A freeze stat is a temperature sensing device for

HVAC Heating, ventilation, and air conditioning (HVAC ) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality. ...

that monitors a

heat exchanger A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contac ...

to prevent its coils from

freezing Freezing is a phase transition in which a liquid turns into a solid when its temperature is lowered below its freezing point. For most substances, the melting and freezing points are the same temperature; however, certain substances possess dif ...

. Freeze stats can be used on both

refrigerant A refrigerant is a working fluid used in the cooling, heating, or reverse cooling/heating cycles of air conditioning systems and heat pumps, where they undergo a repeated phase transition from a liquid to a gas and back again. Refrigerants are ...

-to-

air An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

, and

-to-

liquid Liquid is a state of matter with a definite volume but no fixed shape. Liquids adapt to the shape of their container and are nearly incompressible, maintaining their volume even under pressure. The density of a liquid is usually close to th ...

type heat exchangers and serve different purposes with similar goals for each.

Air coil

The purpose of an air coil freeze stat is to keep the refrigerant-to-air heat exchanger (commonly called '' air coils'') from freezing. This kind of freeze stat is typically used for heating coils which are exposed to outside air and is usually installed on the supply air side of the coil. To accomplish this, they typically shut down the flow of outside air to a mixing box when the temperature reaches a predetermined setpoint. The setpoint for air coil freeze stats is typically about 12°C which is approximately when the

dew point The dew point is the temperature the air needs to be cooled to (at constant pressure) in order to produce a relative humidity of 100%. This temperature depends on the pressure and water content of the air. When the air at a temperature above the ...

temperature of the air starts to drop below freezing point.The Freeze Stat
John Williams Jr., Carrier Corporation, ''The Air Conditioning, Heating, Refrigeration News'', July 2001

Water coil

The water coil freeze stat performs a similar function to the air coil freeze stat, but is used on a different type of HVAC system. Its function is to keep the freon-to-liquid heat exchanger (commonly called ''liquid coils'') from freezing. In practical situations, when an air coil freeze stat is not used, bypassed, or defeated, the air coil can freeze, and this causes a lack of air flow to the facility. By contrast, when a water coil freeze stat is not used, the water coil can get so cold that it can freeze the cooling liquid in the exchanger and burst the exchanger. A ruptured heat exchanger mixes contaminants such as freon and

oil An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) and lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturate ...

into the coolant. The intrusion of coolant into the delicately balanced refrigeration system can cause expensive damage to components such as the freon compressor, and the

reversing valve A reversing valve is a type of valve and is a component in a heat pump, that changes the direction of refrigerant flow. By reversing the flow of refrigerant, the heat pump refrigeration cycle is changed from cooling to heating or vice versa. This ...

Mechanical

Conceptually, mechanical freeze stats are constructed with a diaphragm, a

capillary tube Capillary action (sometimes called capillarity, capillary motion, capillary rise, capillary effect, or wicking) is the process of a liquid flowing in a narrow space without the assistance of external forces like gravity. The effect can be see ...

and bulb, and an electric

switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type o ...

.Combination Temperature Sensors
Trygve Behny, Minco, ''Electronic Design (eepn)'', September 2006 The capillary tube allows gas movement to and from the capillary bulb and the diaphragm with a fall or rise in temperature, respectively. When the temperature reaches a specific setpoint, the pressure in the diaphragm will trip a switch which typically shuts down the flow of outside air while the capillary bulb's temperature is at or below the setpoint. Mechanical freeze stats can have more than one set of contacts, and the contacts can be NO or NC.
Disadvantages * Capillary tube easily kinked during installation limiting effectiveness * Sensing bulb must be mounted horizontally * Diaphragm case must be mounted in a separate, warmer location for effective operation Advantages * Typically inexpensive * Easily understood wiring

Digital

Digital freeze stats, also known as ''electronic'' or ''solid state'' freeze stats, use an electronic circuit,

microprocessor A microprocessor is a computer processor (computing), processor for which the data processing logic and control is included on a single integrated circuit (IC), or a small number of ICs. The microprocessor contains the arithmetic, logic, a ...

, or

microcontroller A microcontroller (MC, uC, or μC) or microcontroller unit (MCU) is a small computer on a single integrated circuit. A microcontroller contains one or more CPUs (processor cores) along with memory and programmable input/output peripherals. Pro ...

in place of the mechanical freeze stat's diaphragm and switch. They also use one or more electric or electronic sensors in place of the capillary tube and bulb for temperature sensing. Digital freeze stats utilizing microcontrollers or microprocessors can also perform more advanced functions than simply opening or closing switch contacts. Digital freeze stats can perform seemingly intelligent operations such as reversing the refrigerant flow and using some building heat to thaw a freon-to-liquid heat exchanger and then restoring the refrigerant flow to its normal direction once the danger of freezing the water coil has been averted.
Disadvantages * Requires power to operate * Typically higher cost than mechanical freeze stats * More complex wiring installation Advantages * Multiple sensors * Sensors can be near or far * No capillary tube kinking issue * No capillary tube leaking issue * Operate in any position * Operate in varying temperatures * Programmable actions and setpoints * Can perform functions other than opening or closing a single switch

References

{{HVAC Heating, ventilation, and air conditioning

Air coil

Water coil

Mechanical

Digital

See also

References