Proper heat pump thermostat wiring is crucial for comfort and system performance. Many homeowners and technicians encounter confusion with the “O” and “B” terminals—small choices with a big impact. This article explores the difference between O and B orientation, how these terminals influence heat pump operation, and how to correctly configure your system for maximum efficiency and manufacturer compatibility.
Heat Pump Terminal Wiring: Overview Table
| Aspect | O Terminal | B Terminal |
|---|---|---|
| Function | Reversing valve in cooling | Reversing valve in heating |
| Common Brands | Trane, American Standard, Goodman | Rheem, Ruud, some older units |
| Thermostat Setting | Select “O” for “Changeover with Cool” | Select “B” for “Changeover with Heat” |
| Typical Comfort Issues | Blowing warm in cool, cool in heat when miswired | Same as O; system runs opposite mode when miswired |
| Diagnostic Tip | Check manufacturer’s guide for correct selection | Always match thermostat Y, O/B logic to unit |
Understanding Heat Pump Reversing Valve Orientation
Heat pumps use a reversing valve to switch between heating and cooling modes. This valve is controlled by a low-voltage wire coming from the thermostat, connected to the “O” or “B” terminal. The orientation—O or B—determines when this valve is energized, which in turn decides whether the system provides heating or cooling at any given time.
What Are “O” And “B” Terminals In Heat Pump Wiring?
The “O” terminal signals the reversing valve to switch to cooling mode when energized. Conversely, the “B” terminal signals the valve to switch to heating mode when energized. Only one of these wires is used per system. The correct selection depends on the heat pump manufacturer’s design.
- O (Changeover with Cooling): Energizes the reversing valve when cooling is needed.
- B (Changeover with Heating): Energizes the reversing valve when heating is needed.
Miswiring these terminals causes the system to run in the opposite mode, leading to comfort complaints and inefficient operation.
Why Does Heat Pump Orientation Matter?
The orientation setting determines whether the system interprets a call for heat or cool correctly. Selecting “O” when your unit needs “B”—or vice versa—causes the system to blow warm air when cooling is requested, and cool air when heating is requested. This misconfiguration wastes energy and leads to uncomfortable indoor environments.
Major Heat Pump Brands: O Vs. B Terminal Chart
| Brand | O or B Terminal? | Typical Default | Check Documentation? |
|---|---|---|---|
| Trane | O | O | Yes |
| American Standard | O | O | Yes |
| Carrier/Bryant | O | O | Yes |
| Rheem/Ruud | B | B | Yes |
| Goodman/Amana | O | O | Yes |
How To Tell If You Need O Or B Orientation
The most reliable way to verify correct thermostat configuration is to reference the heat pump’s manufacturer documentation. Most modern thermostats offer an “O/B” setting in their configuration menu—sometimes titled “Changeover Valve” or “Reversing Valve”—where you specify which terminal gets energized for cooling or heating mode. Never guess—unit performance and warranty depend on correct setup.
Steps For Determining O Or B Setting
- Locate the unit’s brand and model number from the outdoor heat pump.
- Check the official installation manual or manufacturer’s website for thermostat wiring guidance.
- In the thermostat install/setup menu, locate the changeover valve (O/B) option and match it to the heat pump’s requirement.
- If uncertain, consult an HVAC technician or call the manufacturer’s support line.
Practical Example: O/B Terminal Confusion
Suppose a homeowner replaces an old thermostat with a new smart model. During setup, the thermostat asks, “Should the O/B terminal energize in cooling or heating?” If the unit is Trane or Carrier, selecting “Cooling” is correct. For Rheem or Ruud, choose “Heating.” Selecting the wrong option will cause the system to appear “reversed,” and can result in uncomfortable temperatures inside the house.
In these cases, an O/B wiring mistake usually presents as blowing cold air in heating mode, or warm air in cooling mode, despite fans and compressors operating normally. Double-check both wiring and orientation settings to resolve the issue efficiently.
Heat Pump O Vs. B: Modern Smart Thermostat Support
Modern thermostats from brands like Honeywell, Ecobee, and Nest automatically prompt users for the correct O/B (changeover) orientation during installation. They may also display a system test function to confirm that heating and cooling run in the proper direction. Some smart thermostats allow switching the O/B logic via their settings, offering flexibility for multi-brand installations.
- Nest: Offers step-by-step compatibility checks and “heat pump orientation” prompts.
- Ecobee: Lets homeowners manually test and switch O or B orientation from the thermostat’s setup menu.
- Honeywell: Displays O/B changeover options in the guided install wizard.
Always review the thermostat’s installation guide for details, as interfaces vary between models and brands.
Potential Consequences Of Incorrect O/B Wiring
A misconfigured O/B terminal prevents the heat pump from producing the correct output for heating or cooling calls. The most common complaints include:
- Unit blows warm air instead of cool in summer, or cool air during winter.
- Energy consumption increases, but indoor comfort does not improve.
- Compressor short-cycling and higher wear.
- Potential voiding of system warranties due to improper installation.
Prompt correction of O/B wiring is vital for efficient and reliable HVAC system performance.
The Role Of The Reversing Valve In Heat Pumps
At the heart of a heat pump’s ability to provide both heating and cooling is the reversing valve. Controlled via the thermostat’s O or B wire, this electro-mechanical valve changes the direction of refrigerant flow. When the O (or B) terminal is energized, the valve physically shifts, swapping the system from one mode to the other. Ensuring the correct terminal is energized at the right time is critical for the system to respond as intended.
This is why O/B orientation is a fundamental part of heat pump setup and service.
How To Change O/B Orientation On Common Thermostats
- Honeywell: Use the “Installer Setup” and toggle Function 0170 to heat pump; Function 0190 to select O (cooling) or B (heating).
- Ecobee: Settings > Installation Settings > Equipment > O/B Reversing Valve; select “Cool” to energize for cooling or “Heat” for heating.
- Nest: Settings > Equipment > Heat Pump > Orientation, and choose based on brand/manufacturer requirements.
- Emerson Sensi: Guided install prompts user to select O or B based on heat pump requirements.
If you’re unsure, consult the thermostat manual or manufacturer support for guidance tailored to your system.
When To Use Both O And B Terminals Simultaneously
For almost all residential heat pumps, only one of the O or B terminals is connected—never both. Some older commercial or specialized systems may employ unique control sequences, but this is highly unusual in American homes. Connecting both wires at once may cause a short circuit, blown fuse, or damage to the thermostat control board. Stick to manufacturer instructions and use only the officially required terminal.
Wiring Color Codes For O/B Terminals
The O wire is typically orange, and the B wire is generally blue or dark blue. However, color codes are not universal. Confirm wiring by tracing from the unit to the thermostat or using the system wiring diagram, as previous installers may have deviated from standard colors.
- Orange: O (reversing valve – cooling energized)
- Blue or dark blue: B (reversing valve – heating energized)
Never rely solely on color to determine wiring—trace and verify every terminal before installing or configuring a new thermostat.
Heat Pump Orientation In Dual Fuel Or Hybrid Systems
Some homes feature dual fuel systems (heat pump with a gas furnace backup), where proper O/B terminal configuration is even more critical. Incorrect orientation in these systems can result in both the furnace and the heat pump running simultaneously or not at all, leading to rapid component wear or loss of heat/cool altogether. Hybrid systems demand careful attention to both thermostat setup and control board wiring to avoid costly misfires and maintain energy efficiency.
Heat Pump Orientation And Control Board Jumpers
Certain heat pumps include a jumper or dip switch on their control board to designate O or B terminal operation. This setting must match the thermostat’s setup. Never adjust jumpers without consulting the service documentation, as incorrect configuration may disable safety timing, defrost cycles, or system protection mechanisms.
Checklist: Best Practices For O/B Terminal Orientation
- Always consult the heat pump’s installation manual for correct O or B selection.
- Use only one terminal (O or B)—never both. Confirm with wiring diagram.
- Label wires as you remove old thermostats or when installing new HVAC equipment.
- Test system after setup to ensure heating produces warm air and cooling produces cold air.
- Contact a licensed HVAC technician if you remain uncertain about wiring or orientation.
Common Questions About O/B Terminals In Heat Pumps
Can I Use A Standard Thermostat With A Heat Pump?
No—only thermostats with a dedicated O/B terminal or “heat pump mode” can properly control the reversing valve function. Standard thermostats for gas or electric furnaces lack this capability and cannot power the O or B wire at the correct time, resulting in failures to change modes.
What If My System Works Backwards After Thermostat Installation?
This outcome is a sign of incorrect O/B terminal configuration. Return to the thermostat’s installer settings and switch the O/B logic as required. Test both heating and cooling modes after making the change.
How Do I Know If My Heat Pump Uses O Or B?
The best way is to check your unit’s nameplate and consult the official wiring diagram. Most American Standard, Trane, Goodman, and Carrier systems use “O”; Rheem and Ruud systems often use “B.” Always confirm—exceptions exist based on region and age.
Diagnostic Tips: Troubleshooting O/B Wiring Issues
HVAC pros diagnose O/B terminal issues by activating heating and cooling one at a time while observing the compressor and air temperatures. If hot air blows when cool is called (or vice-versa), suspect O/B miswiring. Using a multimeter, verify which terminal is energized during each call. Adjust the thermostat setting if the valve is swapping modes at the wrong times.
Summary Table: Key Takeaways About Heat Pump O Vs. B Orientation
| Key Point | Details |
|---|---|
| Orientation Importance | Prevents system from running in “reverse,” protects comfort and efficiency |
| How To Check | Always use manufacturer documentation, not guesswork |
| Common Defaults | O: Trane, Carrier, Goodman | B: Rheem, Ruud |
| Symptoms Of Misconfiguration | Blowing warm air in cooling or cool air in heating, cycling problems |
| Smart Thermostat Support | O/B logic is selectable in setup menus; test after configuration |
Final Thoughts And Resources
While O and B terminals seem minor, their orientation defines the basic operation of every heat pump. Double-checking orientation saves time and cost, ensures proper comfort, and protects both equipment efficiency and longevity. For complex systems, always reference industry-standard sources or hire a licensed HVAC professional for proper installation and troubleshooting.