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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I found your post on negative subcooling insightful! It's fascinating how we sometimes overlook the importance of precise measurements in HVAC systems. Reflecting on “we become what we behold” I've noticed that my own approach to troubleshooting has evolved as I’ve focused more on detail-oriented practices. Just like in our line of work, our understanding and choices shape the outcome—both for our systems and ourselves. Thank you for shedding light on this critical topic!
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Exactly - negative subcooling isn’t really a thing in a properly functioning system. If your readings show it, it usually means your measurement points are off or your tools need calibration. Subcooling should always be zero or positive, and very low readings can indicate a low refrigerant charge.