Heat pump thermostat wiring diagram: what each wire does

30-second answer

24VAC control wiresverified Sep 2026

A heat pump thermostat wiring diagram maps a ducted system’s 24VAC control wires, one wire per job.^[1] Match wires by terminal letter, never by color.^[2] Heating side first, then the mini-split exception:

• Y runs the compressor. On a heat pump the compressor makes the heat as well as the cooling, so Y is the main heating call.^[1]
• O/B sets the direction. It drives the changeover valve, energized in cool on an O setting or in heat on a B setting.^[1]
• AUX/W2 and E add backup heat. AUX calls auxiliary heat, E is a separate emergency heat call, and nothing goes on W.^[1]
• Most mini splits skip the standard stat. They use their own remote or controller, or a maker’s adapter. MRCOOL does not officially approve any third-party thermostat,^[3] and Mitsubishi announced an interface to connect one.^[5]

Once you know what system you have and the power is off, most homeowners can handle the 24VAC side. The line-voltage supply is a licensed electrician’s job.

What does each wire on a heat pump thermostat do?

On a ducted heat pump, most thermostat terminals switch one job at 24VAC, and the letter beside each terminal names that job, whatever the wire’s color. Honeywell’s T6 Pro manual lists R as 24VAC power from the heating transformer.^[1]

Where DIY ends: the line-voltage side

With the power off at the breaker, moving 24VAC thermostat wires is a job most homeowners can handle. The line-voltage supply is where DIY ends for most people: the breaker, the disconnect and the circuit to the outdoor unit are a licensed electrician’s job, and your local building department decides what needs a permit.

A mini split’s indoor-outdoor cable is line voltage too and must be dead before anyone touches it. MRCOOL says it carries 230V, or 115V on smaller models, and that its wires must match exactly at both ends.^[4] MRCOOL sends anyone unsure about the breaker, disconnect or terminal connections to a licensed electrician.^[4]

Terminal by terminal, heating calls first:

• Y and Y2: compressor stages. Y switches the compressor contactor for stage 1, and Y2 for stage 2 on two-stage equipment.^[1] On a heat pump the compressor also makes the heat, so Y is the main heating call.
• O/B: changeover valve. It switches the heat pump between heating and cooling. In Honeywell’s installer setup, the system type goes to heat pump, then O/B to O, energized in cool, or B, energized in heat.^[1]
• AUX/W2: backup heat. It calls auxiliary heat, or a second stage of heat on equipment that has one.^[1]
• E: emergency heat. It is a separate emergency heat call,^[1] and not every system runs a wire to it.^[2]
• G: fan. It switches the indoor fan.^[1]
• R and C: 24VAC power and common. R brings power from the transformer, and Honeywell’s heat pump diagrams mark C optional when the thermostat runs on batteries.^[1]
• L: fault input. It carries a heat pump fault signal to the thermostat, and Honeywell’s table says it needs the C wire.^[1]
• W: leave it empty. Every T6 Pro heat pump diagram says not to use W for heat pump applications,^[1] and backup heat goes on AUX instead.^[2]

The changeover valve is the heat pump’s reversing valve, which Honeywell’s setup menu calls the reverse valve.^[1] It swaps the refrigerant’s direction so the outdoor coil gathers heat in winter. The guide to how a mini split heat pump works follows the compressor and valve through that loop.

Dual fuel adds a furnace, and Honeywell’s dual fuel diagram adds an outdoor sensor on the S terminals.^[1] The dual fuel heat pump post explains the furnace handoff.

What should you check before moving a wire?

A heat pump thermostat swap starts with the power off, then a 120/240V check, because Honeywell’s install guide says its thermostat will not work on line voltage.^[2] The same guide warns that wrong wiring or setup can cause unintended operation that could lead to property damage, injury or death. If you are unsure what type of system you have, it says to check the equipment literature or call an HVAC professional.^[2]

• Turn the power off at the breaker. Switch off the one that controls the heating and cooling system. Some systems have separate heating and cooling breakers.^[2]
• Check for 120/240V wires. Thick black wires joined with wire nuts, or a thermostat rated 120V or higher, mark a line voltage system.^[2] If you find them, stop: a line-voltage thermostat is a licensed electrician’s job.
• Photograph the terminals. Get the letter beside each wire into the picture.^[2]
• Label each wire by its letter. Tag each one as it comes off, so the label matches the terminal it left.^[2]
• X8S: watch for separate O and B wires. Honeywell’s X8S guide puts the B wire on C, and says to stop and call a qualified contractor if C already has a wire.^[2]

That is an X8S step, so follow your own thermostat’s manual. For a mini split, the DIY mini split installation guide marks which install steps are homeowner work and which belong to a licensed trade.

Does a mini split use a regular thermostat?

Usually not: most mini splits use their own remote or controller, or a maker’s adapter, not a standard thermostat. That is our read of the makers’ own product pages. MRCOOL does not officially support or approve any third-party thermostat and sells its own thermostats instead.^[3] Mitsubishi announced its interface for third-party thermostats in 2015.^[5]

• A same-brand thermostat or controller. This is the path MRCOOL supports, and it will not troubleshoot a third-party thermostat.^[3]
• A maker’s interface. Mitsubishi’s interface lets third-party thermostats control compatible indoor units, those its own MHK1 controller can run.^[5] We could not confirm it is still sold.
• MRCOOL: mode switching rides the interconnect. MRCOOL’s indoor-outdoor cable carries the digital signals for mode switching, compressor speed and fan.^[4]

On any mini split, the maker’s install manual, not a generic heat pump diagram, decides the thermostat wiring. Whether staged thermostat calls cost a mini split its variable-speed behavior is disputed. Mitsubishi’s interface lists conventional one-stage and two-stage heat and cool operation, with two-stage preferred, and Mitsubishi says third-party controllers can still use its inverter-driven compressor.^[5]

Our buyer-research scan, run in August 2026, has two cautions from buyers. One was told a smart thermostat would cost some variable-speed features. Another says Senville warned that wiring the 24V control lines directly between the outdoor unit and air handler could turn heat strips on incorrectly and hamper staging.

The mini split vs heat pump comparison lays out how the two systems differ beyond their controls.

Replacing the ducted heat pump with a mini split instead? The sizing calculator gives a screening load in a minute, and a load calculation settles it.