Information About HVAC Superheat & Subcooling

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Heating, ventilation and air conditioning systems, or HVAC, contain all the elements in your home that provide heating, cooling and air circulation. HVAC technicians monitor these systems through a variety of methods, including superheat and subcooling. These terms relate to temperatures found in systems using refrigerant and compressors, such as an HVAC system. Superheat and subcooling relate not just to temperatures in a system, but also pressure.

Superheat

  • Superheat refers to any heat applied to a substance in its gaseous, or vapor, state. For instance, water boils at 212 degrees Fahrenheit at sea level. As it boils, it changes from a liquid state to a gaseous state -- from water to steam. Steam can rise to temperatures well above 212 degrees if you apply additional heat. Any heat applied to steam qualifies as superheat. For instance, raising the temperature of steam from 212 to 232 degrees Fahrenheit involves an increase of 20 degrees of superheat. Any temperature increase of a gas or vapor constitutes superheat.

Subcooling

  • Subcooling is a temperature change to a liquid or a solid. This change occurs when the substance cools from its saturation point, or the point at which it changes forms. For instance, steam changes to water at 212 degrees Fahrenheit. Cooling water from 212 to 200 degrees Fahrenheit entails the process of subcooling it by 12 degrees. Or, water freezes at 32 degrees Fahrenheit. Changing the temperature of ice from 32 degrees Fahrenheit -- the saturation point -- to 22 degrees Fahrenheit entails subcooling it by 10 degrees. You can subcool any substance by lowering its temperature from its saturation point.

What It Measures

  • HVAC technicians monitor subcooling and superheat temperatures within an HVAC system as a means of monitoring pressure. The saturation temperatures of material depend not only on the material itself, but also atmospheric pressure. For instance, though water boils at 212 degrees Fahrenheit at sea level, it may boil at lower temperatures at higher elevations because the air pressure at these elevations offers less resistance to steam. HVAC technicians monitor superheat and subcooling temperatures as a means of ensuring proper pressure within a system.

What It Means

  • Changes in superheat or subcooling within an HVAC system can mean numerous things. For instance, high superheat levels may indicate improper temperature settings, too little refrigerant in a system or problems with system parts such as filters or caps. Low subcooling levels, on the other hand, only happen for a single reason -- a lack of refrigerant. You can download charts from various HVAC websites indicating specific problems related to low or high levels of superheat and subcooling, though unless you can fix these problems yourself, you should contact an HVAC specialist to interpret your levels.

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