Heat Pump Thermostat Wiring — Full Terminal Guide

Common Symptoms

• Heat pump runs in cool mode when set to heat (reversing valve wired backward)
• No cooling — compressor never starts when Y terminal is called
• Smart thermostat shows no power or won't connect (missing C wire)
• Aux heat runs constantly even on mild days (W2 wired incorrectly)
• Emergency heat light stays on even though thermostat is in normal HEAT mode
• System short-cycles or thermostat display resets frequently

Most Likely Causes

1. 1

   O vs B Terminal Reversed — Wrong Reversing Valve Convention

   Carrier, Trane, Lennox, and American Standard heat pumps use the O terminal — the reversing valve solenoid energizes in COOLING mode. Rheem, Ruud, Goodman, and Amana use the B terminal — the solenoid energizes in HEATING mode. If you wire a Rheem system to the O terminal (or vice versa), the heat pump blows cold air in heat mode and warm air in cool mode. The thermostat must be configured to match the system's convention.

2. 2

   Missing C Wire — Smart Thermostat Won't Power Up

   Smart thermostats (Nest, Ecobee, Honeywell T6/T10, Sensi) require a continuous 24VAC common (C) wire to power their WiFi radio and display. Older heat pump systems with only 7 wires and no C conductor require either running a new wire, using a C-wire adapter kit, or using a power sharing accessory (Ecobee Power Extender Kit, Nest Power Connector). Without C, thermostats steal power from the Y or G circuit, causing intermittent compressor or fan lockouts.

3. 3

   Rc/Rh Jumper Left In — Two-Transformer Systems

   Some systems have two separate 24VAC transformers: one for the air handler (Rh) and one for the outdoor unit or cooling (Rc). The thermostat terminal board ships with a jumper between Rc and Rh. If your system has two transformers and you leave that jumper in, you create a short between two independent 24VAC circuits — blowing both transformers. Remove the Rc-Rh jumper when connecting to a dual-transformer system.

4. 4

   Aux/W2 and E Terminals Confused

   Aux (sometimes labeled W2) triggers the auxiliary electric heat strips when the heat pump alone can't keep up. E (emergency heat) does the same but also locks out the compressor. On many thermostats these are separate terminals. Wiring Aux to E locks out the compressor permanently; wiring E to Aux puts the system in emergency heat mode without the user selecting it.

Quick DIY Checks

1. 1Photograph the old thermostat wiring before removing it. Note the wire color going to each terminal letter. Also photograph the air handler terminal board inside the air handler, which will have terminal labels and the original connections. Use this reference throughout the rewiring.
2. 2Identify your reversing valve terminal — O or B. Check the outdoor unit data plate or installation manual. Carrier, Trane, Lennox, and American Standard use O (solenoid energized in cooling). Rheem, Ruud, Goodman, and Amana use B (solenoid energized in heating). At your thermostat, configure the reversing valve setting to match: Nest → Equipment → O/B Reversing Valve → 'O'; Ecobee → Installation → Wiring → 'O or B'; Honeywell T6/T10 → installer setup parameter 'O/B' terminal function. Getting this wrong causes the heat pump to blow cold air when calling for heat.
3. 3Check for a C wire. Count conductors in your thermostat cable. A standard 8-conductor cable (18/8) has enough wires. The C wire connects from the thermostat C terminal to the C terminal on the air handler control board — completing the 24VAC circuit back to the transformer. Verify 24VAC between the R and C terminals at the air handler board with a multimeter. If no C wire exists in the cable, use the Ecobee Power Extender Kit (connects to PEK terminals on the air handler board) or the Nest Power Connector (wires at the air handler).

1. 4Verify the Rc/Rh configuration. Most residential heat pumps use a single 24VAC transformer. The thermostat terminal board has one R terminal (or Rc and Rh with a factory jumper). If your system has a single transformer, leave the Rc-Rh jumper installed and connect your red wire to Rc. If your system has two transformers (typically commercial or dual-fuel hybrid systems), remove the Rc-Rh jumper and connect the cooling circuit transformer to Rc and the heating/air handler transformer to Rh. Measure 24VAC at each independently before connecting.
2. 5Wire the Y1 terminal to the compressor contactor coil circuit. In a heat pump, Y1 calls the compressor in both heating and cooling. In a two-stage heat pump, Y2 calls the second compressor stage or second refrigerant circuit. Verify at the air handler terminal board: Y1 on thermostat connects to Y1 on the board, which runs through the low-pressure switch and time-delay relay to the outdoor unit contactor coil. Check this connection by setting the thermostat to COOL at 60°F (forcing a call) and measuring 24VAC at Y1 on the air handler board — you should read 24V when the thermostat is calling.
3. 6Wire auxiliary heat and emergency heat correctly. The W1/W2 terminals control electric resistance heat strips in the air handler. W1 (or Aux/W2) energizes sequencers that power the heat strips in sequence (staggered 15–30 seconds apart) when the heat pump is in second-stage heating. E (emergency heat) does the same but the thermostat also sends a signal that locks out the Y terminal — disabling the compressor. On Nest thermostats: Aux terminal on Nest goes to W2 on the air handler board; the Nest handles E internally. On Ecobee: wire W2 to the Ecobee W2 terminal; the thermostat provides a separate lockout relay output.
4. 7Wire the G (fan) and OB (reversing valve) terminals last. G energizes the indoor blower relay for continuous fan mode and during heating/cooling calls. The O or B wire connects from the thermostat O/B terminal to the reversing valve solenoid terminal on the outdoor unit control board (often labeled O or RV). Verify the G circuit by setting the thermostat fan to ON — the indoor blower should run immediately without the compressor. Verify the O/B circuit by switching to COOL at a low set point and measuring 24VAC between the O or B wire and the C common at the outdoor unit board.
5. 8For dual fuel systems (gas furnace + heat pump): the air handler board has a switchover relay that connects the thermostat W1 output to either the gas furnace valve (below the balance point, typically 35–40°F outdoor) or the electric strips (above the balance point). These systems require a thermostat capable of dual fuel control — the Honeywell T10 Pro, Ecobee SmartThermostat Premium, and Carrier Edge thermostats support this directly. Configure the outdoor balance point temperature in the thermostat installer settings to match your system's designed switchover temperature.

Repair vs Replace

Recommended Tools & Parts

• Ecobee SmartThermostat Premium

  Full heat pump thermostat with O/B terminal, Aux heat, emergency heat lockout, and Power Extender Kit (replaces C wire). Compatible with Carrier, Trane, Lennox, Goodman, Rheem, and Amana heat pumps.

  $189–$249

  Buy on Amazon →

• Honeywell Home T10 Pro Smart Thermostat

  Heat pump compatible smart thermostat with dual fuel support, O/B terminal, Aux and Emergency heat. RedLINK wireless compatible.

  $100–$150

  Buy on Amazon →

• Emerson Sensi Touch 2 Smart Thermostat

  WiFi thermostat with full heat pump support including O/B, Aux, Emergency heat. Simple installer setup without contractor required.

  $100–$140

  Buy on Amazon →

• 18/8 Thermostat Wire (50 ft)

  8-conductor 18-gauge thermostat wire for running a new C wire or full cable replacement. Enough for most residential installations.

  $20–$35

  Buy on Amazon →

• Digital Multimeter

  Required for verifying 24VAC at R and C terminals and tracing control circuit voltage. Any basic multimeter with AC voltage range works.

  $15–$30

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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