Heat pump O and B settings: which one your reversing valve needs
O energises the reversing valve in cooling and B energises it in heating. Set the wrong one and the heat pump delivers the opposite mode.
What this means
O and B are two names for the same physical output. The thermostat energises one wire to hold the reversing valve in a particular position, and the setting simply tells the thermostat which mode should energise it. Honeywell Home describes O as energised during cooling and B as energised during heating, and says a large majority of heat pumps energise O in cooling.
Equipment and model context
Cross-brand explanation; product figures come from the model documentation
- Residential air-source and geothermal heat pumps with a single reversing valve output
Which convention applies is a property of the outdoor unit, not of the thermostat, so the equipment wiring diagram or installation instruction is the authority. Geothermal and some older or specialist air-source products use B where the majority use O, and this page does not attempt to list which brands do which.
What this guide covers
- The system blows cold air when heating is called for and warm air when cooling is called for.
- Heating works and cooling does not, or the reverse, with no fault code anywhere.
- The system behaves correctly in one mode and delivers the opposite temperature in the other.
What changes the answer
- Honeywell Home states that the O terminal is energised during cooling and the B terminal is energised during heating.
- Honeywell Home notes that some heat pump manufacturers energise B in heating while a large majority energise O in cooling.
- The Honeywell Home T6 Pro II exposes the choice as installer setup 218, with O in cool as the factory default and B in heat as the alternative.
One wire, one valve, two naming conventions
A heat pump has a single solenoid on its reversing valve. Energise the solenoid and the valve sits in one position; remove power and a spring returns it to the other. There is no third state and no separate heating output, which is why a heat pump thermostat has one changeover terminal rather than two.
Honeywell Home describes the changeover valve as reversing the flow of refrigerant so the same equipment can heat and cool. The valve does not know which mode the house wants. It knows whether its coil has 24 volts on it.
So the manufacturer of the outdoor unit makes a design choice: does the energised position produce cooling, or heating. If energised means cooling, the thermostat should hold the output on during cooling, which the industry labels O. If energised means heating, the output should be on during heating, labelled B.
Why the wrong setting produces such a strange symptom
Setting a thermostat to O on a system that expects B does not disable anything. Every other part of the sequence works: the compressor runs, the outdoor fan runs, the indoor blower runs, and refrigerant circulates at the correct pressures. Only the direction is inverted.
That is why the fault presents as a system delivering the opposite of what was asked rather than as a failure. There is no code, no lockout, and no noise, because from the equipment's point of view nothing is wrong.
It also explains the version of the symptom where one mode works and the other does not. If the wiring or setting leaves the valve permanently in one position, the mode that matches that position behaves normally and the other one delivers the wrong temperature. A reader who tests only in summer can install the fault and not discover it until autumn.
Establishing which convention your equipment uses
The authority is the outdoor unit. Its installation instruction or wiring diagram states which terminal the reversing valve solenoid is fed from and in which mode it is energised, and that statement outranks anything printed on a thermostat.
A second, weaker indicator is the wire that was there before. If a working thermostat had a conductor in a terminal marked O/B and the system heated and cooled correctly, the previous setting was right and is worth reproducing. That is evidence about the installation rather than about the equipment, so it fails whenever the original installation was itself wrong.
The observational test is to run each mode and check the temperature at a supply register, but there is a trap in it. A heat pump reverses into cooling during a defrost cycle, so a brief period of cool supply air in winter is designed behaviour and not proof of a changeover error.
Where the setting lives, and what else lives nearby
On the Honeywell Home T6 Pro II the choice is installer setup 218, offering O in cool as the default and B in heat as the alternative. The same installer menu carries the settings that decide how the system behaves once the direction is right: setup 200 for system type, setup 205 for whether the equipment is air-to-air or geothermal, and setup 255 for whether backup heat is electric or a fuel-burning appliance.
That grouping matters because the changeover setting is the one most likely to be missed on a self-installation. System type and backup heat type produce obvious misbehaviour on the first cycle. A changeover error can sit hidden for a whole season.
The other terminal to get right at the same time is W. Honeywell's wiring documentation carries an explicit warning not to use W for heat pump applications, because on a heat pump the backup heat belongs on Aux or E, and a conductor in W can call for heating that never stops.
| Setting | Output energised during | Valve state that produces cooling |
|---|---|---|
| Set to O | Cooling | Energised, which is the majority convention |
| Set to B | Heating | De-energised, with the coil powered in heat instead |
| Wrong setting selected | The mode the equipment does not expect | Modes are delivered inverted with no fault code |
| No conductor at O/B | Nothing | The valve rests in its spring position in both modes |
The thermostat has one output and the valve has two positions. The setting decides which mode puts the coil under power, and the equipment decides which position produces cooling.
- Thermostat O/B terminalA single 24 volt output, on or off
- Reversing valve solenoidEnergised in one mode, spring-returned in the other
- Refrigerant directionDetermines whether the indoor coil absorbs or rejects heat
- Defrost controlReverses the valve independently, which can confuse a field test
| Source of the answer | How much weight it carries |
|---|---|
| Outdoor unit installation instruction or wiring diagram | Decisive, because the valve belongs to that unit |
| The setting on the working thermostat being replaced | Good evidence about the installation, not about the model |
| Colour of the existing conductor | Weak, because colour conventions are not enforced |
| Supply air temperature in each mode | Useful confirmation, but defrost can mislead in winter |
| Thermostat factory default | Not evidence at all, only a starting point |
Questions people ask about this
Should my thermostat be set to O or B?
It depends on the outdoor unit. Honeywell Home describes O as energised in cooling and B as energised in heating, and says a large majority of heat pumps energise O in cooling, but the equipment wiring diagram is what settles it for a specific installation.
What happens if O and B are set the wrong way round?
The system delivers the opposite mode. Everything else runs normally, so there is no fault code and no unusual noise, and the only sign is warm air on a cooling call or cold air on a heating call.
Can I have a wire in both O and B?
No. There is one solenoid and one output, which is why heat pump thermostats provide a single terminal marked O/B and a setting to say which convention it follows.
Why does my heat pump blow cold air in winter anyway?
A heat pump reverses into cooling during a defrost cycle to clear frost from the outdoor coil, so cool supply air for a few minutes in cold weather is designed behaviour rather than a changeover error.
Where is the O/B setting on a Honeywell T6 Pro?
On the T6 Pro II it is installer setup 218, with O in cool as option 0 and the factory default, and B in heat as option 1. Other thermostats expose the same choice under their own menu numbering.
Do I connect backup heat to W on a heat pump?
No. Honeywell's wiring documentation warns explicitly against using W for heat pump applications and provides Aux and E terminals for auxiliary and emergency heat instead.
Evidence recordHow this page was checked
official manufacturer support article, operation manual · checked 2026-09-08
Every technical claim above was written from primary documentation held in the HVAC Bench evidence record: Resideo Honeywell Home technical literature. Where a source limits a definition to certain models, test conditions, or product classes, that limit is repeated here rather than generalised.
- Documentation class
- official manufacturer support article, operation manual
- Scope of the definition
- Confirm against the exact model manual
- Last checked
- 2026-09-08