What the O/B circuit controls
A heat pump uses the same refrigeration circuit for summer cooling and winter heating. The reversing valve changes the direction of refrigerant flow. The thermostat does not move the valve mechanically; it sends a 24-volt control signal to the reversing-valve solenoid through the O/B circuit.
On most residential systems, the compressor call still travels on Y or Y1. The indoor fan call uses G or equipment-controlled fan logic. Auxiliary heat typically uses AUX, W2, or E. The O/B wire has one focused job: select the refrigeration mode.
O versus B: energized in cooling or heating
| Designation | Typical valve logic | Thermostat action | Common symptom if wrong |
|---|---|---|---|
| O | Valve energized in cooling | Thermostat sends 24 V on O/B during cooling | Heating and cooling operate backward |
| B | Valve energized in heating | Thermostat sends 24 V on O/B during heating | Cooling call produces warm air |
| O/B | Configurable | Installer setup selects O or B behavior | Correct wire, wrong software setting |
| B used as common | Not a reversing-valve signal | Should be verified and usually moved to C on the new control | Blown fuse, no power, or thermostat damage |
Brand tendencies can help a technician form a hypothesis, but they are not proof. Equipment may be replaced while the thermostat cable remains, control boards may use nonstandard labels, and older thermostats sometimes use B for common. The equipment schematic is the controlling reference.
How to identify the correct O/B wire safely
- Turn off power to the indoor and outdoor equipment. Low-voltage shorts commonly blow the 3- or 5-amp fuse on the indoor control board.
- Photograph the existing thermostat terminals. Capture labels, jumpers, wire colors, and any unused conductors.
- Confirm that the system is actually a heat pump. The outdoor unit should operate in heating mode, and its data plate or model documentation should identify it as a heat pump.
- Inspect the indoor equipment board or splice point. Follow the conductor connected to O, B, O/B, or the reversing-valve circuit.
- Read the outdoor-unit wiring diagram. Confirm whether the solenoid is energized in cooling or heating.
- Configure the thermostat for heat-pump equipment. Select the correct number of compressor stages, auxiliary heat type, and O/B logic.
- Test both modes. Verify the voltage signal, outdoor-unit operation, supply-air response, and auxiliary-heat staging.
Symptoms of wrong O/B wiring or configuration
- Warm air from the vents during a cooling call
- Cool or room-temperature air during a heating call
- The thermostat says “Cooling,” but the refrigerant cycle is in heating
- The thermostat says “Heat,” but the outdoor coil behaves as though cooling
- Auxiliary heat runs during mild weather because the heat pump is working against the call
- Very high electric bills
- Long run times or failure to reach set temperature
- Short cycling after a thermostat replacement
- Correct operation in one mode but not the other
- A mode changes only when the O/B conductor is manually disturbed
Related problems can create similar symptoms. Read our guides to thermostat failures, thermostat wiring, C-wire power problems, and setpoint problems.
Nest, ecobee, and Honeywell Home O/B setup
Google Nest
Nest models use an O/B or star terminal depending on the generation. Google warns that a wire marked B can be either the heat-pump reversing-valve conductor or, on some systems, the common wire. Nest setup must match the actual equipment, and the thermostat equipment menu should show the intended terminal function.
ecobee
ecobee thermostats use an O/B terminal and a configurable reversing-valve setting. ecobee's official troubleshooting guidance identifies incorrect O/B energizing as a primary reason a heat pump supplies heat during a cooling call or cooling during a heat call.
Honeywell Home and Resideo
Current Honeywell Home installation documentation distinguishes O as energized in cooling and B as energized in heating. Installer setup on compatible models includes a reversing-valve selection. Heat-pump wiring diagrams also separate O/B from auxiliary and emergency heat terminals.
O/B, auxiliary heat, emergency heat, and dual fuel
O/B does not replace the auxiliary-heat circuit. Electric strip heat normally stages through AUX or W2. Emergency heat may share AUX/E or have a separate E input. Dual-fuel systems require additional logic so the thermostat or equipment control stops the heat pump before energizing a gas furnace.
| Terminal | Typical function | Common mistake |
|---|---|---|
| O/B | Reversing valve | Configured for the wrong energized mode |
| Y / Y1 | Compressor stage 1 | Assuming O/B alone starts the heat pump |
| Y2 | Compressor stage 2 | Missing stage on two-stage equipment |
| AUX / W2 | Backup electric or fossil-fuel heat call | Connected as conventional W without heat-pump setup |
| E | Emergency heat | Confused with ordinary auxiliary staging |
| L / L-A | Equipment fault or monitor input on some systems | Treated as a heating output |
For broader terminal definitions, use our complete thermostat wiring guide.
Professional O/B diagnostic sequence
- Confirm thermostat model, system type, number of compressor stages, and backup-heat configuration.
- Verify that the outdoor equipment is a heat pump and identify valve logic from the unit diagram.
- Check thermostat installer settings for O versus B.
- Measure R-to-C voltage and confirm stable 24-volt power.
- Call for cooling and measure O/B-to-C at the thermostat.
- Repeat at the indoor control board, splice, and outdoor unit to locate an open conductor or voltage drop.
- Call for heating and verify the O/B signal changes as the equipment requires.
- Measure reversing-valve coil resistance with power off and inspect the coil connection.
- Confirm that energizing the solenoid actually shifts the valve and changes refrigerant temperatures and pressures.
- Test defrost, auxiliary heat, emergency heat, and lockout logic where applicable.
If the thermostat produces the correct signal but the valve does not shift, the problem may be a failed coil, stuck reversing valve, damaged wiring, defrost-board fault, or refrigeration-system issue. If the signal is wrong at the thermostat, configuration or thermostat hardware moves higher on the list.
What homeowners can safely check
- Confirm the thermostat is set to the intended mode.
- Wait through normal compressor-protection delays after changing modes.
- Check whether the outdoor unit runs in heating mode.
- Review a photo of the original thermostat wiring.
- Confirm whether the app setup identifies a heat pump.
- Look for an obvious loose wire only after power is shut off.
Repair versus thermostat replacement
| Finding | Likely action | Replacement usually needed? |
|---|---|---|
| Wrong O/B installer setting | Correct setup and test all modes | No |
| Loose conductor or poor splice | Repair and secure connection | No |
| Broken thermostat cable conductor | Use verified spare conductor or replace cable | Not necessarily |
| Failed thermostat O/B relay | Replace compatible thermostat | Usually |
| Failed reversing-valve solenoid coil | Replace coil when serviceable | No thermostat replacement |
| Stuck reversing valve | Evaluate valve repair, refrigerant work, system age, and warranty | Possibly equipment-level repair or replacement |
| Communicating or proprietary system | Use approved control and manufacturer procedure | Generic thermostat may be inappropriate |
Thermostat replacement should solve a proven thermostat problem, not substitute for testing. A new control cannot repair a stuck reversing valve, failed coil, damaged cable, bad transformer, blown fuse, defrost-board fault, or refrigerant issue.
Frequently asked questions
What does the O/B wire do on a heat-pump thermostat?
The O/B circuit controls the heat pump reversing valve, which changes refrigerant flow so the same outdoor unit can provide cooling or heating. The thermostat must be configured for the valve logic used by the equipment.
What is the difference between O and B thermostat wiring?
O normally means the reversing valve is energized in cooling. B normally means it is energized in heating. Many modern thermostats use one configurable O/B terminal instead of separate terminals.
Why does my heat pump blow hot air in cooling mode?
A wrong O/B configuration, loose O/B wire, thermostat setup error, damaged low-voltage wiring, failed reversing-valve coil, stuck valve, or control-board problem can make the system operate in the opposite mode.
Why does my heat pump blow cold air in heating mode?
The thermostat may be energizing the reversing valve in the wrong season, the O/B signal may be missing, or the heat pump may have a refrigerant, defrost, compressor, airflow, or auxiliary-heat problem.
Can a blue B wire be connected to O/B?
Not safely without verification. On some systems B is the reversing-valve wire, but on others a blue wire or B terminal is actually common. Verify the equipment-board terminal and original wiring before moving it.
Is O/B the same as the C wire?
No. O/B is a switched control signal for the reversing valve. C is the 24-volt common return used to power the thermostat and complete low-voltage circuits.
Should a Carrier heat pump use O or B?
Many Carrier-family heat pumps energize the reversing valve in cooling, but the correct setting must be verified from the specific unit wiring diagram or control board rather than assumed from brand alone.
Should a Rheem or Ruud heat pump use B?
Many Rheem and Ruud systems have historically energized the reversing valve in heating, but model-specific documentation and field verification should determine the setting.
Can the wrong O/B setting damage a heat pump?
A brief incorrect setup usually causes reversed heating and cooling rather than immediate damage. Continued operation can create comfort problems, unnecessary auxiliary-heat use, high bills, and compressor stress, so it should be corrected promptly.
Does auxiliary heat connect to O/B?
No. Auxiliary or emergency heat normally uses AUX, W2, E, or a model-specific terminal. O/B controls the reversing valve. The thermostat must also be configured for electric auxiliary heat or dual fuel as appropriate.
How does a technician test the O/B circuit?
A technician verifies the thermostat configuration, measures the 24-volt O/B output in heating and cooling, checks the signal at the indoor and outdoor controls, tests the reversing-valve coil, and confirms the valve actually changes refrigerant flow.
When should I replace the thermostat instead of repairing the wiring?
Replacement makes sense when the thermostat relay or terminal is damaged, the thermostat cannot support the heat-pump stages or accessories, configuration is unreliable, or repair cost approaches the price of a properly compatible control.
Official manufacturer sources
Heat-pump thermostat diagnostics in North Houston
AC Repair Expo Heating & Cooling Inc diagnoses heat-pump thermostats, reversing-valve circuits, low-voltage wiring, defrost controls, auxiliary heat, and complete HVAC operation throughout Spring, The Woodlands, Tomball, Cypress, Conroe, Humble, Kingwood, and nearby North Houston communities.
Call 832-479-2727 or book online. Texas HVAC license TACLB43277C.