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Freeze Stat

January 16, 2014

 

“Freeze stats” (or “low temperature cut-out thermostats”) are highly reliable, low cost devices which provide freeze protection to expensive heating and cooling coils in heating, ventilation and air conditioning (HVAC) systems.  Protection is required because the expansion that occurs when a fluid freezes in a confined space can cause stress damage or catastrophic bursting of thin-walled heat exchanging coils even after only one low temperature event.  Although electronic versions with digital processors are available, the vast majority are simple electromechanical devices that require no external power to operate.

Freeze stats measure the air temperature passing a liquid filled coil in an air duct or heat exchanger and switch their internal relay or relays when the air temperature drops below a pre-set level (the “setpoint”).  Because temperature gradients within an air stream are common, in order to fully protect the coil from freeze damage the freeze stat must be able to react to localized cold spots rather than simply the average cross sectional temperature or the temperature at a single measurement point.

The most common location for installing the freeze stat in the air handling system is on the downstream face of the heating coil.  Typical air handling system sequence of operation would call for the hot water pump to be running and the hot water valve to the heating coil to be open well before the air temperature at this location approaches freezing.  A trip of the freeze stat often indicates a failure of either the hot water pump or valve.  The freeze stat’s relays are typically wired into control circuits that turn the supply air fan off, close the supply air damper and annunciate the fault.

Freeze stats may be equipped with either automatic or manual reset.  Units with “automatic reset” return their relays to the normal state when the low temperature condition is no longer present while “manual reset” models remain in their tripped state until the condition clears and a reset button is pushed by an engineer or technician who has visually confirmed that the pumps, valves, dampers and coils are in proper working condition.