What Does a Freezestat Do in an Air Handler?

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The air handler of an HVAC -- heat, ventilation and air conditioning -- unit is the part of the system that moves the heated or cooled air and is often incorrectly referred to as a blower or fan. Located inside or immediately before the air handler, the freezestat senses the temperature and activates a protective sequence intended to prevent the coils from freezing up.

Location and Purpose

  • Freezestats in air handlers are unique to commercial air conditioning installations. They are installed immediately before the coils, either in the duct work or in the air handler, and are intended to protect the coils from the effects of unusually cold temperatures. When the freezestat trips, it stops the blower, closes the external dampers and opens valves that cause the pumps to run; the pumps send water through the system, and the water protects the coils from damage. There are two types of freezestats for each type of coil-cooling design.

Freon-To-Air Heat Exchangers

  • The freezestat in a Freon-to-air heat exchanger is called an air coil freeze stat; it keeps the air-cooled coils from freezing up. The device is used in air conditioners where the heating coils are cooled by outside air. The freezestat in this situation closes down the flow of outside air when a preset minimum temperature is reached inside the mixing box. That preset temperature is typically around 35 degrees Fahrenheit.

Freon-To-Liquid Heat Exchangers

  • The freezestat in a Freon-to-liquid heat exchanger is called a water coil freeze stat; it keeps the liquid-cooled coils from freezing. If the water-cooled coils freeze, the cooling liquid freezes as well, and the heat exchanger bursts. Freon is expelled and water enters the refrigeration system; this usually destroys the compressor.

Available Designs

  • Freezestats are available in two types: mechanical and digital. Mechanical freezestats have a gas that changes volume in accordance with varying temperatures; the gas flexes a diaphragm that, in turn, activates a switch. Mechanical freezestats are relatively cheap but are vulnerable to damage and have a strict set of installation requirements. Digital freezestats use a sensor to read the temperature and a microprocessor to activate the switch. Although more expensive than the mechanical version and requiring a power supply to operate, digital freezestats are less vulnerable to accidental damage, can be fitted almost anywhere and have variable temperature settings.

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