Post by kdgrayson on Dec 30, 2016 15:59:38 GMT -8
The HVAC thermostat above,left uses a "safe" low voltage of 24 volts that is reduced from line voltage, either 120v or 230v by the Transformer.
The basic heat/ac Thermostat has 5 terminals,
Red or R = 24v "hot leg" of the 24 v
White or W = Heat circuit
Yellow or Y = Cool circuit
Green or G = Fan
Common or C = 24v "neutral leg" of the 24v.
The Transformer reduces line voltage to 24 volts.
The 24v hot leg enters the Thermostat on it's R terminal, upon a call for heat a switch closes between
the R & W thermostat terminals.
Upon a call for ac mode a switch closes between
R & Y
Upon a call for fan mode a switch closes between
R & G.
Red & Common are the 2 "legs" of the 24v supplied.
Red is the "hot leg" akin to Positive on a cars battery.
Common is the "neutral leg" akin to Negative.
It is called Common as every 24v circuit returns to "Common" to complete the circuit.
The picture above right demonstrates the various circuits for a gas furnace with AC.