O/B Terminal — Reversing Valve Wiring by Brand

Common Symptoms

• Heat pump blows cold air when thermostat is set to HEAT
• Heat pump blows warm air when thermostat is set to COOL
• House temperature continues dropping while heat pump is running
• System appears to work but heating and cooling modes are swapped
• Thermostat was recently replaced and now heating or cooling fails

Most Likely Causes

1. 1

   O/B Convention Mismatch — Wrong Terminal Setting in Thermostat

   The reversing valve solenoid is a simple electromagnetic coil that, when energized with 24VAC, shifts the valve from one position to the other. O-convention systems (Carrier, Trane, Lennox, American Standard) energize the solenoid in COOLING mode — when you call for cool, 24V appears on the O wire, shifting the valve to cooling position. B-convention systems (Rheem, Ruud, Goodman, Amana) energize the solenoid in HEATING mode — 24V appears on the B wire when the system heats. If your thermostat is configured for O but your system uses B, the valve is in the wrong position for every call.

2. 2

   Wire Connected to Wrong Terminal Letter on Thermostat

   During a DIY thermostat swap, the reversing valve wire (often orange on Carrier systems or dark blue on Rheem systems) may be connected to W (heat) instead of O, or to C (common) instead of B. This energizes the wrong circuit or provides no control signal at all. The symptom is typically no mode switching — the system stays locked in one refrigerant flow direction.

3. 3

   Reversing Valve Solenoid Failure (Mechanical vs. Electrical)

   A reversing valve solenoid that has failed open will never energize even when 24V is applied — the valve stays in one position. A solenoid failed closed has the coil shorted, drawing excessive current and potentially tripping the transformer fuse. Check solenoid coil resistance: a healthy solenoid reads 10–35 ohms. Open circuit (infinite resistance) = failed coil.

Quick DIY Checks

1. 1Identify your system's convention from the brand. Carrier, Trane, Lennox, and American Standard heat pumps use the O terminal (solenoid energized in cooling). Rheem, Ruud, Goodman, and Amana heat pumps use the B terminal (solenoid energized in heating). Write down which convention applies to your outdoor unit — this is printed on the data plate or in the installation manual.
2. 2Run the field voltage test to confirm the convention electrically. Set the thermostat to COOL at a temperature below current room temperature to force a cooling call. At the air handler terminal board (inside the air handler cabinet), measure voltage between the O wire terminal and the C common terminal. Use a multimeter set to AC voltage. If you read ~24VAC during a cooling call — it's an O-convention system (solenoid energizes in cooling). If you read ~0V during cooling (but 24V would appear in heating) — it's a B-convention system.
3. 3Configure your thermostat to match your system's convention. On Nest Learning Thermostat: go to Settings → Equipment → Continue → select your equipment type → find 'Reversing Valve' and choose O (energizes in cooling) or B (energizes in heating). On Ecobee: Main Menu → Installation → Equipment → Wiring → select 'Heat Pump' → set 'O or B' to match your system. On Honeywell T6 Pro / T10 Pro: enter installer setup (hold Menu for 5 seconds) → navigate to parameter 'HtPmp' or 'O/B' → set to match. On Emerson Sensi: use the app → Installation → Equipment → reversing valve setting.

1. 4Verify the reversing valve wire is connected to the correct physical terminal. At the thermostat base, the reversing valve wire (often orange on Carrier/Trane, blue or brown on Rheem/Goodman) must be in the O terminal (not W, C, or Y). At the air handler terminal board, trace that same wire to the O or RV terminal on the board — this connects to the outdoor unit's reversing valve solenoid wires. On Carrier air handlers: the outdoor reversing valve wires connect to terminals labeled RVS or O on the integrated control board.
2. 5Test the solenoid coil resistance if the valve still won't switch after correct wiring. Disconnect power. At the outdoor unit control board, disconnect the two wires going to the reversing valve solenoid (typically connected to O and C terminals on the outdoor board). Set a multimeter to resistance (Ω). Measure across the two solenoid wires. A working reversing valve solenoid coil reads 10–35 ohms. Infinite resistance (OL) indicates a failed open coil — the solenoid must be replaced. Near-zero resistance indicates a short — also replace.
3. 6Verify mode switching with the thermostat after completing wiring. Set to HEAT at a high set point. Go outside and place your hand near the outdoor unit air discharge (the top of the unit). In heating mode, the outdoor unit rejects heat in cooling and absorbs it in heating — so the air coming off the top in heat mode should be cooler than ambient (the unit is extracting heat from outside). In cooling mode, the discharge air is warm. If this is reversed, the O/B setting in the thermostat is still incorrect.

Repair vs Replace

Recommended Tools & Parts

• Reversing Valve Solenoid Coil (Universal 24V)

  Replacement solenoid coil for heat pump reversing valves. Clips onto the existing valve body — no refrigerant recovery required. Verify coil body diameter and mounting style before ordering.

  $25–$60

  Buy on Amazon →

• Digital Multimeter with AC/DC Voltage

  Required for the O/B field voltage test and solenoid coil resistance check. Any multimeter with 200VAC range and resistance mode works.

  $15–$35

  Buy on Amazon →

• Nest Learning Thermostat (4th Gen)

  Compatible with heat pump O/B terminal. Configure reversing valve convention in equipment setup. Requires C wire or Nest Power Connector.

  $149–$229

  Buy on Amazon →

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

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