hal,as far as i know the boiler starts on outside air temp and return water temp.
I'm not going to get into a long dialog with you but around here in the north east boilers are either maintained temperature or cold start. Most are the latter to save energy and will only run when a thermostat calls for heat and will cycle on the high limit aquastat until the thermostats are satisfied. There are some high end boilers and controllers that will modulate the water temperature based on the outside temperature but most operate the same way.
Not all valves have spring return, some have 24v at the valve all the time and use a signal provided through a electronic controller or t-stat to tell the valve which way to go.
Sure and there are DDC (direct digital control)systems and networked systems with (GASP!) CAT5 wiring also but that't not what the original post is about now is it.
r is one side of 24v w is for heating and i have never heard of t&t. i think you may be the one with bad info. i highly doubt that a end switch on a valve is going to start the boiler, unless maybe the boiler is in florida where they dont need heat.
The end switches control the boiler the same way as a low voltage thermostat would with only one zone.
-Hal
