Heat Pump Thermostat Setting: Choosing O or B for Proper Reversing Valve

Most modern heat pumps use a reversing valve that switches the system between heating and cooling. Thermostat manufacturers label the control for that valve as either O or B, and selecting the correct orientation is essential for efficient operation and occupant comfort. This article explains the difference between O and B, how to identify the correct setting, troubleshooting tips, and best practices for common U.S. installations.

Setting	Reversing Valve Energized	Typical Use
O	Energized In Cooling	Most Residential Heat Pumps
B	Energized In Heating	Some Older Or Specific Manufacturer Systems

What The O And B Thermostat Settings Mean

Thermostats that control heat pumps include a terminal for the reversing valve, labeled either O or B depending on the manufacturer’s convention. O means the reversing valve is energized when the heat pump is in cooling mode. B means the valve is energized when the heat pump is in heating mode.

Functionally, the reversing valve changes refrigerant flow so the outdoor coil becomes either the condenser or the evaporator. The correct O or B setting ensures the valve is actuated at the correct time, preventing reverse operation and potential heating or cooling failures.

Why Choosing O Or B Correctly Matters

An incorrect selection can cause the heat pump to operate in reverse of the thermostat command, making the home uncomfortable and potentially causing the auxiliary heat to run excessively. This raises energy bills and can create unnecessary wear on the system.

In some systems, improper reversing valve logic may also confuse diagnostic indicators or prevent defrost cycles from operating correctly, which can reduce system lifespan, particularly in cold climates.

How To Identify The Correct Orientation For A Specific Heat Pump

Check The Manufacturer Documentation

The most reliable method is to consult the heat pump and thermostat installation manuals. Manufacturers specify whether the reversing valve is O or B type. HVAC spec sheets and wiring diagrams often show the expected energization behavior.

Observe System Behavior During Heating And Cooling Commands

One practical test is to set the thermostat to cooling and confirm the outdoor unit runs in cooling mode: the outdoor fan should run and the outdoor coil should feel warm as it rejects heat. If the reversing valve is in the wrong position, the system may try to heat while cooling is requested.

Look At The Valve Wiring And Coil Color Coding

Some reversing valves have model numbers or wire color codes that indicate normally energized position. While not universal, red or orange wires and labels sometimes hint at manufacturer defaults. Still, this approach should be combined with documentation or testing.

Step-By-Step Field Test To Determine O Or B

1. Set thermostat to cooling and lower the setpoint below room temperature to force cooling.
2. Observe the outdoor unit: the fan should run and the outdoor coil should be warm to the touch when cooling is active.
3. If the outdoor coil is cold and indoor temperature rises, the reversing valve is likely reversed relative to the thermostat setting.
4. Next, set thermostat to heating and raise the setpoint above room temperature to force heating.
5. Confirm the outdoor unit runs and the outdoor coil feels cold during heating (system reversing valve correct for typical installations).

When results don’t match expected behavior, switch the thermostat O/B setting and repeat tests. Only change settings through the thermostat’s setup menu; do not rewire the valve without consulting manuals.

Common Scenarios And Recommended Settings

Standard Residential Heat Pumps

Most residential heat pumps (especially newer models) use an O reversing valve that energizes in cooling. Thermostat manufacturers also default to O in many heat pump setups, so new installations often work correctly out of the box.

Systems That Use B

Some older or specialized systems use B, energizing the reversing valve during heating. This is less common but important to identify, particularly when replacing thermostats or retrofitting controls with different manufacturer conventions.

Dual-Fuel Or Hybrid Systems

In dual-fuel setups where a gas furnace supplements a heat pump, correct O/B orientation remains critical because the thermostat coordinates valve operation and furnace lockouts for efficient operation.

Thermostat Wiring Guidelines And Safety

Thermostats typically have a dedicated terminal labeled O/B to connect the reversing valve. The O/B terminal sends 24V control signals supplied by the HVAC control circuit. Ensure power is off at the breaker before accessing wiring to minimize shock and equipment damage risks.

Replacing a thermostat requires transferring the O/B wire to the same labeled terminal on the new thermostat. If the original thermostat used a different label or combined terminals, consult wiring diagrams or take photos to preserve wiring relationships.

Troubleshooting Common Problems Related To O And B

System Runs Only In One Mode

If the heat pump only heats or cools regardless of thermostat commands, the reversing valve may be stuck, the thermostat setting (O/B) may be incorrect, or the control wiring may be faulty. Check the O/B setting first, then inspect the valve coil and wiring continuity.

Frequent Short Cycling Or Excessive Auxiliary Heat

Incorrect reversing valve operation can cause the system to misinterpret temperatures, increasing reliance on auxiliary electric heat. Verify proper valve orientation and run defrost cycles if ice buildup occurs to restore normal cycling.

Defrost Cycle Issues

Improper reversing valve behavior can prevent effective defrosting, causing ice accumulation on outdoor coils in cold climates. Ensuring the valve energizes in the correct mode fixes many defrost problems.

When To Call A Professional HVAC Technician

If changing the thermostat O/B setting doesn’t fix reversing valve behavior, or if the valve coil appears damaged, contact a licensed HVAC technician. Professionals can test coil resistance, check 24V control signals, and safely replace a stuck or leaking reversing valve.

Complex issues such as refrigerant leaks, compressor faults, or control board failures require diagnostic tools and should not be handled by inexperienced homeowners.

Best Practices For Thermostat Installation And Configuration

• Always confirm the heat pump model and manufacturer recommendations before configuring the thermostat’s O/B setting.
• Document wiring before removing an old thermostat: take clear photos and label wires.
• Use a thermostat that supports heat pump systems and provides an O/B terminal for reversing valve control.
• After installation, perform both heating and cooling tests and verify outdoor coil and fan behavior.
• Keep manuals and wiring diagrams accessible for future service visits.

Energy And Comfort Implications Of Correct O/B Settings

Correct O/B selection supports efficient heat pump operation and reduces run-time for backup heat, lowering energy bills. It also improves occupant comfort by ensuring the system responds properly to thermostat setpoints.

Misconfigured reversing valves force the system to work against its intended refrigerant flow, often causing longer recovery times, higher energy use, and potential premature component wear.

Examples And Manufacturer Notes

Different manufacturers sometimes ship reversing valves energized in opposite modes. For example, some older Carrier or Bryant systems used B-style logic while many contemporary Daikin, Mitsubishi, and Goodman systems use O-style logic. Always check specific model documentation for accurate orientation.

Smart thermostats often detect system type and recommend O or B during setup. If the smart thermostat makes an incorrect guess, the installer should override the selection and verify operation manually.

Key Diagnostic Tools And Metrics

Useful tools include a multimeter for verifying 24V control presence, an inline clamp meter for current draw, and infrared thermometers to check coil temperatures during operation. A technician may also use refrigerant gauges to confirm normal pressure differentials during heating and cooling cycles.

Track runtime, short cycles, and auxiliary heat activation frequency as measurable metrics that indicate correct or incorrect reversing valve behavior.

Frequently Asked Questions About O And B Settings

Can The O/B Setting Be Switched Without Wiring Changes?

Yes. Most thermostats allow switching the O/B logic in the settings menu. This flips which mode energizes the reversing valve without moving wires. This is the recommended first step for correcting orientation errors.

Is One Setting More Efficient Than The Other?

No. Efficiency is determined by the heat pump’s design and installation. O vs B simply defines when the valve is energized. Correct selection ensures efficient operation; using the wrong selection reduces real-world efficiency.

Will The Reversing Valve Fail If Set Incorrectly?

Setting the thermostat incorrectly does not immediately damage the valve, but prolonged operation with reversed logic can cause system stress and increased wear on components, increasing the likelihood of failure over time.

Maintenance Tips To Preserve Reversing Valve Functionality

• Schedule annual HVAC inspections to verify reversing valve operation and overall system health.
• Keep outdoor coils clean and clear of debris to prevent strain on the system during mode changes.
• Monitor and replace worn wiring or connectors that control the valve to avoid intermittent energization issues.
• Address refrigerant leaks promptly; improper refrigerant charge stresses compressor and valve operation.

Resources And References For Further Reading

Consult the heat pump manufacturer’s technical bulletins, thermostat installation guides, and recognized HVAC trade resources such as ACCA (Air Conditioning Contractors of America) for wiring standards and best practices. Contractor training materials and OEM wiring diagrams provide model-specific guidance.

For safety and warranty concerns, always follow manufacturer instructions when adjusting thermostat settings or ordering replacement parts.

Keywords Used: Heat Pump Orientation O or B, O or B thermostat, reversing valve, heat pump thermostat setting, O vs B, heat pump wiring.