I'm just switching the thermostat, so no large current load. The thermostat will be inactive during the daytime so there will be no activity at the boiler or pumps. Basically, this just interrupts the current flow to the thermostat.
And what does the thermostat control? Is it line voltage or low voltage? There has to be some sort of large contactor somewhere, to control a 50 amp load. If that contactor is within the heater unit, then you should be good to go.