Freeze Stat

A freeze stat is a temperature sensing device for HVAC

HVAC

HVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...

 that monitors a heat exchanger

Heat exchanger

A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

 to prevent its coils from freezing

Freezing

Freezing or solidification is a phase change in which a liquid turns into a solid when its temperature is lowered below its freezing point. The reverse process is melting....

. Freeze stats can be used on both freon-to-air, and freon-to-liquid type heat exchangers and serve different purposes with similar goals for each.

Air coil freeze stats

The purpose of an air coil freeze stat is to keep the freon-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. 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 38 degrees Fahrenheit which is approximately when the dew point

Dew point

The dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.The dew point is...

 temperature of the air starts to drop below freezing point.

Water coil freeze stats

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 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

Reversing valve

thumb|right|293px|border|A reversing valve removed from an HVAC heat pump for replacement.A reversing valve 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...

.

Mechanical Freeze Stats

Conceptually, mechanical freeze stats are constructed with a diaphragm, a capillary tube and bulb, and an electric switch

Switch

In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....

. 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 Freeze Stats

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

Microprocessor

A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...

, or microcontroller

Microcontroller

A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...

 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 seeminly 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

See also

Freeze Stat Uses

• Heat pump
  Heat pump
  A heat pump is a machine or device that effectively "moves" thermal energy from one location called the "source," which is at a lower temperature, to another location called the "sink" or "heat sink", which is at a higher temperature. An air conditioner is a particular type of heat pump, but the...
  
  
• Geothermal heat pump
• Air handlers/mixing boxes

HVAC Information

• HVAC
  HVAC
  HVAC refers to technology of indoor or automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer...
  
  
• Refrigeration cycle
• Freon

Other HVAC Components

• Thermostat
• Freon compressor
• Heat exchanger
  Heat exchanger
  A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
  
  
• Thermal expansion valve
  Thermal expansion valve
  A thermal expansion valve is a component in refrigeration and air conditioning systems that controls the amount of refrigerant flow into the evaporator thereby controlling the superheating at the outlet of the evaporator...
  
  
• Reversing valve
  Reversing valve
  thumb|right|293px|border|A reversing valve removed from an HVAC heat pump for replacement.A reversing valve 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...