A bypass circuit must be installed if the boiler manufacturer requires one, or specifies that a minimum flow rate has to be maintained while the boiler is firing.The installed bypass circuit must then include an automatic bypass valve (not a fixed-position valve). Automatic by-pass valves are especially necessary if your heating system includes lots of Thermostatic Radiator Valves (TRV's). The automatic by-pass valve will maintain correct water flow through the boiler when the TRV's start to close. Automatic by-pass valves will also be likely to reduce noise levels in systems which are noisy due to excess water velocities.