5-Wire Thermostat Wiring Diagram: Understanding R, C, W, Y, and G Terminals

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A comprehensive reference for wiring a five-wire HVAC thermostat, explaining the function of each terminal — R (power), C (common), W (heat), Y (cool), and G (fan) — with practical connection guidance.

A five-wire thermostat installation is the standard wiring configuration for a conventional (non-heat-pump) split-system HVAC installation with a forced-air furnace and central air conditioning. The five wires connect to five labelled terminals on both the thermostat and the air handler or furnace control board, and understanding each wire's function is essential before making or changing any connections.

The R terminal (Red wire) carries 24 V AC power from the HVAC system's step-down transformer to the thermostat. Some systems split this into two terminals — Rh (red, heating) and Rc (red, cooling) — with a removable jumper between them. In a combined system, the jumper is left in place and only one red wire is needed.

The C terminal (Common wire, typically blue or black) completes the 24 V AC circuit back to the transformer's secondary winding. It provides the return path for the control current, and without it, smart and WiFi thermostats cannot operate because they draw continuous power from the 24 V circuit. Many older installations lack a C wire — a common source of frustration when upgrading to a smart thermostat.

The W terminal (White wire) is the heating call output. When the thermostat's set temperature exceeds the room temperature, it closes the circuit between R and W, sending 24 V to the furnace or boiler's heating relay, which opens the gas valve and ignites the burner.

The Y terminal (Yellow wire) is the cooling call output. When room temperature exceeds the set point in cooling mode, the thermostat closes R to Y, energising the condenser unit contactor and starting the compressor.

The G terminal (Green wire) controls the air handler blower fan independently. When G is energised (R to G closed), the fan runs. In cooling mode, the system energises both Y and G simultaneously. In heating mode, modern furnaces manage the fan internally via the furnace control board — the thermostat G signal may or may not be used, depending on the system.

Always consult the furnace/air handler wiring diagram and thermostat installation manual for the specific equipment before disconnecting any wires.

How to wire 5 wire thermostat wiring diagram

Photograph the existing wiring before removal Before disconnecting anything, photograph the existing thermostat wiring clearly — capture both the wire colours and the terminal labels they connect to. If the wire colours are faded or difficult to read, use tape labels on each wire. This photograph is your reference if wiring needs to be restored or if commissioning reveals an error.
Switch off the HVAC system Set the thermostat to 'Off'. Switch off the circuit breaker supplying the furnace or air handler at the electrical panel. Also switch off the outdoor condenser unit's disconnect (if replacing a thermostat with the system running). Confirm the system is de-energised — the furnace control board should have no LED activity.
Remove the old thermostat faceplate and note wire connections Remove the old thermostat from its base plate. Note and record which wire is connected to which terminal (R, C, W, Y, G — or Rh, Rc on split-R systems). Tape each wire to the wall so it cannot fall back into the wall cavity while the new thermostat is being fitted.
Connect wires to the new thermostat base Mount the new thermostat base plate. Strip approximately 10 mm of insulation from each wire end. Connect each wire to the corresponding terminal on the new thermostat base: Red to R (or Rh/Rc as applicable), Blue or Black to C, White to W, Yellow to Y, Green to G. Tighten all terminal screws firmly — a loose connection in the R wire is the most common cause of a non-functional thermostat installation. If the new thermostat uses push-in terminals, insert the stripped wire fully until it clicks.
Verify the C wire is connected Before fitting the thermostat faceplate, verify the C wire is connected at both ends — at the thermostat terminal and at the furnace or air handler control board. A C wire present at the thermostat but not connected at the control board will not function. Locate the control board in the air handler, identify the C terminal (often labelled 'C' or 'Com'), and confirm the other end of the blue or black wire is securely attached.
Restore power and configure the thermostat Restore the circuit breaker and any local disconnects. Fit the thermostat faceplate. Follow the thermostat's setup wizard to configure: system type (gas/electric heat, conventional or heat pump), number of stages, fan control, and temperature display units. Set a test heating call (set point above room temperature) and verify the furnace fires. Set a test cooling call and verify the compressor starts. Confirm the fan operates in 'Auto' and 'On' modes.
Test all functions and confirm normal operation Run the system through a complete heating and cooling cycle. Listen for unusual noises, check for airflow at all registers, and verify the thermostat displays the correct room temperature. Allow the system to reach the set point and confirm it cycles off correctly. Check that the C wire is providing stable power by navigating the thermostat's settings menus — a thermostat drawing insufficient power will exhibit sluggish display response or random restarts.

Specifications

R terminal — supply voltage	24 V AC from furnace/air handler step-down transformer secondary
C terminal — common/return	24 V AC circuit return (transformer secondary common) — provides continuous power path for smart thermostats
W terminal — heating call	Closes R-to-W circuit to energise furnace/boiler heating relay; 24 V AC signal
Y terminal — cooling call	Closes R-to-Y circuit to energise condenser unit contactor; 24 V AC signal
G terminal — fan call	Closes R-to-G circuit to energise air handler blower motor relay; 24 V AC signal
Thermostat cable specification	18 AWG (0.75 mm²) multi-conductor thermostat cable; 300 V low-voltage rated
Transformer rating (typical residential)	24 V AC, 20–40 VA — verify against number of thermostats and zone controllers connected
Applicable standard (US/Canada)	ANSI/ASHRAE 135 (BACnet), NEC Article 725 (Class 2 wiring); local building code

Safety warnings

ISOLATE BEFORE WIRING: Switch off the furnace circuit breaker and the outdoor condenser disconnect before disconnecting or connecting any wiring. While thermostat cables carry only 24 V AC (which is unlikely to cause electrocution), the furnace control board is connected to 120 V or 230 V AC mains — contact with the control board wiring at mains voltage is dangerous.
HEAT PUMP SYSTEMS DIFFER: The five-wire R/C/W/Y/G configuration described on this page is for conventional (non-heat-pump) systems. Heat pump systems require additional terminals (O/B for reversing valve, AUX for auxiliary heat). Wiring a heat pump system with a conventional thermostat wiring configuration will result in incorrect or damaging operation. Identify your system type before proceeding.
DO NOT JUMPER R AND C: The R and C terminals are the two sides of the 24 V AC supply. Connecting R to C directly (accidentally or intentionally) short-circuits the transformer, which will blow the transformer fuse or damage the transformer. Inspect your wiring before restoring power.
TRANSFORMER OVERLOAD: Multiple HVAC zones connected to a single transformer can overload it. If you are adding a new thermostat to an existing system with zone dampers or multiple air handlers, verify the transformer's VA rating is sufficient for the combined load of all thermostats and zone controllers.
QUALIFIED HVAC TECHNICIAN: While thermostat wiring is low-voltage, it interfaces with gas-fired furnaces and high-voltage electrical equipment. If you are uncertain about system type, wiring configuration, or if the system has not been serviced recently, engage a qualified HVAC technician. Incorrect wiring of a gas furnace's control circuit can prevent safety limits from operating correctly.

Tools needed

Insulated screwdrivers (flat-blade and Phillips, for terminal screws)
Wire strippers (sized for 18 AWG thermostat cable)
Digital multimeter (for 24 V AC verification at thermostat terminals)
Non-contact voltage tester (for verifying circuit de-energisation)
Smartphone or camera (for photographing existing wiring before removal)
Electrical tape and wire labels (for identifying wires)
Ladder (for accessing air handler in ceiling or rooftop installations)

Common mistakes

Not photographing the original wiring before removal — if you make a mistake, you have no reference to return to the known-working state.
Forgetting to connect or verify the C wire at the control board end — the C wire must be connected at both the thermostat and the furnace control board C terminal. A C wire dangling free inside the air handler provides no return path.
Accidentally connecting the R wire to the C terminal and the C wire to the R terminal — this reverses the polarity of the 24 V circuit and may damage the thermostat's internal electronics.
Using the wrong thermostat for a heat pump system — the O/B reversing valve terminal is absent on a conventional thermostat, leaving the heat pump unable to switch between heating and cooling modes.
Ignoring the Rh/Rc jumper requirement — some dual-transformer systems (separate heating and cooling transformers) require the jumper to be removed. Leaving it in place on a dual-transformer system creates a short circuit between two out-of-phase transformers.
Not testing all modes (heat, cool, fan) before considering the installation complete — a wiring fault in the G wire only becomes apparent when the fan is tested independently.

Troubleshooting

Thermostat display is blank or thermostat unresponsive after installation: Cause: No C wire connected (smart thermostat drawing no power), R wire not connected or loose, circuit breaker off, or blown 3 A control fuse on the furnace control board Fix: Verify the furnace circuit breaker is on. Check the furnace control board for a blown 3 A fuse (a common casualty of accidental R-to-C shorts during wiring). Verify the R wire is securely connected at both ends. Confirm the C wire is connected at both the thermostat and the control board C terminal.
Heating works but cooling does not start: Cause: Y wire loose or disconnected at thermostat or control board, outdoor condenser disconnect switched off, or condenser contactor fault Fix: Verify the Y wire is connected at both ends. Check the outdoor condenser unit's disconnect is in the 'on' position. With the thermostat calling for cooling, use a multimeter to measure 24 V AC between the Y terminal and C terminal at the air handler control board — if 24 V is present, the control signal is reaching the board and the fault is in the condenser wiring or contactor.
Fan runs continuously regardless of thermostat fan setting: Cause: Thermostat 'Fan' setting left on 'On' instead of 'Auto', G wire connected to R terminal, or furnace control board fan relay stuck closed Fix: Verify thermostat fan setting is 'Auto'. With the system switched off at the circuit breaker, measure resistance between G and R at the thermostat base — it should read open circuit (infinite resistance) in 'Auto' mode with no call for heat or cool. If the fan runs with the thermostat disconnected from its base, the furnace control board fan relay is stuck — this requires a control board replacement.
Smart thermostat keeps losing WiFi or rebooting: Cause: Insufficient power — C wire not providing adequate return path, or C wire connection has high resistance due to corrosion or loose terminal Fix: Measure 24 V AC between the R and C terminals at the thermostat (with the thermostat removed from its base). Should read 24–28 V AC. If low or zero, trace the C wire. Clean and retighten the C terminal connection at the control board. If no C wire exists, install a C-wire adapter or run new five-conductor cable.
System short-cycles (turns on and off frequently): Cause: Thermostat set point too close to current room temperature, anticipator setting incorrect (on older mechanical thermostats), thermostat mounted near a heat source or cold draught, or refrigerant charge issue causing the compressor to trip on high pressure Fix: Verify thermostat location is on an interior wall, away from supply registers, windows, and direct sunlight. On programmable thermostats, check minimum run-time and cycle-rate settings. For cooling short-cycling, check condenser coil for blockage and system refrigerant charge — this requires a licensed HVAC technician.

Frequently asked questions

What is the C wire and why does my smart thermostat need it?

The C (common) wire completes the 24 V AC control circuit, providing a continuous return path. Smart and WiFi thermostats draw constant low-level power from the 24 V circuit to run their electronics, display, and WiFi radio. Without a C wire, they cannot draw continuous power and may use battery backup, exhibit erratic behaviour, or refuse to operate.

My existing thermostat has only 4 wires — how do I add a C wire?

Options include: running a new five-wire thermostat cable from the air handler/furnace to the thermostat location; using the G wire as a C wire if you are willing to lose independent fan control (use a control board adapter kit); or fitting a C-wire adapter kit that uses the existing two-wire or four-wire cable to simulate the C wire function. Each approach has trade-offs — consult the smart thermostat manufacturer's compatibility guide.

What happens if I wire Y and W terminals in reverse?

The heating and cooling calls will be reversed — the furnace will fire when you call for cooling, and the compressor will run when you call for heat. In cooling mode with the furnace running simultaneously, this can overheat the air handler and trip safety limits. Identify and correct the wiring before running either system.

Why is my furnace fan running constantly even when there is no heating or cooling call?

The G terminal is energised continuously — either the thermostat's 'Fan' switch is set to 'On' instead of 'Auto', or the G wire is accidentally connected to the R terminal (or another always-on terminal) at the thermostat or control board. Verify the thermostat fan setting and check the G wire connection at both ends.

Can I use a standard five-wire thermostat for a heat pump system?

Generally no. Heat pump systems require different terminal functions — particularly the O/B wire that switches the reversing valve between heating and cooling modes. A heat pump thermostat also manages auxiliary/emergency heat (AUX/E terminals). Wiring a heat pump system with a conventional thermostat will result in incorrect reversing valve operation. Always use a thermostat specifically designed and configured for heat pump use.

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