The Du144 automatic bypass valve controls water flow in the heating circuit according to the water pressure across it and is used to maintain a minimum flow rate through the boiler and to limit circulation pressure when other water paths are closed. A bypass circuit must be installed if the boiler manufacturer requires one or specifies that a minimum flow rate has to be maintained whilst the boiler is firing. The use of automatic bypass valves becomes particularly important when heating systems include large numbers of thermostatic radiator valves (trvs) - whilst the trvs are open the automatic bypass valve remains closed, however, as the trvs start to close, the automatic bypass valve starts to open maintaining the required water flow through the boiler. Using an automatic bypass valve is also likely to reduce noise in systems caused by excess water velocities. Because manual or fixed position valves do not regulate the flow and allow water to bypass even when it is not necessary, building regulations require bypass circuits to use automatic bypass valves and not fixed position valves.