Indoor Thermostat

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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.

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.