A freeze stat is a sensor located in your HVAC system that is designed to minimize the potential of freezing a water or steam coil in your air distribution system. A true freeze stat is directly interlocked to shut down equipment and requires a manual reset if it trips. This is different than a low temperature sensor that might be used for alarm notification only. Freeze stats are typically copper capillary tubes that are strung diagonally across the face of the coil you want to protect. Gas within the capillary tube expands and contracts based on temperature. If a section of the tube drops below the setpoint of the freeze stat, the gas movement will trip a switch that will shut down your HVAC equipment. A freeze stat could protect you from expensive repairs due to equipment damage and water damage, but it can also be a nuisance if it is not installed correctly. Following are some common reasons for freeze stat nuisance trips:
The temperature at which the freeze stat will trip is manually adjustable, with a dial or slider bar, and is typically set around 38 degree F. The freeze stat will trip if the capillary tube senses air at, or below, this temperature. Sometimes maintenance personnel increase the setpoint thinking that “warmer” is better, when in reality they are increasing the range in which the freeze stat will trip.
The freeze stat is frequently installed in the wrong location. Generally, freeze stats should either be located on the downstream side of the first heating coil or on the upstream side of the first non-heating coil that is to be protected.
Most air handling units include a mixing box where cold outdoor air is mixed with warm return air. Depending upon the configuration of your unit, the two air streams can remain unmixed and allow cold outside air to hit the freeze stat and cause a trip. Solving stratification problems can vary from the simple to the complex depending upon the specific project conditions.
Unfortunately, frequent nuisance tripping can lead to the freeze stat being disabled by maintenance personnel. This can leave your HVAC system and building at risk of damage. A tripped freeze stat should not be ignored.