HVACopediaLennox · Heat pump · Defrost
Demand defrost cycle
What happens, in order
- Control tracks outdoor ambient, coil temperature and heating run time
- When coil temperature is below 37 F, heating run time is counted as frost accumulation time
- When the accumulation target is reached, defrost starts
- Compressor goes off for 30 seconds going into defrost if the DELAY jumper is on
- Control energizes the reversing valve, turns the outdoor fan off, and puts 24 V on W1 for auxiliary heat
- Defrost ends when the coil sensor reaches the termination temperature, when 14 minutes have run, or when the thermostat demand ends
- If the thermostat ends it, defrost is held until the next demand; if the coil is still below termination, the control continues the defrost
- Compressor goes off for 30 seconds again coming out of defrost if the DELAY jumper is on
- After defrost the control adjusts the next accumulation time up or down by 30 minutes based on how long the last defrost took
- If the outdoor temperature is below 10 F the accumulation time is set to 360 minutes
Timings and thresholds
| Item | Value |
|---|---|
| Initial frost accumulation target | 90 min |
| Accumulation adjustment | plus or minus 30 min |
| Cold climate accumulation below 10 F | 360 min |
| Maximum defrost length | 14 min |
| Termination temperature choices | 50, 70, 90, 100 F (factory 50 F, 90 F if shunt missing) |
| Nominal defrost time shunt P3 | 60, 100, 140 (default), 165 s |
| Compressor off at defrost entry and exit | 30 s with DELAY jumper (shipped on) |
Codes you may see during this
Coil sensor fault, ambient sensor fault (defrost board LEDs)
Applies to: ML14KP1 and EL16KP1 heat pumps
Models
More Lennox heat pump sequences
- Heat pump first-stage heating · Heating
- Heat pump cooling call · Cooling
Related
Lennox error codes · Lennox tips · Lennox parts · All Lennox sequences
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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.