Suction Line (Low Side, large/insulated) Temperature

  • Normal operating range: The suction line temperature measured about 6–12 inches from the compressor inlet should typically be between 40°F and 60°F under normal running conditions[1][2][3].
  • Absolute minimum: The suction temperature should never drop below 35°F entering the compressor, as this risks freezing the evaporator coil and returning liquid refrigerant to the compressor, which can cause serious damage[3][4].
  • Typical “good” value: Most systems aim for a temperature safely above freezing, ideally in the 40°F to 60°F range, with a superheat (temperature of vapor above its boiling point at that pressure) of about 20°F to 30°F being ideal[1][2].
    • If a liquid-to-suction line heat exchanger is installed, suction line temperatures up to 65°F at the compressor inlet are still acceptable[1].

“Suction line temperature…should be somewhere in the range of 40° to 60°. …If a liquid line/suction line heat exchanger is installed, a suction line temperature of 65° at the inlet of the compressor is acceptable. Air temperature leaving the evaporator should be 37° to 41°.”[1]

Liquid Line (High Side) Temperature

  • Relative minimum: The liquid line temperature should not be below outdoor ambient temperature. A significantly colder liquid line may indicate a restriction or sensing issue[5][6].
  • Normal “approach”: Typically, the liquid line temperature should be 4–15°F above the outdoor ambient temperature in normal operation[6].
  • If the liquid line is colder than the ambient air, check for restrictions or measurement errors[5][6].

“The liquid line can’t be any colder than the medium (outdoor air) to which the condenser is rejecting its heat in a traditional residential split A/C unit. … The temperature should be between 4 and 15 degrees warmer than the outdoor temperature.”[5][6]

Additional Notes

  • Evaporator coil freeze protection: The evaporator coil temperature must stay above 32°F to prevent freezing, so suction line temperatures below this point are never acceptable for standard air conditioning[1][3].
  • Operating conditions, system design, refrigerant type, and ambient temperatures will influence these values—always consult manufacturer specifications for final guidance.

Summary Table: Minimum Acceptable AC Refrigerant Line Temperatures

Line Type Minimum Acceptable Temp Normal Range Notes
Suction Line 35°F (never below) 40–60°F 65°F max if heat exchanger installed
Liquid Line Not below ambient temp 4–15°F above ambient Colder indicates problem or measurement error

 

Failing to stay within these ranges can result in poor performance or system damage, particularly to the compressor on the suction side[1][3][4].

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  1. https://s3.amazonaws.com/system-evaluation-manual-17-2/Sample_1_14_2017.pdf
  2. https://www.reddit.com/r/HVAC/comments/8k9uj1/refrigerant_line_maxmin_temperature/
  3. https://www.contractingbusiness.com/service/media-gallery/20870989/testing-refrigerant-charge-without-gauges
  4. https://www.inspectorsjournal.com/topic/3365-cold-high-pressure-line/
  5. https://www.youtube.com/watch?v=XClJ74NQx20
  6. http://www.hvacrschool.com/podcasts/short-89-liquid-line-temperature-2/