Before wiring a thermostat we first need to know the type of system in question. Most homes today have an HVAC system with a furnace (oil, gas or electric) and an A/C unit. While these two systems typically have similar wiring properties, other systems such as heat pump system, have different properties and will be wired differently.

When wiring a thermostat, follow the standardized diagrams and wiring colors. If you are re-wiring an existing system, acknowledge that whoever did it before may not have used common standards. In that case, the colors may vary from the examples in this article. determine if the existing wire scheme is standardized or not before disconnecting any wires.

To avoid any confusion later on, take a photo of the wiring prior to disconnecting them so you can go back and see what color went to which terminal.

Most HVAC systems with an old-style thermostat, are also compatible with programable thermostats. A heat pump system works differently than a standard HVAC system and may not be compatible with a programable thermostat. Before you set up for the task, research your unit to verify compatibility.

Once you are ready to start, look at the table below. It shows the most common terminals/ color codes and which wire is used in which circuit. Most terminals do not have all these termination points. Use this table to determine color codes and connection points for your unit.

 

Marking

Wire Color

Description

A

 

Heat and cool active to any relay

B

Blue

Change-over relay to heat relay

C

Black/Blue

Common of 24 Vac transformer

E

Brown

Emergency heat relay

G

Green

Fan relay

L

 

Emergency heat active lamp

O

Orange

Change-over relay to cool relay

P

 

Defrost control lamp

R

Red

Power, 24 Vac transformer

Rc

Red

Power from cooling, 24 Vac transformer

Rh

Red

Power from heat, 24 Vac transformer