Understanding Condenser Subcooling

In the May 7 issue of The NEWS, Professor John Tomczyk asserts in his article that insufficient condenser airflow, as well as a dirty condenser coil, will cause condenser subcooling to increase. He goes into a very thorough explanation of why this happens, but I’m afraid that explanation might be lost on many technicians. Here’s a better indicator:

Modern residential air conditioning units (13 SEER or higher) generally have a design delta T on the condenser coil of approximately 20 degrees, meaning that the condensing temperature, and thus the head pressure, needs to be about 20 degrees higher than the outside air temperature. Most TXV-equipped residential air conditioning equipment calls for approximately 10 degrees of subcooling, so the liquid line should be about 10 degrees cooler than the condensing temperature. If the condensing temperature is 20 degrees higher than the outside air temperature, that would make the liquid line temperature about 10 degrees higher than the outside air temperature.

Thus, an increase in subcooling due to inadequate airflow across the condenser coil would become obvious with a comparison of liquid line temperature to outdoor air temperature. If the difference is substantially more than 10 degrees, regardless of subcooling measured, you have a heat rejection problem in the condenser coil.

Greg Goater
Director of Training and Safety
Isaac Heating and Air Conditioning Inc.
Rochester, N.Y.

Publication date: 6/25/2012