Thermostat Wiring Diagrams
For Every System Type

How to Read a Thermostat Wiring Diagram

A thermostat wiring diagram maps each wire from your thermostat to your HVAC equipment. Every diagram has three key elements you need to understand before doing any wiring work:

How to Wire a Thermostat Step-by-Step

2-Wire System (Heating Only)

The 2-wire system is the oldest and simplest thermostat wiring configuration. It is commonly found in homes with a single gas furnace, boiler, or wall heater where the only function is to turn heat on and off. When the thermostat calls for heat, it closes the circuit between R and W, sending a 24V signal to the furnace to ignite.

There is no separate fan control wire because the furnace manages the blower fan internally — it turns the fan on after the heat exchanger warms up and shuts it off after the burner stops. This setup cannot control air conditioning.

Smart thermostat note: If you want to upgrade to a smart thermostat like Nest, Ecobee, or Honeywell, you will need to add a C wire for continuous power. With only 2 wires, most smart thermostats either won't work or will experience chronic battery drain. See our C wire guide for options.

3-Wire System (Cooling Only)

The 3-wire configuration is found in homes with central air conditioning but no furnace — typically in warmer climates where heating is provided by a separate system (like a wall heater or radiant floor). The R wire may be labeled Rc on the thermostat, indicating it is powered from the cooling system's transformer.

The Y wire signals the outdoor AC compressor to start, while the G wire independently controls the indoor blower fan. This separation allows you to run the fan without the compressor for air circulation. When cooling is called, both Y and G are typically energized together so the fan distributes the cooled air through the ductwork.

This system is relatively uncommon in modern installations. Most homes with central air also have a furnace, making a 4-wire or 5-wire system more typical.

4-Wire System (Heating + Cooling)

The 4-wire system is the most common residential thermostat wiring configuration in the United States. It supports a standard gas furnace paired with a central air conditioning unit — the combination found in the majority of American homes. The R wire provides power, W calls for heat from the furnace, Y activates the outdoor AC compressor, and G controls the indoor blower fan.

With 4 wires, you have full control over heating, cooling, and fan-only modes. The thermostat acts as a switch for each function: when you set it to heat, it connects R to W; when set to cool, it connects R to Y and R to G simultaneously.

Smart thermostat limitation: Without a C wire, many smart thermostats will not work reliably on a 4-wire system. The Nest can attempt to trickle-charge through the HVAC system, but this frequently causes "delayed" messages and battery drain. Ecobee includes a Power Extender Kit to solve this, while Honeywell smart models require a C wire. If you have an unused wire in your cable bundle, you can repurpose it as a C wire.

5-Wire System (With C Wire)

The 5-wire system is the "gold standard" for modern thermostat installations, especially if you are using or plan to upgrade to a smart thermostat. It includes all four functional wires (R, W, Y, G) plus the C (common) wire, which provides a continuous 24V return path to power the thermostat.

The C wire completes the 24V circuit, allowing the thermostat to draw constant power for its Wi-Fi radio, display, sensors, and processor. Without it, smart thermostats must rely on battery backup or trickle-charging — both of which cause reliability issues over time.

Compatibility: The Ecobee requires a C wire (but includes a Power Extender Kit). The Google Nest strongly recommends a C wire to prevent battery drain. The Honeywell T-series requires a C wire. If your system has only 4 wires, check out our complete C wire guide for options to add one.

Heat Pump Wiring (6-8 Wires)

Heat pump wiring is more complex than conventional systems because the heat pump uses the same compressor for both heating and cooling. A reversing valve inside the outdoor unit switches the direction of refrigerant flow, and the O/B wire controls that valve.

The W2/AUX wire activates auxiliary heat strips, which are electric resistance heaters that supplement the heat pump when outdoor temperatures are too low for the heat pump to be efficient (typically below 30-35°F). Emergency heat mode forces the system to use only the heat strips, bypassing the heat pump entirely. The Y2 wire is only present in two-stage heat pump systems where the compressor can run at a lower speed for mild conditions and full speed for extreme temperatures.

Dual Transformer Systems (Rh and Rc)

In most homes, a single 24V transformer powers both the heating and cooling systems, and you have one R wire going to the thermostat. However, some homes — particularly older builds or properties where the heating and cooling systems were installed at different times — use two separate transformers: one for heating and one for cooling.

When you have dual transformers, the thermostat has two separate power terminals:

• Rh (R-heat): Receives 24V from the heating system's transformer (usually at the furnace)
• Rc (R-cool): Receives 24V from the cooling system's transformer (usually at the air handler or outdoor unit)

The jumper wire: If you have a single transformer (most common), there should be a small jumper wire or metal bridge connecting the Rh and Rc terminals together. This tells the thermostat to use the same power source for both heating and cooling. If you only have one R wire and your thermostat has separate Rh/Rc terminals, install the jumper and connect your R wire to either terminal.

Important: Never connect two separate transformers to the Rh and Rc terminals without removing the jumper first. Running two transformers into a jumpered connection will cause a short circuit and blow fuses on one or both control boards.

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