The first thing that must happen is that the control thermostat must provide a voltage signal to the furnace heating unit. The thermostat can be most understood as simply a switch that turns a low voltage 24 volt power on and off to the respective terminals.

In the case of the thermostat calling for heat the thermostat switches on the power from the “R” or in some cases the “RH” terminal which is the 24 volt power source to the “W” terminal on the thermostat which is in essence the call for heat. These terminals are located on the back or on the wall mounted face plate of the thermostat.

All the wires connected to the thermostat will usually always go directly to the furnace and if wired correctly the specific terminals that the wires are connected to will match at both the thermostat and at the furnace. For instance the wire connected to the “W” terminal should be the same wire at both the thermostat and the furnace heating system.

To learn more just go to our website for more information http://www.asi-heating-airconditioning.com/About_T-Stat_Call_Heat.html