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A Full Guide To Thermostat Wiring: Types and Color Coding
Thermostat wiring is a tricky topic due to the complex colour-coding of wires that go into the thermostat. This blog provides an easy-to-understand summary of all things thermostat wiring.
What is a thermostat?
The thermostat is a control device that regulates the temperature in a building or designated space. Thermostats are used in individual heating, central heating, water heating, heat pumps, HVAC systems, and other equipment.
The most popular thermostat brands in the North American market are Lowe's, EcoBee, Nest, Honeywell, and Emerson. Prices for ordinary circular thermostats vary between $90 and $250. There are various thermostats, but electronic ones are by far the most popular, followed by programmable electronic thermostats.
What is a thermostat wire?
A thermostat wire is a low-voltage solid copper wire used for different purposes within and in the systems connected to the thermostat. Usually, thermostat wires are plenum-rated unless they don't have to run through air-circulation spaces in buildings.
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 in thermostats, similar to a standard Honeywell thermostat found in most American homes.
- 2 Wire Thermostat is used in furnaces and other simple systems where only power and heat wires are needed.
- 3 Wire Thermostat is for boilers and water heaters. An additional third wire usually controls the fan.
- 4 Wire Thermostat is used in standard HVAC systems and heat pumps. It is a common form of a contemporary smart thermostat, similar to the Nest brand. A fourth wire is for cooling.
- 5 Wire Thermostat is the most common thermostat for most HVAC systems and is 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 for a continuous power supply.
- 6 Wire Thermostats are used in some air-conditioning systems, most complex 2-stage heating systems, and heat pumps with complicated heating and cooling systems that require a reversing valve. The reverse valve is the component of heat pumps that allows the switching between cooling and heating modes.
- 7 Wire Thermostat is also a part of some air-conditioning systems, 2-stage heating systems, and control alarm systems. The 7th wire supports the second stage of heat.
- 8 Wire Thermostat is used in some complex heat pumps with multiple heating and cooling stages. The 8th wire supports the second stage of cooling.
Thermostat Wire Color Coding
Here is the colour-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 four is used in 4-wire thermostats and all consecutive thermostats, and so on.
- 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. It is sometimes split into Rh (heating) and Rc (cooling) in older thermostats or systems with separate heating and cooling transformers.
- White Wire is used for heating. Depending on the structure, it can be connected to W or W1. For systems with multiple stages of heat, W2 is used for the second stage. White wire connects the thermostat to the furnace.
- Green Wire is for an indoor fan. It is connected to the G terminal. The green wire activates the fan independently of heating or cooling.
- Yellow Or Blue Wire is for cooling. It is connected to the Y terminal. Yellow is the most common colour 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. Yellow is the most common color for cooling wires, but Blue is also used in some systems. The cooling wire connects the thermostat to a compressor.
- 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 colour, with black and blue being equally common. C-wires are essential for powering smart thermostats that operate 24/7. Some older systems may not have a C-wire, but adapters are available.The colour of the common wire is blue if the heat pump is involved. If there is no heat pump, it can be black.
- 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. It switches the heat pump between heating and cooling modes. The O/B wire's function can vary based on the manufacturer. It is essential to consult the system's manual to determine if it controls the heating or cooling mode for the heat pump.
- 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.
- Other uncommon wires may include auxiliary heat, dehumidifier control, or humidifier control. These wires are not typically found in basic HVAC systems but in more advanced configurations.
Where to Buy Thermostat Wiring?
Nassau National Cable offers all kinds of thermostat wire at great prices. Aside from regular low-voltage thermostat wiring, the company also offers cables for line-voltage thermostats with a higher voltage of 120 or 240 volts, used in space heaters and other devices.
Questions and Answers
How does the thermostat work?
A thermostat works by regulating the temperature of a space. Here's a concise overview:
- The thermostat senses the ambient temperature using a built-in sensor.
- You set a desired temperature on the thermostat.
- The thermostat compares the current temperature to the set temperature.
- If the current temperature is below the set point, the thermostat signals the heating system to turn on.
- If the current temperature is above the set point, it signals the cooling system to turn on.
- The thermostat continuously monitors the temperature and turns the heating or cooling system on and off to maintain the desired temperature.
Smart thermostats also learn preferences, can be controlled remotely, and optimize energy usage.
What is a C wire on a thermostat?
The C-wire, or common wire, provides a continuous flow of 24V power to the thermostat. Its primary role is to power advanced features in modern thermostats, such as Wi-Fi connectivity, displays, and other electronic functions.
How to tell if a thermostat is bad?
Here are the main signs that the thermostat is bad:
- The thermostat does not respond to changes in settings.
- The displayed temperature does not match the actual room temperature. The temperatures throughout the house vary too much.
- The HVAC system runs continuously or not at all.
- There is short-cycling.
- Programmed settings are getting deleted by themselves.
- The room temperature fluctuates more than usual.
- The thermostat is old and outdated.
- The display is unresponsive or blank.
- No clicking sound when changing the temperature settings.
You can restart the thermostat to check if the issues are permanent.
