Updated: Sep 12, 2019
To determine the Target Superheat for an air conditioning system with a fixed orifice (such as a piston or capillary tube) measure the indoor WB (wet bulb) temperature with a digital psychrometer and the outdoor DB (dry bulb) temperature with a standard digital temperature reader. Input these temperatures in a superheat chart, calculation, app, or digital manifold set in order to determine the Target Superheat at that moment. Remember that the target superheat will change as the building lowers in WB and while charging refrigerant. The outdoor DB will general stay the same while checking the charge but it may fluctuate some.
Set the Actual Superheat as close to the Target Superheat as possible to have an accurate refrigerant charge.
Measure the indoor WB as close as possible to the inlet of the evaporator coil, preferably in the return duct a few feet prior to the coil. Measure the outdoor DB roughly one foot away from the inlet of the outdoor coil in the shade.
Example 1: Indoor WB 66° F, Outdoor DB 90° F= Target Superheat of 13 ° F
Example 2: Target Superheat Calculation (This calculation will get you close to manufacturer superheat chart results)
WB is 64° F, DB is 96° F
Target Superheat Formula = [(3 x WB) – 80 – DB] /2
[(3 x 64) – 80 – 96] /2 = Target Superheat
3 x 64=192, 192-80=112, 112-96=16 16/2=8° F of Target Superheat
8° F of Target Superheat
If you want to see a video on finding target superheat using the chart above- https://youtu.be/bIL9BrKCujc
If you want to see a video finding target superheat using a digital manifold see this video- https://youtu.be/YPitQHSgvog
Check out our book “Refrigerant Charging and Service Procedures for Air Conditioning”.
The full outline is available at https://www.acservicetech.com/the-book
If you have already purchased our book, be sure to tell local HVACR Instructors about our book and what you think of it. We would love to get the book into the hands of the next generation of HVACR Technicians!
Author: Craig Migliaccio
About the Author:Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 15 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel