lockshield
lockshield valve

Turn off the lockshield valve then open it gradually again until the difference between the two thermometers is about 20 degrees Farenheight.

This is achieved by the use of lockshield valves which can restrict flow through the most favoured radiators.

Open the handwheel valve at one end of the radiator, and remove the cap from the lockshield valve at the other end.

Each radiator has a control valve - the one you use to turn it on or off, and a lockshield valve - the one with a cover which you don't normally turn.

It is important however to note how far the lockshield valve is open, by counting the number of turns, to ensure that upon reinstating the radiator this valve is reset to its original position.

Close the handwheel valve at one end, and remove the cover of the lockshield valve at the other end so you can close it by turning its spindle with pliers or a small spanner.

When the radiator is full, close the air vent, reset the lockshield valve to its original setting, replace its cover, and open the handwheel valve to allow water to circulate right the way through it.

The mandatory use of TRVs has also resulted in a new generation of plumbers and heating engineers who have no interest in the traditional skill of balancing a central heating system by adjusting the lockshield valves at the opposite ends of the radiators to the main valves.