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Lennox · Heat pump troubleshooting

Low capacity or abnormal pressures - Lennox EL16KP1 / ML14KP1

What is going on

Unit runs but heats or cools poorly, or gauge readings do not match the normal operating pressures.

Likely causes

  • Wrong charge
  • Dirty or blocked indoor/outdoor coil or filter
  • Low indoor airflow
  • Wrong or failed indoor expansion valve
  • Old mineral oil left in a reused line set or coil
  • Scroll compressor running unloaded on its internal vacuum protector

Check in this order

  1. Make sure filters, indoor coil and outdoor coil are clean (aluminum outdoor coil: water under 50 psi or a cleaner with pH 5-9).
  2. Check indoor blower speed and pressure drop over the coil to confirm airflow.
  3. Compare liquid and vapor pressures with the Normal Operating Pressures table (liquid +/-10 psig, suction +/-5 psig) for the size and outdoor temperature.
  4. Check charge by subcooling cooling mode at 60°F outdoor and above, heating mode below 60°F; use the target subcooling for the indoor matchup.
  5. If the compressor hisses and suction is below 20 psig, the scroll's internal vacuum protector has unloaded it; it resets when suction rises above 40 psig. Find the low-side cause - do not replace the compressor.
  6. If the line set or coil was used with mineral oil or the coil is older than January 1999, it must be flushed.
  7. Confirm the indoor expansion valve is an approved kit and the bulb is mounted correctly (never on the bottom of the line).

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, System Operation p.26, Maintenance p.33, Charging Information p.39. 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.