In a worst case scenario, cold radiators could indicate complete boiler failure. Of course, it could also mean a simple fuse has blown. There are a number of reasons that your radiators might be cold:
- Thermostat is set too low
- Timer or programmer isn’t working
- Gas boiler's pilot light is out
- Build-up of rust and sludge
- Air pockets in the system
Thermostat is set too low
Check that the room thermostats and boiler thermostats are set correctly. For instructions on the correct setting, see your manufacturer's manual.
Timer or programmer not working
Check that the timer or programmer is switched on and set correctly. For instructions on the correct setting see your manufacturer's manual.
Gas boiler's pilot light is out
Check the pilot relighting instructions supplied with the boiler (these are usually printed on the back of the front panel).
Build-up of rust and sludge
The method of solving this problem will depend on whether you have an open vent system or not.
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I have an open vent system
If you have an open vent system, you can use a heating system sludge remover. This can be bought at most DIY stores. Add the liquid to the feed and expansion tank. After a few days you will then need to empty and refill the system.
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I don't have an open vent system
If you don't have an open vent system, you'll need to flush through the radiators with a hosepipe. See your manufacturer's manual for instructions on how to do this.
Air pockets in the system
In some cases it's likely to be air in the system and not just an individual radiator. You can tell when this is happening as the radiator becomes cold as the air pockets move around your heating system.
Try bleeding the radiators. This will get rid of build-up and balance the radiators and could even flush out old dirty water, replacing it with new water.
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