I am replacing my boiler it has three zones (24volt stats) am using a taco SR503 switching relay the wiring diagram does not tell me if it is voltage (110 or 24 volt) for stats does say 110 to pumps also does not state voltage to tt going to boiler can it be assumed it is 24volt

That is why they make electric testing meters. NEVER hook up something before you test for the correct voltage. If I told you any different I would be guessing and that is not a good way to work on HVAC equipment..