HVACopediaLennox · Heat pump · Cooling
Heat pump cooling call
What happens, in order
- Thermostat set to cooling puts 24 V on O and energizes the reversing valve
- Y1 demand from the thermostat
- 24 V energizes the compressor contactor K1
- K1 closes and the compressor and outdoor fan start
- Demand satisfied Y1 drops, K1 opens, and the compressor and outdoor fan stop immediately
- The defrost board holds a 5 minute anti-short-cycle delay after power-up, a safety trip or the end of a thermostat demand; the TEST pins override it
Timings and thresholds
| Item | Value |
|---|---|
| Anti-short-cycle delay | 5 min |
Codes you may see during this
Defrost board LED states: low pressure fault/lockout, high pressure fault/lockout, ambient sensor fault, coil sensor fault
Applies to: ML14KP1 and EL16KP1 heat pumps (R-454B)
Models
More Lennox heat pump sequences
- Heat pump first-stage heating · Heating
- Demand defrost cycle · Defrost
Related
Lennox error codes · Lennox tips · Lennox parts · All Lennox sequences
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Sequences of operation, error codes and tips work offline in the HVACopedia app.
Source: Lennox ML14KP1 service manual 100108 (03-2026), Defrost System pp.29-32 and ML14KP1 Operating Sequence p.37; Lennox EL16KP1 service manual 100195 (03-2025), same sections. Written in our own words; timings can differ by model and software version, so check the manual for your exact unit.
For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.