The easiest way to understand a dual thermostat is to think of it as being two thermostats in one. At the simplest level imagine having two mechanical thermostats, one set at 45 and one set at 72. You would have both thermostats connected to your furnace and when you are at the building you would have the furnace running off of the thermostat set at 72, then when you leave have the furnace running off the the thermostat set at 45. You do this by installing a switch which both thermostats are connected to and the furnace is connected to on the other end, you would just flip the switch to change thermostats. Dual thermostats are essentially all of that, in one compact package, and instead of having to stand there and flip a switch they offer an input which be triggered using a Sensaphone 400 or Freeze Alarm Deluxe.