If the lower half of the radiator is hot, but the top is cool, then air is trapped in the system. Use a radiator-bleeding key to open the air vent at one end of the top of the radiator. Make sure you have a piece of rag ready to collect the water once it starts to flow. Do not open the vent completely. One turn is usually sufficient to hear the hissing of the escaping air. Once water starts to appear at the vent, close it up. The water can be very hot, so it is safer to turn the heating off while you complete this job.

If you find that radiators, or a particular radiator needs bleeding on a regular basis, then air is entering the system. This problem can cause corrosion and there may be a problem with the system that needs expert guidance. It could be caused by a build up of sludge in the bottom of the radiator, inhibiting the circulation of water. There is a sludge removal liquid available. Add the liquid to the feed and expansion cistern. Two days later it will be necessary to drain and refill the system.

If the airlocks are more serious, it could be due to lack of water in the system, leaks or a design defect somewhere in the pipework. Bleeding the radiators doesn't help if the problem is more serious. Locate the pump and find the flow regulator. Make a note of the setting, before using a screwdriver to turn it off and on very quickly for a few times. If this is still unsuccessful, try bleeding the pump itself. The bleed valve should be visible on the top of the pump casing. Open and close it with a screwdriver very quickly. If you hear the sound of air hissing - you have found your fault.
 

 
This problem is probably due to a build up of sludge throughout the system. The sludge - black iron oxide, is caused by corrosion on the inside of the pipes and radiators, resulting in poor circulation of water. The system needs to be thoroughly cleaned with a special chemical. There is a sludge removal liquid available. Add the liquid to the feed and expansion cistern. Two days later it will be necessary to drain and refill the system.
 

 
It is probably due to the pump failing. Turn the central heating system off, wait for the pump to cool and try to start the pump manually. In the middle of the pump, remove the screw and turn the pump's manual starter - the impeller.

No luck? Try tapping the pump two or three times with a mallet, gently but sharply.

No luck? Try removing the pump and use a hosepipe to flush it through with clean water. The pump should not be submerged in water.

No luck? Remove and replace the pump.
 

 
You will need to check the complete system, as it is not properly balanced.
 

 
If the heating is working downstairs but is cool upstairs this often indicates that the feed-and-expansion cistern is empty. Refill the cistern allowing just enough water to float the ball when the water in the system is cold. The extra space is needed to accommodate the expansion of the water as it is heated up. Another problem could be a faulty ball valve.
 

 
If the system is gravity driven, hot water from the hot water cylinder fails to reach the upstairs radiators when a gravity-check valve switches off the heating. When the heating is switched on, hot water will rise naturally above cooler water. The gravity-check valve can be found on the flow pipe to the upstairs radiators. If it is stuck in the 'open' position, the pipe on either side of the valve will be warm. It needs replacing and it might be time to call in a central heating engineer.
 

 
Plumbing > Problems associated with noises from the central heating system
 

 
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