HVACopedia

Lennox · Heat pump troubleshooting

Outdoor unit will not start - Lennox EL16KP1 / ML14KP1

What is going on

Thermostat calls for heat or cooling but the outdoor compressor and fan do not run.

Likely causes

  • 5-minute anti-short-cycle delay
  • No 24 VAC to the defrost control or no Y1 call
  • Pressure switch lockout (5 strikes)
  • Compressor thermal protection (S173) or internal overload open
  • Contactor K1 not pulling in
  • No line voltage

Check in this order

  1. Read the defrost control LEDs both off = no 24 VAC; alternating slow flash = 5-minute delay (jumper TEST pins to skip); red or green steady/flashing = pressure or sensor fault (see LED codes).
  2. Check 24 VAC between R and C and on Y1 from the thermostat.
  3. Check that Y1 OUT energizes the compressor contactor K1 and that K1 closes.
  4. Check line voltage at the disconnect; it must be within the nameplate range (operating range is +10% / -5% of rated voltage).
  5. If locked out, fix the cause and clear the lockout with the TEST pins or by cycling 24 VAC.
  6. If the compressor is hot, give the internal or S173 thermal protector time to reset.
  7. After a long power outage in cold weather, leave the thermostat on Emergency Heat for about 6 hours so the crankcase heater can warm the compressor before restarting the heat pump.

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

More heat pump guides

Related

Lennox error codes · Lennox parts · Lennox tips · Calculators and reference tables

Take it to the job

All guides, error codes and tips work offline in the HVACopedia app.

Open the app

Source: Lennox EL16KP1 Unit Information 100195 / ML14KP1 Unit Information 100108, System Operation p.26, Table 3 p.29, Test Pins p.31, Table 4 p.32, Operating Sequence and Start-Up 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.