Indoor Thermostat
A building thermostat is a device to control the heating and air-conditioning systems in a home, office, or other indoor environment.
The majority of modern heating/cooling/heat pump thermostats operate on 24 volts A/C.
Contents
4 Wire 24V HVAC Thermostat Wire Diagram
W : White Wire : Heat
Y : Yellow or Black Wire : Cool
G : Green : Fan
R : Red : 24v
Jump W + R will turn on heater on a gas operated HVAC system. On the gas system the fan is triggered automatically at the heater when a certain temperature is reached.
Jump G + R to force the fan to turn on
Honeywell's round mechanical mercury thermostat "The Round" has 6 connectors. Only 4 are necessary for a typical HVAC system.
B : black
G : green : Fan
Y : yellow : Cool
W : white : Heat
R : red : 24v
O : orange
A modern electronic thermostat may have as many as 8 connectors. To connect this to a 4 wire 24v system use the following:
W : white : heat
Y : yellow or black wire : cool
G : green : fan
B :
O :
RC : red wire : 24v - bridge to RH
RH : red wire : 24v - bridge to RC
C :
Hot Wire Testing
Shorting or Hot Jumping is risky and can in face damage your system. Certain brand and models work differently from what is commonly known. Use this as a general reference but do not proceed unless you have research your system brand and model.
5 Wire Red, Blue, White, Green, Yellow
- Red + Yellow - turns on the Air Conditioner
- Red + Green - turns on the blower fan
- Red + White - turns on heat
Common HVAC Thermostat Wiring
(R) Red - Voltage
Transformer provided DC voltage
(G) Green - Blower Fan
Many thermostats (G) and (Y) terminals are connected together at all times when the fan switch is in the "Auto" mode.
(Y) Yellow - Compressor
The Air Conditioner. When voltage supplied it activates cooling. On a Heat Pump system it can also activate heat via the heat pump.
(W) White - Heat
Activate heat. Heat including gas heat, electric heat, or auxiliary heat on a heat pump.
(B) Blue - Common (exceptions exist)
What is called 'common' and is typically not used on modern thermostats.
Needed on some electronic thermostats or if the system uses indicator lamps.
On some Rheem & Ruud systems it is not 'common'. May be Blue or Orange.
