the S350 is a switch it has no power running out of it unless you wire power into it.. I ran an extra pair TT wire(s) to my s350 off of my LR zoneR845a relay TT terminals. if zones not running and s350 switches on, will turn on mt LR zone circulator, and primary loop circulator by using LR TT voltage in R845a. If LR zones already on for heat call..., in theroy the boiler should be dumping heat because LR zones on and NOT go into an overfire situation which would activate S350 JC relay . And this has held true. I also have oil boiler, set another S350 on end of JC relays, to comtrol primary circulators for oil boiler/coal boiler so if coals unable to keep up with load and temp drops to setpoint, it shuts of coals primary circulator and switches on oils primary pump and oil heats till coal gets back up to reset temp(differentials) & (offsets) it works excellent.