Reasons your radiators aren’t heating up
If none of your radiators are heating up properly, it could be a result of a more serious problem with your central heating system or boiler. But, there are a number of smaller faults that should be ruled out first:
1) Thermostat is set too low
If no radiators are heating up, check that the room thermostat is set correctly. Also check the boiler thermostat. For instructions on the correct setting, see your manufacturer’s manual. 
2) Timer or programmer isn’t working
If all radiators are cold, ensure the timer or programmer is switched on and set correctly. Your manufacturer’s manual will provide details on the correct settings.
3) Gas boiler pilot light is out
If you find that the gas boiler pilot light has gone out, consult the relighting instructions that can usually be found printed on the back of the boiler’s front panel or in the manufacturer’s instructions.
4) A build-up of rust and sludge
The method for solving this problem will depend on whether you have an open vent system or not. If you suspect that there’s been a build-up of rust or sludge in your radiators, there are two ways to remove it, depending on the type of system you have: For open-vent systems For heating systems that are conventional, unpressurised and tank fed, you can use a heating system sludge remover. This can be bought at most DIY stores and must be used as stated in the manufacturer’s instructions. Simply add the liquid to the feed and expansion tank and after a few days, empty and refill the system. For non-open vent systems If you don’t have an open vent system, you’ll need to flush your radiators with a hosepipe. Your manufacturer’s manual will provide further information on how to do this correctly.
5) Air pockets in the system
In some cases, the problem is likely to be air in the system and not just an individual radiator. You can tell when this is happening because the radiators will become cold as the air pockets move around your heating system. To remove any air that’s become trapped within your central heating system, you may need to bleed a radiator. 
6) Circulation Fault
If none of the above tips help to get your radiators warming up, this might mean that there is an issue with your circulatory pump. At this point, it may be necessary to take advice from a registered heating professional.
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Despite boiler having been checked over and air locks having been eliminated, the central heating rads do not receive sufficient hot water to be effective especially when the hot water system is running. Our heating engineer believes the problem is the pipe work is too small. We don’t know how old the radiator system is but we have been here for 8 years with no problem till now.
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J P Warlow