Low Suction Pressure Cheat Sheet

If I had a nickel for every time someone asked what causes low suction pressure, I’d have…well, I’d have a bunch on nickels. Ok, time to get serious.

Low suction pressure is high on the top ten list of air conditioning issues that technicians run across.

So, what can cause low suction pressure? Unfortunately, a bunch of things, and to diagnose the problem we need to grab a few operational readings.

Tools needed - a set of refrigeration gauges and a temperature probe. Take the following readings:

  1. Return Air Temperature

  2. Supply Air Temperature

  3. Liquid line temperature before and after filter drier

  4. Evap coil Delta-T (A minus B above)

  5. Super-heat

  6. Sub-cooling

Now, answer YES or NO to each of the following questions:

  1. Is the Return Air temperature lower than 70 degrees?

  2. Is the Delta-T less than 10 degrees?

  3. Is the Delta-T more than 20 degrees?

  4. Is the Super-heat more than 20 degrees?

  5. Is the super-heat less than 5 degrees?

  6. Is the Sub-cooling more than 15 degrees?

  7. Is the Sub-cooling less than 5 degrees?

  8. Is the outlet of the drier 3 degrees colder than the inlet?

  9. Is the TXV body sweating or frosted?

Place your YES or NO answer under questions 1 through 9, the probable cause is on the left. A dash (-) means the answer is irrelevant, or it could be either.

Low Load- Could be dirty filters, not enough air flow, dirty evap coil, RAT too low, a bypass damper is open, etc.

Closed TXV - The TXV could be restricted, mis adjusted, or the sensing bulb capilary is broken.

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