A Differential Bypass Valve provides a constant minimum water flow around a heating system. It also protects the Circulating Pump and the system and improves balancing to Thermostatic Radiator Valves. When the pressure increases, the valve opens thereby providing a constant water flow through the boiler. It ensures a recirculation proportional to the number of valves that close, thus avoiding noise whilst at the same time maintaining the pump head pressure at a constant value. This valve is very effective in systems where various heating loads may simultaneously be excluded from the circuit by the closure of zone control valves and thermostatic or manual radiator valves. Fitted between flow and return, it helps prevent unnecessary cycling of the boiler.