All radiators remain cool while boiler operates normally.
If the boiler appears to be functioning correctly but the radiators are still not getting hot the problem may well lie inside the pipework. Over a period of time the water inside the pipes and radiators cause the metal to corrode leaving sludge deposits throughout the system. This sludge gathers in certain areas of the system and prevents the free flow of hot water to the radiators. The only solution for this is to chemically clean or flush through the system to remove these deposits.

Another reason for the failure of hot water to circulate through the systemcould be due to a defect with the pump. This can be checked by locating the pump which should be near to the boiler or could be actually inside of it. The pump should makea whirring noise and should be vibrating as the motor forces the water through the impellers. If the pump is not working then it may be jammed in which case it may be possible to free it with either a light tap from a heavy object such as a hammer or by removing the cover(ensuring that the power is turned off to the pump) and locating a large screw in the centre of the pump housing. If this screw is removed another screw shouldbe revealed beneath it, by turning this second screw you may be able to release any slight jamming of the pump. Once the screw feels to be moving freely then reassemble the pump and try again. Alternatively if the pump appears to be functioning correctly then air may have worked it's way into the pipework causing an airlock.

You can learn more about how to cure airlocks by reading the bleeding radiators section.  Finally, if bleeding has no effect then a blockage may have occurred in the pump, this can only be cleared by removing the pump to clean or replace it. As wiring and pipework is involved we would suggest calling in a professional.

If the pump is not running then you could try a manual re-start. Switchoff the pump and look for a large screw in the middle of it. Remove the screw and you will find another screw underneath, turn this until the spindle feels free then replace the top screw and turn the pump back on.
 

Cool radiators furthest from boiler:
If the radiators get cooler at a greater distance from the boiler, this indicates that your central heating system is not properly balanced. This occurs on feed and return systems whereby the pipe flowing to every radiator is from the main flow pipe. At the furthest radiator, the main flow pipe turns back toward the boiler and becomes the return pipe, into which pipes from every radiator feed. The water does not flow in and out of each radiator in turn, but comes from the main pipe that feeds them all, and goes into the main pipe returning from them all. If the system is not balanced, you will not get the right feed to the furthest radiator.

When the radiators are first installed they are given a lockshield valve which allows for balancing adjustment. Once installed and balanced, they should not require further altering unless there are changes to the radiators or the pipework.

Balancing -
Before you begin, turn off the central heating and give it plenty of time to cool down.

Locate the lockshield valve at one end of the radiator under a push on cover. Using an adjustable spanner, open the lockshield and the control valve (at the other end of the radiator) on all the radiators.

Attach radiator thermometers to the inlet and outlet pipes of the radiator nearest to the boiler, not to the main flow and return pipes.
Now switch on the central heating system.

Partially close the lockshield valve on the nearest radiator. As the temperature rises, slowly open the valve until the temperature difference between the two thermometers is about 20°F (12°F).

Attach the thermometers to the next radiator along from the boiler. Repeat the process as above.

Continue along all the radiators until they have been balanced.
The effectiveness of balancing the radiators can be affected by external temperature. If the external temperature is higher than the system design value, the heat disseminated by each radiator will be less than the design intends. Consequently, the temperature drop across each radiator will be less than 20°F. If the balancing occurs on a summer's day, adjust the lockshield valves to get a lower temperature difference.

Bleeding the radiators.
Before you try to bleed the radiators you should firstly turn off the pump and the boiler.
Every radiator has a small square bleed valve at one of it's top corners. You should have a special key which will fit over this valve but if you do not they are available from all good DIY shops.

Insert the key into the valve and turn it anticlockwise about a quarter of a turn. You should also have a small container and a cloth to avoid any spillage or spurting from the valve. This will only occur if the valve is opened too far as only air should be able to escape if correctly done.

As the air is released you should hear a hissing noise, when the hissing stops and water begins to appear close the valve firmly.

If no water or air escapes when the valve is opened then you should check to make sure that there is water in the feed and expansion tank, if there is water present then the valve is probably blocked with either paint or debris from inside the radiators. This is a relatively simple operation but one which may require assistance from a professional.

Automatic bleed valves
If you are having to bleed radiators on a constant basis then it might be to your advantage to have an automatic bleed valve installed.