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. Lastly, these thermostat wiring diagrams will help you wire a new thermostat.

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

Heat Pump Thermostat Wiring Diagram


Air Conditioner - Thermostat Wiring Diagrams

Thermostat Wiring Diagrams for Heat Pumps

Heat pumps are different than air conditioners because heat pumps use 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.

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

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 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. 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 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 - wire system - this is usually a home heating system
  • 3 - wire system - also possible only a heating system
  • 4 - wire system - sometimes, when changing over from an older mechanical thermostat you will find the four wire systems.
  • 5 - wire systems - this is typically air conditioning and heating with a common wire for powering the thermostat
  • 6 or more wires are typically heat pumps.

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 manufacturers website. High Performance HVAC

Thermostat Wiring Diagrams

Ecobee WiFi Thermostat App for Smartphone
Ecobee Smart WiFi Thermostat