Heat-Pump Thermostat O/B Setting: Reversing-Valve Logic
The O/B thermostat terminal controls a heat pump’s reversing valve on conventional 24-volt systems. Some equipment energizes the valve in cooling; other equipment energizes it in heating. The thermostat installer setting must match the outdoor unit. If it is wrong, the system may deliver heat during a cooling call and cooling during a heating call.
O and B are not two wires that should be energized together on most modern thermostat terminals labeled O/B. They describe alternate changeover logic. The equipment wiring diagram and thermostat guide determine the correct choice.
Understand what the reversing valve changes
A heat pump reverses refrigerant flow so the indoor and outdoor coils exchange roles between heating and cooling. The reversing-valve solenoid moves that valve when its control condition is met. The thermostat’s O/B output provides the command; it does not power the compressor by itself.
The outdoor control may add timing and protection. Hearing a valve shift does not confirm the full refrigeration cycle, and lack of an obvious sound does not prove the valve failed.
Use the outdoor-unit documentation
Thermostat menus may label the choices “energize on cool,” “energize on heat,” O, B, or by equipment brand. Select from the heat-pump installation instructions, not a generic online chart. Product families and communicating controls can differ.
If a contractor replaced only the thermostat, preserve the old wiring photographs and installer settings. Thermostat compatibility with new equipment explains why control type and stages should be identified together.
A wrong setting creates a clear mode reversal
After the normal compressor delay, a cooling call may produce warm supply air while a heating call produces cool air. Confirm that the thermostat is actually in the selected mode and allow enough time for the coil to change temperature. Auto changeover and defrost can otherwise confuse a quick test.
Do not repeatedly switch modes. Each change can introduce compressor protection, and rapid operation is not a safe diagnostic method.
Auxiliary heat can hide the error
During a heating call with incorrect changeover logic, electric auxiliary heat may warm the supply air while the heat pump operates in cooling. The home can appear to heat, but energy use and system operation are wrong. A thermostat may also display auxiliary heat without revealing the reversing-valve command.
A technician should test compressor and auxiliary stages separately, using the documented installer procedure and respecting time delays. The equipment sequence matters more than the final register temperature.
Wiring faults can mimic configuration trouble
A loose O/B conductor, damaged cable, incorrect splice, failed thermostat output, or outdoor-board issue can prevent the command from reaching the valve. A configuration change will not repair an open circuit. Conversely, replacing a reversing valve will not correct a wrong installer setting.
With power off, an owner may compare visible terminal labels to the installation record. Live-voltage testing, opening outdoor controls, and energizing wires manually are professional tasks.
Defrost is not evidence that O/B is wrong
Heat pumps temporarily change refrigeration operation to remove frost from the outdoor coil. Indoor airflow may pause or auxiliary heat may operate. This is initiated by equipment controls and can involve the reversing valve even when the thermostat remains in heat mode.
Judge the thermostat setting during stable heating and cooling operation, not during defrost. Record outdoor weather, displayed mode, and fault messages if behavior is inconsistent.
Communicating systems use different signals
A proprietary thermostat may send digital commands rather than a conventional O/B voltage. Its setup process can identify the outdoor unit automatically or use model-specific menus. Converting to a conventional thermostat may require an approved interface and could remove advanced functions.
Do not attach an O/B wire based solely on color when the original control used a communication bus. The equipment manual is the authority.
Commission both seasons after correction
Verify a steady cooling call, a steady heating call, fan behavior, each compressor stage, and auxiliary heat where present. Record which O/B option was selected and the documentation supporting it. The outdoor unit should respond after its normal delay without fault codes.
If the unit receives the correct changeover command but still conditions in the wrong direction, the technician must evaluate the valve, coil temperatures, control board, and refrigeration circuit. The general heat-pump operating overview can orient owners, while equipment-start diagnosis applies when the outdoor section never begins.
Account for dual-fuel controls
In a dual-fuel system, the thermostat or interface may stop the compressor and start a furnace according to outdoor temperature or control logic. The reversing valve is irrelevant while the furnace provides heat, so a brief observation during changeover cannot validate O/B behavior.
Test heat-pump heating when the controls permit it, then verify the furnace transition separately. Outdoor sensor accuracy, lockout settings, and stage ownership should be documented; copied balance-point values can produce unnecessary fuel use or inadequate heat.
Record the final terminal sequence
The commissioning sheet should show O/B state in cooling, heat-pump heating, off, and defrost where service data makes it observable. Add Y stages, G behavior, and auxiliary calls to the same chart. This gives future technicians a model-specific truth table rather than a brand assumption.
Sources and verification
The technical statements in Heat-Pump Thermostat O/B Setting: Reversing-Valve Logic were cross-checked against the primary references below. Final sizing, airflow, electrical, combustion, and refrigerant decisions must also follow the exact equipment instructions and applicable local code.