There are 4 ways to calculate Subcooling:
1). Liquid Line pressure converted to Saturation Temperature minus liquid line temperature at the service valve: LP(sat) – LT
4). Condenser Coil Surface Temperature (measured in the middle third of the coil – hopefully in lots of places) minus liquid line temperature at the exit of the condenser coil (liquid line): CoilSatTemp – LT
As many of you know, most packaged systems use a discharge port instead of a liquid line port for high-side pressures. In many programs technicians have been instructed to subtract 15 psi from discharge pressures to approximate liquid line pressures before computing Subcooling values. This really is good in the absence of OEM literature – or really high temps, however it can be quite wrong in the case of residential packed units, systems with relatively small coil systems, or at relatively low outside ambient conditions (< 80 degrees).
In the instance where a TxV is part of a system, the OEM will (hopefully) supply SC target information..
Moreover, on many packed systems, the OEM will often provide a chart for SC Targets at different outdoor temperatures. Therefore when a tech indicates the SC aim – we would have to know if it is one point specified by the OEM, an average derived from a chart or graph on a wide range of outdoor temperatures, or a genuine value used for a particular temperature (i.e. at 95 degrees). Subcooling values and aims can and will change by about 3 degrees over the normal outdoor temperature range, and if one is not conscious of the SC aim relativity, then an “evaluator” could fail work. The contractor could be also caused by this misunderstanding / technician to mischarge the occupation by utilizing the wrong SC calculation or goal.
Put another manner, if the OEM specifies a SC aim and doesn’t provide a liquid line service valve, then they are most likely specifying Discharge Line Subcooling
not Liquid Line Subcooling.
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