Heat Pump O vs B: Which Wire Controls the Reversing Valve

Choosing between the “O” and “B” thermostat terminals matters when wiring or troubleshooting a heat pump because these terminals control the reversing valve, which determines heating or cooling mode. This article clarifies the differences between Heat Pump O or B, explains wiring conventions, describes common thermostat configurations, and offers troubleshooting tips to ensure the system operates reliably and efficiently.

Terminal	Typical Function	Common Use
O	Energize For Cooling	Most Carrier/Trane Systems And Many Thermostats
B	Energize For Heating	Common On Rheem, Some Older/International Systems
Orange Wire	Reversing Valve Control	Often Connects To O Or B Terminal

What Are O And B Terminals On A Heat Pump Thermostat

The O and B terminals are dedicated thermostat connections that control the heat pump reversing valve. The reversing valve switches refrigerant flow so the system provides heating or cooling. Which terminal is energized (O or B) determines whether the reversing valve is activated to make the system heat or cool.

Manufacturers and thermostats may use different conventions: some energize O for cooling, others energize B for heating. Understanding which your equipment uses prevents incorrect operation and potential damage.

How The Reversing Valve Works

The reversing valve is a solenoid-operated valve in the heat pump refrigerant circuit. It changes refrigerant flow direction, switching the indoor coil between evaporator and condenser functions. In cooling mode the indoor coil is the evaporator; in heating mode it becomes the condenser.

The thermostat energizes the reversing valve via the O or B terminal, supplying 24VAC to the valve coil. Some systems use a spring return so the valve rests in one position when unpowered and moves when powered.

Typical Wiring Conventions And Manufacturer Differences

Wiring labels can vary by brand and region. The following outlines common conventions but always verify with documentation before wiring.

• O Terminal: Frequently used to energize the reversing valve for cooling mode. Common on many North American thermostats and manufacturers like Trane and Carrier.
• B Terminal: Often used to energize the reversing valve for heating mode. Seen on brands such as Rheem and some international systems.
• Orange Wire: Typically carries the reversing valve signal and attaches to O or B on the thermostat according to system design.

Some modern thermostats detect whether O or B is needed based on system configuration, but manual setup is still common. Mistakes in selecting O vs B cause the heat pump to run in the wrong mode until corrected.

How To Identify Which Terminal Your System Uses

Before wiring, identify the heat pump manufacturer, model, and the reversing valve type. Use these steps to determine whether to use O or B:

1. Check The Manufacturer’s Wiring Diagram: The outdoor unit’s wiring panel or manual often indicates polarity and whether the valve is energized in heating or cooling.
2. Inspect The Valve Body: Some reversing valves are labeled with A/B or arrows indicating flow direction; match with manual guidance.
3. Look At The Thermostat Labeling: Many thermostats label O/B as a combined terminal and allow selection in setup menus.
4. Measure With A Multimeter: With the system calling for heat or cool, measure 24VAC at the O/B terminal to see which mode energizes the terminal.

Thermostat Settings: O/B Configuration And Auto-Detection

Modern programmable and smart thermostats often include a setup option that asks “Is the changeover valve energized in heating or cooling?” or a two-choice setting: O/B with selection for “O=Cool” or “B=Heat.” Input the correct option based on the outdoor unit documentation.

Some thermostats auto-detect the valve type during installation. However, confirmation by testing system operation after configuration is prudent to avoid reversed heating/cooling operation.

Common Wiring Scenarios And How To Handle Them

The wiring scenario depends on whether the thermostat has a single combined O/B terminal or separate O and B terminals.

Single Combined O/B Terminal

Most residential thermostats have one O/B terminal. Connect the orange wire from the outdoor unit to this terminal and configure the thermostat for whether the valve is energized in heat or cool.

Separate O And B Terminals

Some thermostats provide both O and B terminals. In that case, connect the reversing valve wire to the correct terminal only. Leave the other terminal unused. Connecting the wire to both can backfeed and cause malfunction.

Heat Pump With Auxiliary Heat

Systems with auxiliary electric or fossil fuel heat use additional terminals such as W, AUX, or E. Ensure these are wired correctly and that the O/B selection remains accurate to prevent simultaneous heating and cooling cycles.

Troubleshooting Reversing Valve And O/B Issues

If a heat pump runs opposite the desired mode or the reversing valve fails to switch, follow these troubleshooting steps:

• Verify Thermostat Configuration: Ensure the thermostat’s O/B setting matches the outdoor unit requirement.
• Check Wiring Connections: Confirm the orange (or designated) wire is securely connected to the O or B terminal and to the outdoor unit’s valve lead.
• Test For 24VAC Signal: With the system calling for heating or cooling, check for 24VAC at the thermostat terminal to confirm signal is present.
• Listen For Valve Activation: When the system switches modes, listen at the outdoor unit for a click that indicates valve movement. No click may indicate a faulty valve solenoid or absence of control voltage.
• Inspect The Reversing Valve Coil: If 24VAC is present but the valve doesn’t actuate, the coil may be burned or the valve mechanically stuck.

Safety And Best Practices For Wiring

Always turn off power at the air handler and outdoor disconnect before working on thermostat wiring. Use the following practices to avoid damage and ensure safe operation:

• Label Wires Before Disconnecting: Mark wires at both ends to avoid confusion during reassembly.
• Use Proper Gauge Wire: Thermostat wire is typically 18 AWG; use multi-conductor cable for more complex systems.
• Follow Manufacturer Diagrams: Adhere to outdoor unit and thermostat manuals for terminal assignments.
• Test After Power Restoration: After wiring and configuration, test heat and cool modes to verify reversing valve operation.

Examples Of Manufacturer Conventions

The following examples illustrate how different manufacturers use O and B labels; these are common patterns, not absolute rules:

Manufacturer	Typical Reversing Valve Energized
Carrier/Trane	O Energized For Cooling
Rheem/Ruud	B Energized For Heating
Goodman/Amana	Often O For Cooling, Verify Model
Daikin/Mitsubishi	May Use Different Controls Or Proprietary Wiring

Because practices vary, always refer to the unit’s spec sheet. If uncertain, contact manufacturer technical support or a licensed HVAC technician.

How Incorrect O/B Wiring Affects System Performance

If the reversing valve is set to energize in the wrong mode, the heat pump will operate in reverse: calling for heat will cool the house while calling for cooling will heat it. This creates discomfort, wasted energy, and can stress auxiliary systems.

Beyond comfort issues, long-term incorrect operation may increase wear on components and inflate energy bills. Correct configuration is a simple step that avoids costly service calls.

When To Call A Professional

DIY checks and configuration can resolve many O/B issues, but call a licensed HVAC technician when encountering any of the following:

• No 24VAC at thermostat terminal despite correct wiring and settings.
• Reversing valve is powered but fails to move, indicating a stuck valve or failed coil.
• Uncertainty about manufacturer-specific wiring or when working on complex multi-stage systems.
• Fear of damaging controls or violating warranty by incorrect wiring.

Modern Thermostats And Smart Controls

Smart thermostats often simplify O/B setup with step-by-step configuration and compatibility checks. During setup, the thermostat may query the user about system type and reversing valve behavior or attempt auto-detection.

Even with smart thermostats, verify operation manually after installation. Some systems require firmware updates or specific adapter kits to work with non-standard reversing valve arrangements.

Key Takeaways For “Heat Pump O Or B”

O vs B Controls The Reversing Valve — The O and B terminals tell the thermostat which mode energizes the reversing valve.

Manufacturer Variations Matter — Different brands use different conventions; always consult documentation.

Test Before Finalizing — Confirm which terminal energizes during heating or cooling with a multimeter or by observing system behavior.

Safety First — Turn off power when wiring and consider a professional if uncertain.

Helpful Resources And Further Reading

• ENERGY STAR: Heat Pumps — Overview of heat pump efficiency and guidelines.
• ASHRAE — Technical standards for HVAC design and operation.
• Manufacturer Installation Manuals — Access individual unit wiring diagrams through manufacturer websites.

Understanding whether to use Heat Pump O Or B allows homeowners and technicians to configure thermostats properly and ensure reliable, efficient heat pump operation. Proper wiring and verification reduce service calls and improve comfort. When in doubt, document the system and consult the manufacturer or a licensed HVAC professional.