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HVAC system troubleshooting is very important.  In the context of the HVAC industry, the term “negative subcooling number” has an important meaning. Yet there are plenty of people who work in the field who can’t explain exactly what the number means and why is important. Let’s take a loo at what the negative subcooling number is all about with some HVAC troubleshooting.

Air conditioning repairman working on a compressor and giving a thumbsup.

HVAC System Troubleshooting: Subcooling and Superheat

To understand what negative subcooling means, you must first understand subcooling as well as superheat. In general, the terms “subcooling” and “superheat” relate to the manner in which cooling systems take advantage of the properties in air conditioning refrigerant. Refrigerant makes the creation of cool air possible.


In the context of the HVAC system, subcooling is a generally used term that refers to cooling refrigerant to a temperature below the point at which it would vaporize. It is the act of lowering the temperature of a cooling material to a level that is beyond its saturation point. The saturation point is the temperature where a material can change from one state to another. As an example, water boils at 212 degrees Fahrenheit when at sea level. Removing the heat source from water that is at a temperature of 212 degrees is a form of subcooling. So water that must be cooled from 212 degrees to 211 degrees will need one degree worth of subcooling.

HVAC System Troubleshooting: Why Negative Subcooling Occurs?

Performing subcooling usually creates positive values. In the above example, the subcooling value was positive one. In instances when HVAC systems have negative subcooling values, it is an indication that there will be a move opposite of standard subcooling (or an increase in temperature). When there is a negative subcooling result, there is a good chance that the system is actually superheating its refrigerant rather than subcooling it.


In order for superheating to occur, the opposite process of subcooling must take place. This is the act of applying heat to a substance that is already at its saturation point. As an example, water turns to steam at 212 degrees Fahrenheit. Applying heat to steam when the water is 212 degrees is superheating the steam. Therefore, heating the steam from 212 degrees to 217 degrees will require 5 degrees worth of superheat. In a nutshell, when a cooling system superheats refrigerant instead of cooling it, the result is the negative subcooling value.

HVAC System Troubleshooting: Consider the Possible Causes of the Negative Subcooling Value

When a negative subcooling value exists, it is a reflection of the cooling system’s failure to properly decrease the refrigerant’s temperature throughout the subcooling process. There are all different possible causes for a negative subcooling value including an undercharged system, poor air flow by the condenser, an overfeeding/maladjusted meter etc. Go ahead and give your condenser coil a thorough clean if you have a negative subcooling value. It will boost air flow and help combat ineffective subcooling.

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If you would like more information about HVAC troubleshooting, reach out to us today. Our HVAC specialists are here to help you repair your system, perform preventative maintenance or complete a comprehensive installation.