve.

  • Don't fill it up completely, as there must be enough room for the water to expand.

Once the cistern has filled you will need to bleed the affected radiators. Check the overflow for the next couple of days to ensure the ball valve is working correctly, if it is dripping call a professional plumber to check and replace as necessary.

Cold radiators downstairs

If your downstairs radiators are failing to heat up then try turning the upstairs radiators off, there may be an airlock in the system, if this fails there's a good chance that your pump is broken or on its way out. You'll need to call on the skills of an expert plumber.

Remember, cold radiators could lead to frozen or burst pipes in winter, so they need to be tackled quickly.

Top of radiator cold

This probably means there's air in the system. You will need to bleed the radiator so that this trapped air can be released.

Middle of radiator cold

This may mean a build-up of rust or sludge is obstructing the flow of water through the radiator.

Build-up of rust and sludge

The method of solving this problem will depend on whether you have an open vented system or not.

I have an open vent system

If you have an open vented system, you can use a heating system sludge remover. This can be bought at most DIY stores. Follow the instructions provided with the product. Please ensure that the manufacturer’s instructions are followed as some chemicals may void any guarantee with your central heating appliance.

I don't have an open vent system

If you don’t have a open vent system there are a number of products available at most DIY stores that enable a chemical cleaner to be injected into the system via an applicator gun. Please follow instructions provided and check the manufacturer’s instructions of the central heating boiler as some chemicals will void any guarantee.

With either system and if confident, you can turn off your radiator from both valves, drain it until empty, ensure you cover the open ends so no sludge drips out and use a hosepipe to clean the inside of the radiator.

Cleaning your heating system can be a big job, if in doubt get a professional plumber out.

Air locks in the central heating system

In some cases it's likely there’s air in the system and not just an individual radiator. You can tell when this is happening because your radiators will go cold alternately as air locks move around the heating system. Try bleeding the radiators first before getting in professional help.

Close the valve

Radiators heat up in turn, which means those nearest the boiler will often be warmer. By partially closing the lockshield valve on those radiators you can allow more hot water to flow to the ones further away.

Use a radiator thermometer

Ideally, you need to use radiator thermometers and adjust each radiator in the order in which they heat up. Turn off the lockshield valve, place a thermometer at either end of the radiator and open the valve slowly until the temperature at either end is almost the same.

Leaking radiators and pipes

When you have one or more leaking radiators, there are some actions you can take before the problem escalates. A pinhole leak on a radiator can be a sign of internal corrosion. This can happen when debris that collect during installation aren’t properly removed (and this can be within weeks of a system being fitted) or because of air being drawn into the system.

To fix the problem, try the following:

  • Shut off your boiler and let it cool

  • Place a bowl and dustsheet under the leak, sometimes plastic bags can be used to direct the flow.

  • Turn off the valves at each end of the radiator to relieve pressure.

  • Drain the radiator down

  • Remove the radiator and leave the rest of the system running. If in doubt get a professional plumber

  • Flush out the system using a non-acidic cleaner.

  • Refit the radiator.

Leaking pipes

Leaky pipes are a nuisance, and can grow into bigger problems if left unchecked.

Leaky connection or joint

If water is leaking from a connection or joint then it should be easy to tighten the joint with a spanner or wrench. However if the joint has been soldered you won't be able to tighten it and the pipe may need replacing.

Leaking from pipe

If the pipe is leaking from a section which has no joint or connection then the pipe will need replacing. A short-term solution is to tie a rag around the pipe, or use specialist sealant from a DIY store. You may wish to get a professional plumber to replace the pipe as it can be a messy and complex job.

Noisy boiler

If your boiler makes all sorts of odd banging and bumping noises then there may be a lack of water in the feed and expansion tank. This is usually located in the loft, or if you have a sealed system it may need re-pressurising, the gauge should read 1 bar minimum.

Another issue could be a build-up of sludge and scale in your boiler. Air in the boiler can also cause similar banging noises. Don't try and tackle these problems yourself, call in a Gas Safe Register professional heating engineer.

Water gurgling

Water gurgling noisily through the central heating system is a pretty clear sign that you have air in the radiators. This problem can be easily resolved by bleeding your radiators.

Don’t forget

If you have a major problem with your boiler or heating, call in a GasSafe Register engineer. Remember that all boilers and heating systems should be regularly checked and serviced by a professional. 

Read our disclaimer