Thermostat Wire Color Codes

THERMOSTAT WIRE COLOR CODING

Here is the color-coding system for 24V thermostats. Wires 1 and 2 are used in all thermostats. Wire 3 is used in 3-wire thermostats and all consecutive thermostats; wire 4 is used in 4-wire thermostats and all consecutive thermostats, and so on.

1. Red Wire is a part of every type of thermostat. It is a power wire that connects the R terminal for 24V power. In new thermostats, this Wire is designated by the letter R. In old thermostats, the letter combination is RC. 
2. White Wire is used for heating. Depending on the structure, it can be connected to W or W1.
3. Green Wire is for an indoor fan. It is connected to the G terminal.
4. Yellow Or Blue Wire is for cooling. It is connected to the Y terminal. Yellow is the most common color for this type of wire. In most cases, the terminal is called Y1, but Y2 is the name for some air-conditioning systems with complex 2-stage compressors.
5. Black (Or Blue) Wire is the so-called C wire, or common wire. It is a 24V common wire that is linked to the transformer. Its goal is to complete the 24v electrical circuit. Common wires vary in color, with black and blue being equally common.
6. Orange (Or Dark Blue) Wire is designated for reverse valve cooling or reverse valve heating. This wire has a connection to the condenser of the heat pump outdoor part.
7. Light Blue Wire, Brown Wire, and Others. The 7-wire and 8-wire thermostats, in which these wires are used, are by far the less common ones. Light blue Y2 wire is used in 7-wire and 8-wire thermostats for second-stage cooling systems. Brown wire with W2 is used for second-stage heating systems. Brown wire with the letter E is used for emergency heat. Other types of wires are possible but uncommon.

HOW MANY WIRES ARE THERE IN A THERMOSTAT? WHAT ARE DIFFERENT THERMOSTATS USED FOR? 

An electronic thermostat may have a different number of wires, anywhere between 2 and 8. A 5-wire structure is the most common one found in thermostats like a standard Honeywell thermostat found in most American homes.

• 2 Wire Thermostat is used in furnaces.
• 3 Wire Thermostat is for boilers and water heaters.
• 4 Wire Thermostat is used in HVAC systems and heat pumps. It is a common form of a contemporary smart thermostat like the ones by the Nest brand.
• 5 Wire Thermostat is the most common thermostat for most HVAC systems responsible for all kinds of things, including furnaces, heat pumps, and air conditioners of a new generation. It is used in newer smart thermostats that require a C-wire to function.
• 6 Wire Thermostats are used in some air-conditioning systems and most complex 2-stage heating systems, as well as heat pumps with complicated heating and cooling systems.
• 7 Wire Thermostat is also a part of some air-conditioning systems 2-stage heating systems, as well as control alarm systems
• 8 Wire Thermostat is used in some complex heat pumps.