HVACopediaLennox · Heat pump · Heating
Heat pump first-stage heating
What happens, in order
- Thermostat set to heating leaves O de-energized, so the reversing valve is off
- Y1 demand from the thermostat energizes contactor K1, which starts the compressor and outdoor fan
- While heating below 37 F coil temperature, the control adds up run time as frost accumulation time
- Demand satisfied K1 drops and the compressor and fan stop right away
- Two low pressure switch trips in a row in heating start a defrost cycle (for freezing rain)
- Low pressure switch is ignored for 90 seconds after compressor start, during defrost and 90 seconds after it, during TEST, and for 10 minutes of compressor run when outdoor temp is below 15 F
Timings and thresholds
| Item | Value |
|---|---|
| Low pressure switch ignore after start | 90 s |
| Low pressure ignore below 15 F | 10 min |
| Strikes to lockout per fault | 5 |
Codes you may see during this
Low pressure fault/lockout, high pressure fault/lockout (defrost board LEDs)
Applies to: ML14KP1 and EL16KP1 heat pumps
Models
More Lennox heat pump sequences
- Heat pump cooling call · Cooling
- 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.