Thermostat Wiring Diagrams - Heat pumps are wired for HVAC control far differently than air conditioning systems so make sure you know the difference and correctly identify the type of HVAC system you have installed. Additionally, before you decide to change your thermostat, make sure you have the correct tools especially a screwdriver and wire pliers.

The charts listed here will help you with a heat pump thermostat and an air conditioner thermostat. Either of these can be a regular digital thermostat or a programmable thermostat. Lastly, these thermostat wiring diagrams will help you wire a new thermostat. An air conditioner cooling system is different than a heat pump heating and cooling system.

Thermostat Wiring Diagrams

With the top thermostat wiring diagram showing an air conditioning system. The second wiring diagram showing a heat pump system. Finally, the third thermostat diagram showing the average type of split system with an air conditioner or gas or oil furnace used for heating.

The heating system could also be a boiler, however, is it necessary to ensure with split systems and the type of heating systems described that you do not have dual transformers in the system. Or to at least be aware that the system has dual transformers. Lastly, whenever in doubt call a professional.

Thermostat Wiring Diagrams - HVAC Control

HVAC Control - Heat Pump Systems

Air Conditioner Thermostat Wiring Details and Color Code

  • R Terminal is Connected to the Red Wire or R Wire - this is 24-volt power for the thermostat and controlled devices. Origin is the control transformer and then the R Terminal.
  • G Terminal is connected to the Green Wire or G Wire - This is for the blower fan in the air handler.
  • Y Terminal is connected to the Y Wire or Yellow Wire - the yellow wire is for cooling and originates at the thermostat, runs to the air handling unit, and then to the condensing unit outside.
  • W Terminal is connected to the W Wire or White Wire - This is for the heating circuit and control heat.
  • C Terminal is connected to the Blue wire many times referred to as the C Wire - this is normally used for common and completes the circuit between R and C with the thermostat being the load for a basic circuit.

COVID 19 Home Protection

UVC Light to Kill Viruses and Other Airborne Harmful Things that Affect Health

For additional protection, you can also use UV Light that will kill harmful viruses and bacteria inside the airflow of the air handler. In labs, researchers used a spectrum of ultraviolet light called UVC to kill viruses. This product requires professional installation so it will turn on and off with the blower fan in your air handler but it will offer the protection you need for you and your family from any viruses.

Click the image to the right for purchase options of the UV light to protect your family.

Heat Pump Thermostat Wiring Diagram


Air Conditioner - Thermostat Wiring Diagrams

Thermostat Wiring Diagrams for Heat Pumps - Heat Pump Thermostat Wire Diagrams

Heat pumps are different than air conditioners because a heat pump uses the process of refrigeration to heat and cool. While an air conditioner uses the process of refrigeration to only cool, the central air conditioner will usually be paired with a gas furnace, an electric furnace, or some other method of heating.

Therefore, while the air conditioner is used extensively in warm or hot weather, the heat pump is used year-round. A heat pump will use the yellow wire to turn on the condenser and the orange wire to shift the reversing valve in the condenser to reverse the flow of refrigerant for heating.

Basic Heat Pump Thermostat Wiring and Wire Colors for Thermostat Wires

  • R Red Wire to the R Terminal
  • G Green Wire to the G Terminal
  • Y Yellow Wire to the Y Terminal
  • W White Wire to the W Term.
  • O Orange Wire to the O Terminal for the reversing valve although a few manufacturers use the B terminal for the reversing valve (Rheem and Ruud)
  • C Blue Wire to the C Terminal (Sometimes you will this referred to as the C Wire)

This only for basic heat pump thermostat wiring to get the normal components working. Some manufacturers have additional wires for various functions. Always consult with the heat pump thermostat wiring guide that comes with your thermostat. Many times, wiring the new thermostat will mirror exactly how the old thermostat was wired.

The heat pump system has different controls than an air conditioner. It has its heat pump sequence of operation. Therefore, a heat pump works differently than an air conditioner. Additionally, it will require a different thermostat and different wiring than an air conditioner.

Sometimes, depending on the thermostat and the manufacturer, the thermostat can be a dual heat pump thermostat. Or it can be used for an air conditioner and gas furnace system depending on how the thermostat is wired. Lastly, always make sure you know what type of system you have before doing any wiring.

Honeywell Diagrams - Programmable Thermostat Wiring - Non-Programmable Thermostat Wiring

single stage heat pump thermostat wiring diagram

That is a basic Honeywell thermostat wiring diagram for a single-stage heat pump. If you have a two-stage heat pump, then you will also utilize terminal Y2 for the second stage. Furthermore, this thermostat wiring diagram is specifically for a system with two transformers. Your system likely only has one transformer, as most typical residential systems only use a single transformer for control. If you do have two transformers, then your old thermostat had a red wire on the RC terminal.

Additionally, it had a red wire on the RH terminal. Single transformer systems typically use a jumper for thermostats. To account for dual transformer systems, thermostats that have two R terminals (RC and R or RC and RH). Another diagram of thermostat wiring for two transformers. Lastly, the thermostat wiring diagram below utilizes the Y2 terminal meaning the system has two stages.

honeywell thermostat wiring diagram two transformer wiring

Thermostat Wiring Colors and Terminals

Thermostat Wiring Diagrams Furnaces

Heating Only Thermostat Wiring Diagrams If you only have a furnace such as a gas furnace, oil furnace, electric furnace, or a boiler. Therefore, you will use the following color code for simple thermostat wiring:

  • Terminal R or terminal RH for the red wire. This originates from the transformer. If the thermostat has an RC terminal it is not needed for this application. It does not matter if you leave the jumper in the thermostat or take the jumper out if a factory jumper is installed between RH and RC provided there is no cooling for this particular system.
  • Terminal W for the heating relay. for hot water boilers this will likely go to your aquastat on the boiler, however, it could also control a zone valve if you have a zoning system. After the zone valve opens it closes a switch and tells the boiler to maintain a temperature set point for the water.
  • Terminal C if the thermostat is digital and has no batteries. Finally, the common originates from the transformer and is required to power up the thermostat.

“Cool only” thermostat wiring diagram will mirror the above instructions except you use the Y terminal instead of the W terminal. Lastly, the terminal points where you connect the wire is typically in the wall plate and not on the thermostat itself.

White Rodgers Diagram

Thermostat Wiring Diagrams 

White-Rodgers shows a slightly different type of wiring diagram that mirrors a ladder logic diagram. It is clear and understandable. At least to an expert. This diagram shows what each terminal designation that goes to the HVAC system. Finally, the illustration below is for a system with a single transformer. The O Terminal in this diagram is for use for a heat pump thermostat installation for the reversing valve. A heat pump thermostat will use the orange wire for the reversing valve in the condenser.White Rodgers Thermostat Wiring Diagram

Smart Thermostats - WiFi - Smartphone Apps and Wiring

Many new room thermostats are digital thermostats and are considered smart thermostats. Additionally, these are simply temperature control devices that can help you reduce your energy bill and save energy. Furthermore, many of these newer thermostats are WiFi thermostats that can be connected via your smartphone and/or computer. Despite the newer technology, the thermostat wiring remains the same.

Additionally, there is no difference between wiring a smart or WiFi thermostat over wiring a plain digital one. For non-technical types who get confused over smartphones, apps for smartphones, WiFi, and other modern technical additions to the thermostat, the wiring will be the same. However, the hard part, if you want one of these smart thermostats, is in connecting to it via WiFi. Finally, simply follow the instructions that came with your thermostat for making these options and features work.

Thermostat Wire and Electrical Power

Thermostat Wiring Diagrams Air Conditioners

The wire you use to wire a thermostat should be 18 gauge solid wire. Furthermore, the wire should be in a bundle and have different colors for a color code. Additionally, unless you have a millivolt system or electric baseboard heating (gas logs typically) then your system will be low voltage. This low voltage ranges from 23 volts to 30 volts. This reduced voltage originates from line voltage through a transformer located typically, in your air handler.

Furthermore, it is important to find the breakers for your heating and cooling systems and turn the power off before wiring. And yes, there could be more than one break that provides voltage to your HVAC unit. Heating and air conditioning systems are usually separate systems and have their own breakers. Importantly, this is especially true if you have an air conditioner with a hydronic (boiler) system. Remember, before wiring, disconnect the power. Combinations include:

  • 2 wires system - this is usually a home heating system
  • 3 wires system - also possible only a heating system
  • 4 wires system - sometimes, when changing over from an older mechanical thermostat you will find the four-wire systems. 4 wires thermostat systems are not typical with digital or programmable thermostat wiring.
  • 5 wires systems - this is typically air conditioning and heating with a common wire for powering the thermostat
  • 6 wires or more are typically heat pumps. Heat pumps utilize extra controls over an air conditioner such as reversing valves and electric heat strips. Always follow the suggested color for your particular brand of HVAC equipment.

Smart and Programmable Thermostats

Thermostat Wiring Diagrams Honeywell

Your heating and cooling system, if a modern system, likely has a home thermostat that is a digital thermostat over the older mechanical thermostats. Furthermore, thermostat installation for the newer energy-efficient thermostats will offer better energy saving. Additionally, better home comfort along with reducing your energy costs. As a homeowner, to install a new thermostat properly with no disasters, simply follow the instructions. Finally, a few of the better thermostats include:

  • Ecobee - I personally have this one in my home and love it.
  • Honeywell Lyric
  • Emerson Sensi
  • Nest Learning Thermostat
  • And a few other brands


Many of these thermostats are touchscreen. Furthermore, they are also controllable with an app through your smartphone or via the internet from a laptop or desktop. This offers control from a remote location. If you went to work and forgot to change the temperature of the thermostat, simply log on and change it or turn it off. They reduce energy usage thereby reducing your energy bills. Furthermore, these thermostats do require a WiFi connection to make them work remotely.

Furthermore, this home technology has taken leaps and bounds at reducing home energy use. Finally, Thermostat Wiring Diagrams for these thermostats can be found here and on the manufacturer’s website. High Performance HVAC

Thermostat Wiring Diagrams

Ecobee WiFi Thermostat App for Smartphone
Ecobee Smart WiFi Thermostat