A lockshield or a lockshield valve is a type of radiator valve. It connects a radiator to the return pipe to the boiler. It is one of two valves normally found on radiators, the other being the TRV (thermostatic radiator valve). A lockshield can easily be identified by its small white plastic cap. However, unlike a manual radiator valve (which also has a small white plastic cap), it cannot be adjusted without tools.
Lockshield valve overview
The purpose of a lockshield is to regulate the flow of water through a radiator. Water pumped through the central heating circuit by the boiler will take the path of least resistance when it returns to the the boiler via the return pipe. This means that, generally speaking, the further away from the boiler a radiator is, the less hot it will be, and the longer it will take to heat up.
By adjusting the lockshields accordingly, the occupant can set the radiators to heat up evenly. This ensures that the radiators heat up at the same rate and that the temperature in each room is equal. This is known as balancing the radiators. Generally speaking, the further away a radiator is from the boiler, the more open the lockshield valve needs to be, and vice versa.
Lockshields are typically 15 mm compression fittings. They are available as angled or straight fittings. For a standard radiator with an inlet and an outlet at each side, this will mean angled for pipes coming out from the floorboards, and straight for pipes which run parallel to the floor.
As mentioned, a lockshield is easily identifiable by its small white plastic cap, underneath which is a two-sided spindle. Once the radiators are balanced, the lockshield no longer needs to be turned. The plastic cap should be replaced to prevent the valve from being adjusted again. Even if the cap is removed, however, the spindle can only be adjusted with a pair of pliers or an adjustable spanner.