What happens
The thermostat is configured for a different equipment type, changeover logic or number of stages.
Typical symptoms
Heat during a cool call after replacement; a two-stage system never requests full capacity.
Checks that distinguish this cause
Compare the installed equipment manual with the thermostat installer settings, O/B logic, stage count and terminal assignments. Do not assume every heat pump energizes O/B in cooling.
What can look similar
An open O/B or Y2 wire can produce identical symptoms even with correct settings.
Repair direction
Configure the supported functions and correct wiring according to both manufacturers' instructions.
Verify the repair
Verify mode change and stage transitions at the equipment, including correct indoor airflow.
Equipment-specific limits
Confirm the equipment type and supported stages. Anti-short-cycle delays, schedules and lockouts may be normal behavior.
Practice this diagnosis
These simulated calls exercise this component or repair family. The article describes a broader diagnostic process; a call may represent a different cause within that family.
References and applicability
Original HVACSim explanations with manufacturer and instrument-maker references for further reading. These resources support the subject; their model-specific examples are not specifications for every system.
- Resideo: RTH6360 thermostat installation and setupExamples of heat-pump O/B and staging setup. Menu numbers vary by thermostat.
- Fluke: HVAC and industrial electrical testing applicationsElectrical measurement principles, including control-circuit voltage references.
Editorial review: 2026-09-29. Verify the current manual for the installed model.