When all the TRVs are open, the by-pass valve remains closed,
allowing the full boiler output to circulate around the heating
system. As the TRVs sense that the selected room temperatures are
reached and begin to close, the by-pass valve starts to open,
maintaining optimum flow through the boiler, thus eliminating
possible damage to the boiler and pump. Installation of a by-pass
valve also minimises the noise often experienced when flow rates
through the boiler decreases