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.
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.
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