I think Hal gave a more detailed explanation of what I was describing. I differ a little with him in that I think you usually just start moving fluids with the switches on the zone valves. I'm used to seeing the boiler doing its own thing, running between its low and high temperature set points. Thermal mass, delta Ts, flow rates, # of zones calling for heat, all that good stuff, will eventually cause the boiler to kick in at its low temperature setpoint.
I should've mentioned that we're talking low voltage, current limited here. The switches tend to connect to a module that contains its own 24 volt transformer and the relay/contactor that controls the circulating pump.
Any air handlers/heat exchangers at the far end might well have their own high and low temperature set points (or just 1 set point with a dead band or differential.) This way, you don't start blowing cold air around or stop the fan while the coils are still hot.
Joe
