Furnace thermostat wiring diagram terminal letters on a thermostat and what they control the hot wire 24 volts usually red from the transformer is the main power wire to turn on or off a furnace components.
													
																	Thermostat wiring heat pump gas furnace. 
									
	
		
	
																	It corresponds to the chart below to explain the thermostat terminal functions. 
																	Y compressor stage 1 cooling y2 compressor stage 2 cooling g fan. 
																	See the diagram below for what each wire controls on your system. 
																	Before uninstalling the old thermostat take a picture of the wiring with your cell phone before removing the wires. 
															
													
									
	
		
	
																	Terminal r or terminal rh for the red wire. 
																	This way you have a reference. 
																	The basic heat pump wiring for a heat pump thermostat is illustrated here. 
																	When the red wire is disconnected from w the furnace should turn off. 
															
													
									
	
		
	
																	This originates from the transformer. 
																	Heating only thermostat wiring diagrams if you only have a furnace such as a gas furnace oil furnace electric furnace or a boiler. 
																	For example if the red wire is connected to w on the thermostat the furnace should turn on. 
																	My paperback and e book. 
															
													
									
	
		
	
																	The thermostat uses 1 wire to control each of your hvac system s primary functions such as heating cooling fan etc.