By-Pass Valves (BPVs) are designed to be fitted in all heating systems where Thermostatic Radiator Valves (TRVs) are installed so as to maintain a minimum flow rate in the system. When all TRVs are fully open the BPV will remain closed and the full boiler output will circulate through the system. As the TRVs close the BPV will start to open thus maintaining optimum flow rate through the boiler. Installing a BPV will minimise the noise and vibration often experienced when flow through the boiler diminishes. The BPV should be fitted between the flow and return side of every system. As the TRVs close, the by-pass valve opens to deliver supply water to the return line.