CSLB C-20 HVAC ContractorControls, Electrical and SafetyMedium
On a heat pump thermostat wiring system, the reversing valve solenoid is wired to the O terminal (energized in cooling mode) on most manufacturers' equipment. If a technician mistakenly wires the reversing valve to the B terminal instead, what will happen when the system calls for cooling?
- AThe blower will fail to start because O and B share the same relay
- BThe reversing valve will remain de-energized during cooling, causing the unit to run in heating mode
- CNo change will occur since O and B terminals are functionally identical
- DThe compressor will short cycle due to low voltage feedback
Show answer & explanationAnswer & explanation
Correct answer: B. The reversing valve will remain de-energized during cooling, causing the unit to run in heating mode
O and B terminals control the reversing valve but with opposite logic depending on manufacturer convention: O energizes the valve in cooling, while B energizes it in heating. If wired to B on an O-type valve, the valve stays de-energized during a cooling call, causing the system to operate backward (in heating mode) when cooling is selected.
Why the other options are wrong
- A. O and B are not shared relays; they are distinct terminals for reversing valve logic.
- C. O and B terminals are not interchangeable—they represent opposite energization conventions.
- D. Reversing valve wiring errors affect mode selection, not compressor cycling directly.
Heat Pump O/B Terminals
O and B terminals control the reversing valve solenoid; O-type valves energize in cooling, B-type valves energize in heating, depending on manufacturer.
- O = Energized in Cooling (common convention)
- B = Energized in Heating (opposite convention)
- Wiring mismatch causes system to run in wrong mode
Memory trick: 'O' is for 'On in cOOling'; mismatched wiring flips the season.