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Lennox · Heat pump troubleshooting

Heat pump not defrosting / outdoor coil iced - Lennox EL16KP1 / ML14KP1

What is going on

Ice builds up on the outdoor coil in heating and the unit does not clear it, or defrost ends before the coil is clear.

Likely causes

  • Coil sensor fault (alternating fast flash) - no defrost at all
  • Coil sensor not clipped in the right spot
  • Termination temperature set too low for a cold climate
  • Reversing valve or its 24 VAC output (O OUT) not working
  • Low charge or poor airflow making frost build faster

Check in this order

  1. Read the defrost control LEDs first alternating FAST flash = coil sensor problem (no defrost); simultaneous FAST flash = ambient sensor problem (time/temperature defrost only).
  2. Check the coil sensor is clipped on the tube between the TXV outlet and the distributor and plugged into P4.
  3. Know the timing the control counts heating run time while the coil is below 37°F; the first target is 90 minutes, then it adjusts up or down by 30 minutes based on the last defrost length. Below 10°F outdoor it waits 360 minutes; it goes back to 90 minutes when outdoor rises above 15°F.
  4. Defrost ends when the coil reaches the termination setting (P1: 50, 70, 90 or 100°F; factory 50°F; 90°F if no jumper) or after 14 minutes maximum.
  5. In cold climates, move the P1 jumper to a higher termination temperature if the coil is not cleared during defrost.
  6. Force a defrost by jumpering the TEST pins; in defrost the control energizes the reversing valve, stops the outdoor fan and sends 24 VAC to W1 (auxiliary heat).
  7. If the unit does not change over, check 24 VAC at O OUT and the reversing valve solenoid.
  8. Check charge (subcooling) and indoor airflow; two low pressure trips in a row in heating will also start a defrost.
  9. Return the test jumper to the desired termination pins when finished.

Applies to: Lennox EL16KP1 (Elite) and ML14KP1 (Merit) R-454B heat pumps, sizes -018 to -060, demand defrost control A108 (CMC1)

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Source: Lennox EL16KP1 Unit Information 100195 / ML14KP1 Unit Information 100108, Defrost System pp.30-31, Table 3 p.29, Table 4 p.32, Operating Sequence p.37. Written in our own words; check the manual for your exact model.

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.