Updated: Jun 18
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: Indoor WB 62° F, Outdoor DB 100° F= Target Superheat of X ° F
This means that the Target Superheat is too low to set the system to because the Target Superheat would be below 5° F. Your Target Superheat result may only be this low in a dry climate. A good example of this is in the state of Arizona. In this case, the technician would need to set the Actual Superheat to an inefficiently higher number in order to keep the compressor safe. Otherwise, the fixed orifice could be replaced with a TXV metering device or an accumulator tank could be added to protect the compressor. To learn more about charging refrigerant in a dry climate, read this article!
Example 3: Target Superheat Calculation instead of the Target Superheat Chart
(This calculation will get you close to the target superheat chart results but it may not be exactly the same.)
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
If you want to learn more about all the fine details on charging methods and troubleshooting, check out our book which is available on our website and on amazon. The full outline and sample pages are available here. We have a 1,000 question workbook with an answer key that you can use to apply your knowledge as well.
Check out our free quizzes to test your knowledge here!
If you want to learn the full Total Superheat Charging Method, check out this article!
If you want to learn the full Subcooling Charging Method, check out this article!
If you want to learn about Delta T, check out this article!
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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