What happens
The full-capacity request is absent because of settings, a conductor fault or controller logic.
Typical symptoms
A two-stage installation operates at low capacity during high demand.
Checks that distinguish this cause
Verify thermostat stage support and settings, then trace Y2 through documented checkpoints while accounting for staging delays. Confirm actual airflow and compressor state.
What can look similar
Correct staging cannot overcome undersizing, poor charge or restricted airflow.
Repair direction
Correct the approved setup or repair the identified stage-command conductor.
Verify the repair
Verify the intended transition and matching airflow without bypassing timers.
Equipment-specific limits
A valid Y2 call is only one part of staging. Solenoid voltage, timing and operating-stage logic depend on the exact compressor and controller.
Practice this diagnosis
This cause is a reference topic; it does not yet have a dedicated simulated call. Browse the practice board.
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.
- Copeland: Two-stage scroll compressorsLow-voltage solenoid modulation on applicable two-stage scroll models.
- Resideo: RTH6360 thermostat installation and setupExamples of heat-pump O/B and staging setup. Menu numbers vary by thermostat.
Editorial review: 2026-09-29. Verify the current manual for the installed model.