lockshield

A lockshield is a flow-control valve fitted to the return pipe of a radiator, usually at the opposite end from the thermostatic valve (TRV). Unlike the TRV, which adjusts to room temperature, the lockshield restricts flow by a fixed amount set during commissioning. Its purpose is to balance water distribution across multiple radiators in a heating system so that all rooms reach design temperature without some radiators becoming too hot while others stay cold.

The valve body contains a tapered needle or plunger that screws down to reduce the internal bore, limiting flow rate in cubic metres per hour. The 'lock' refers to a small plastic or metal cap that covers the adjustment screw once set, protecting it from accidental tampering. This cap must be removed to adjust the valve, which involves turning a special key or hex wrench while observing flow through a small bypass port or using a calibrated balancing meter. Most lockshields are integral to the radiator valve union or fitted as a separate inline component on the return pipe.

Balancing a system requires measuring the pressure drop across each radiator and calculating the valve opening needed to restrict flow proportionally. Without this, the radiators closest to the boiler pump receive more hot water and heat up faster, starving distant radiators of adequate flow. In practice, installers often use approximate settings based on radiator output and distance, then fine-tune by feel or rough measurement over several heating cycles.

The lockshield is most useful in systems with 10 or more radiators, long pipe runs, or where radiators are at different heights. In small two or three radiator systems, balancing may be unnecessary. Sludge or corrosion inside the valve can cause it to stick or fail to restrict flow properly, requiring replacement. Because the adjustment is permanent until changed, a faulty lockshield that has opened too wide cannot be fixed by occupants and will cause cold radiators elsewhere in the system.