6-Wire Thermostat Wiring Diagram: Terminal-by-Terminal Connection Guide
This is a free printable 6 wire thermostat wiring diagram: download the diagram as SVG or open it and print to paper or PDF.
A 6-wire thermostat wiring diagram maps each conductor to its HVAC terminal, covering heating, cooling, fan, common, and auxiliary or reversing-valve functions.
A 6-wire thermostat installation is common in systems that combine heating and cooling with auxiliary or emergency heat — most typically a heat pump with an air handler. Each of the six conductors connects to a specific terminal on the thermostat sub-base, and the same terminal designations are used at the HVAC equipment control board.
The standard low-voltage thermostat circuit operates at 24 V AC, supplied by a step-down transformer inside the furnace, air handler, or heat pump unit. The six conductors carry different control signals:
- R (or Rh / Rc): 24 V AC supply from the transformer. Some systems have a jumper between Rh and Rc; others separate heating and cooling power supplies. - C: The common conductor, completing the 24 V circuit back to the transformer. This conductor allows smart thermostats and programmable devices to draw continuous low-level power without batteries. - G: Controls the indoor air handler fan (blower). Closing this circuit energises the fan relay independently of heating or cooling. - Y: Cooling signal. Energising this terminal activates the compressor contactor on the outdoor condensing unit or heat pump. - W (or W1): Heating signal. On a furnace system this triggers the gas valve or electric heat. On a heat pump this activates auxiliary (backup) heat strips. - O or B: Reversing valve terminal for heat pumps. O energises the reversing valve in cooling mode (most common, used by many heat pump brands). B energises the reversing valve in heating mode (used by some manufacturers). Never assume — check the equipment documentation.
Additional terminals sometimes encountered on a 6-wire run include E (emergency heat), L (system fault indicator), or S1/S2 (outdoor sensor). Always identify the specific equipment model and consult its wiring diagram rather than relying on generic colour coding, as wire colours are not standardised across manufacturers.
All thermostat wiring is low-voltage (typically 24 V AC) and is therefore lower risk than mains voltage, but the HVAC equipment itself contains mains-voltage components. Always isolate the equipment at the main power disconnect before accessing the air handler, furnace, or outdoor unit.
How to wire 6 wire thermostat wiring diagram
- Photograph and label the existing wiring before disconnecting Before removing the old thermostat, photograph both the thermostat sub-base and the control board in the air handler or furnace. Label each wire with its terminal designation using masking tape and a marker. This prevents costly errors if wires fall into the wall cavity.
- Isolate all HVAC equipment at the main power disconnect Turn off the air handler or furnace at its dedicated circuit breaker and at the service disconnect switch on the unit itself. Turn off the outdoor unit at its disconnect. Although thermostat wiring is low-voltage, the equipment control boards are connected to mains voltage and must be de-energised before opening the air handler cabinet.
- Remove the old thermostat and pull the cable forward Detach the thermostat from its sub-base and release each wire from its terminal. Secure the cable so it cannot fall into the wall cavity — a pencil taped across the cable bundle works well. Thread the thermostat cable through the new wall plate base.
- Connect wires to the new thermostat according to terminal markings Match each conductor to the corresponding terminal on the new thermostat: R (or Rh/Rc) to the red wire, C to the common wire, G to the green fan wire, Y to the yellow cooling wire, W or W1 to the white heat wire, and O or B to the orange or blue reversing-valve wire. Tug each wire gently after insertion to confirm it is secure.
- Verify connections at the HVAC control board Open the air handler or furnace cabinet and confirm each conductor terminates at the correct labelled terminal on the control board, matching the terminal designations used at the thermostat. Inspect for any bare conductor strands that might bridge adjacent terminals.
- Restore power and configure the thermostat Restore power to all equipment. Enter the thermostat installer configuration menu and set the equipment type (heat pump or conventional), auxiliary heat type, and reversing valve polarity (O or B). Test each operating mode: fan only, cooling, heating, and auxiliary heat.
Specifications
| Thermostat circuit operating voltage | 24 V AC (nominal), typically 22–28 V AC acceptable range |
|---|---|
| Control transformer output (typical) | 24 V AC, 40 VA to 75 VA |
| Thermostat cable conductor size | 18 AWG or 20 AWG (18 AWG preferred for runs over 15 m) |
| Standard terminal designations | R (or Rh/Rc), C, G, Y, W (or W1), O or B; additional: E, L, Y2, W2, S1, S2 |
| Low-voltage control fuse (typical) | 3 A or 5 A, equipment-specific — refer to HVAC unit service manual |
| Maximum conductor run (18 AWG, 24 V AC) | Approximately 50 m before voltage drop becomes significant; use 18 AWG for longer runs |
| Applicable standards | UL 873 (USA thermostats), NEC / NFPA 70 Article 725 (low-voltage wiring), CSA C22.2 No. 24 (Canada) |
Safety warnings
- Although thermostat wiring is low-voltage (typically 24 V AC), the HVAC equipment to which it connects contains mains-voltage components (typically 120 V, 208/230 V, or 240 V AC). Always isolate the air handler, furnace, and outdoor unit at their respective circuit breakers and service disconnects before opening any equipment cabinet or accessing the control board.
- Confirm the equipment is de-energised with a calibrated non-contact voltage tester before touching any conductors inside the HVAC unit. Mains voltage is present at the contactor, transformer primary, and heat strips even when the thermostat is in the off position.
- A shorted thermostat conductor (two bare wires touching adjacent terminals) can blow the 3 A or 5 A fuse on the control board or damage the transformer. Inspect for stray wire strands before restoring power. Replacement fuses must match the original rating exactly.
- Do not exceed the VA rating of the control transformer when adding accessories such as zone controllers, humidistats, or smart thermostat modules. Each additional 24 V device draws current from the same transformer secondary; exceeding the transformer VA rating will cause overheating.
- Electrical work on HVAC control wiring may require a licensed HVAC technician or electrician depending on local regulations. Confirm permit and licensing requirements with your local authority before undertaking any work beyond thermostat replacement.
Tools needed
- Digital multimeter (AC voltage, DC voltage, and continuity functions)
- Non-contact voltage tester
- Insulated screwdrivers (flathead, sizes to match thermostat and control board terminals)
- Wire strippers (rated for 18–22 AWG thermostat wire)
- Needle-nose pliers
- Smartphone or camera (to photograph existing wiring before removal)
- Masking tape and permanent marker (for labelling conductors)
Common mistakes
- Reversing the O and B terminals on a heat pump system, causing the system to heat in cooling mode and cool in heating mode. Always confirm the reversing valve polarity (O for cooling-energised or B for heating-energised) against the heat pump manufacturer's documentation.
- Leaving the Rh-to-Rc jumper wire in place when using a system with two separate transformers. This creates a transformer-to-transformer short on the 24 V AC secondary, which will blow fuses and potentially damage one or both transformers.
- Connecting the C wire at the thermostat but not at the control board (or vice versa), resulting in the thermostat losing power or displaying erratic behaviour.
- Pulling thermostat cable through a wall and allowing it to fall inside the cavity when the old thermostat is removed. Always secure the cable with tape or a temporary anchor before disconnecting.
- Using the wrong thermostat type for the system — for example, configuring a conventional heat/cool thermostat on a heat pump system. This will operate only the auxiliary heat strips rather than the compressor for heating.
- Overtightening terminal screws on the thermostat sub-base, which can crack the terminal block on plastic thermostats. Tighten firmly but without excessive force.
Troubleshooting
- Thermostat display is blank or thermostat will not power on after installation
- Cause: C wire not connected at both ends, or a blown low-voltage fuse on the control board Fix: Verify the C wire is seated in the C terminal at both the thermostat and the HVAC control board. Check the low-voltage fuse (typically a 3 A mini-blade or 5 A glass fuse on the control board) and replace if blown. Measure 24 V AC between the R and C terminals at the thermostat.
- System heats when cooling is called, or cools when heating is called
- Cause: O/B reversing valve terminal configured incorrectly for the heat pump model in use Fix: Access the thermostat installer configuration menu and toggle the reversing valve setting between O and B. Alternatively, swap the conductor between the O and B terminals. Confirm the correct setting against the heat pump outdoor unit documentation.
- Fan runs continuously regardless of thermostat setting
- Cause: G terminal conductor shorted to the R terminal, or thermostat fan mode set to 'ON' instead of 'AUTO' Fix: First check the thermostat fan setting and change it to AUTO. If the fan still runs, isolate the system and inspect the G terminal wiring for stray wire strands contacting the R terminal. Confirm the HVAC control board fan relay is not mechanically stuck.
- Cooling operates but auxiliary heat does not engage on a heat pump
- Cause: W1 or W2 conductor not connected at the control board, or auxiliary heat lockout active Fix: Verify the W terminal wire is connected at both the thermostat and the air handler control board. Check whether the thermostat has an outdoor temperature lockout set for auxiliary heat, and adjust the threshold. Confirm the heat strips are energised by checking for 240 V AC at the strip terminals (by a qualified technician).
Frequently asked questions
What does the C wire on a thermostat do?
The C wire (common wire) completes the 24 V AC circuit between the thermostat and the transformer in the HVAC unit. It provides continuous low-voltage power so that smart thermostats and programmable models can operate their displays, Wi-Fi, and other electronics without relying solely on batteries.
What is the difference between the O and B terminals on a heat pump thermostat?
Both O and B control the reversing valve in a heat pump. The O terminal energises the reversing valve in cooling mode (the more common configuration). The B terminal energises it in heating mode. The correct setting depends on the heat pump manufacturer's specification — always check the equipment documentation before configuring.
Why does my thermostat have separate Rh and Rc terminals?
Rh and Rc allow the thermostat to be connected to separate 24 V transformers for the heating system and the cooling system. Many installations use a single transformer and install a jumper wire between Rh and Rc. Remove the jumper only if the system uses two separate transformers.
Can I use any colour wire for thermostat connections?
Thermostat wire colours are not standardised by code, only by industry convention. The commonly followed convention is: red (R), white (W), green (G), yellow (Y), blue or black (C), orange (O), and brown or blue (B). Always label conductors at both ends and verify terminal assignments against the equipment wiring diagram rather than colour alone.
My thermostat shows a fault code after rewiring — what should I check first?
First verify that no wires are touching adjacent terminals (creating a short). Then confirm the C wire is connected at both the thermostat and the control board. Check that R is connected and the system transformer is producing 24–28 V AC. Consult the specific fault code in the HVAC equipment service manual.
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