Can I measure a Negative Subcooling When Checking the Charge? (Quick Tip)
- Craig Migliacco
- Jun 29, 2020
- 1 min read
Updated: Mar 2
The answer is no because the refrigerant rejects heat in the condenser so the pressure and temperature of the refrigerant should lower. The refrigerant starts off in the condenser as a superheated vapor and after it rejects heat, it changes from a vapor state to saturated. After it rejects enough heat, the refrigerant changes to a subcooled liquid. In this picture, we see a saturated temperature of 105°F and a line temp of 93°F.
105 - 93 = 12°F of Subcooling
It is possible for the refrigerant to stay saturated instead of subcooling if the system is extremely low on refrigerant. A system that is very low on refrigerant may read 0-3 degrees of subcooling. Typically, there will be at least a small amount of subcooling measured. If the refrigerant is rejecting heat at the condenser, there is no way for the pressure or temperature of the refrigerant to increase as it makes its way through the condenser. It will either exit the condenser as saturated (saturated is liquid and vapor refrigerant in the same location) in a low refrigerant scenario or the refrigerant will be subcooled (subcooling is the lowering in temperature of the liquid refrigerant). If you are measuring negative subcooling, make sure your measurement locations are correct, otherwise you may want to recalibrate your tools!
Be sure to check out the full Subcooling Charging Method Article here!
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That line about refrigerant staying saturated instead of subcooling when low really clicked for me—it makes the whole condenser process feel more intuitive. I also ended up opening Weeklyadhub weekly ad afterward because it carries the same thread a little further.
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Excellent technical explanation. HVAC topics can sometimes be difficult to understand, but this article broke down the concept of negative subcooling in a very practical and accessible way. The troubleshooting insights and real-world context were particularly valuable. The clarity of the explanation reminded me of a discussion I recently encountered through https://gocollectiv.com/, where professionals were sharing different perspectives on system diagnostics. Content like this is extremely helpful for technicians looking to strengthen their understanding and avoid common mistakes.
The article I saw was interesting because of the presentation.
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