Thermostatic Unions

Joe Brix

as an option instead of a bypass to prevent condensation. I guess they restrict flow until the temp comes up right? Shown installed on the supply right outside the boiler before expansion tank and circ pump. Would this be better then a bypass? I would think restricting flow until the boiler heats up would be better then allowing any cold return. But would this cause the pump to cavitate or heat up since flow was restricted? Any chance for these to get stuck closed?