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 :