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Air Handler Water Coil Freeze Stat - The HVAC Air Handler Freeze Stat is a low-temperature limit control and it is used mainly in commercial systems in the ductwork on economizers or in air handlers just before any coil especially water coils. It is a safety and protection control to prevent extremely cold air from reaching the hot water or chilled water coils inside the air handler where the air enters from the economizer.
Under ideal control setup if the freeze stat trips the economizer dampers close and the blower fan stops. This prevents coils from freezing. A frozen coil can bust open and will need to be replaced if it freezes. This is the purpose of the freeze stat in HVAC air handler applications which utilize economizers for free cooling and other purposes.
Air Handler Water Coil Freeze Stat
There are some instances where HVAC maintenance departments have problems with nuisance trips on freeze stats and the HVAC maintenance department decides to jump this device out. This can cause severe damage to the equipment if the economizer feeds freezing air through the air handler. If the HVAC maintenance department experiences nuisance trips that cannot be adjusted through the range settings on the freeze stat device then the freeze stat should be replaced with a new freeze stat rather than jumping out the device. It would be better to disable the economizer than to jump out of the freeze if a problem occurs.
Air Handler Water Coil Freeze Stat - Sequence of Operation
The sequence of operation for any freeze stat inside an air handler will include the following:
- Stop the blower
- Close all outside air dampers
- Open the return air dampers
- Open all valves serving chilled water or hot water coils inside the air handling unit
- This event also sends an alarm through the DDC system to notify building managers
When these valves open the appropriate pumps should energize. The pumps respond so that the water will flow through the coils. This prevents a freezing coil which will likely burst open if it freezes. Another spelling for it is freezestat.
Some freezestats have two contacts on them. A normally open (NO) contact and a normally closed (NC) set of contacts. These contacts are typically used by the DDC controls to initiate the sequence of operation in the event of a freezestat trip. This can be tested by manually tripping the freezestat. And depending on how the system is wired will depend on whether the NO contacts are used or the NC contacts are used. It is possible the controls contractor technician used both sets of contacts.
Air Handler Water Coil Freeze Stat - Conclusion
One of the biggest problems with freeze stats is nuisance trips. Often, in some cases, this causes the maintenance department to decrease the trip temperature. They change the temperature trip setting of the freeze stat to try to prevent nuisance trips. It is important that the freeze stat trips before the coil freezes. This should be avoided. Other methods should be used rather than lowering the temperature on the freeze to prevent nuisance trips. An adjustment to the economizer system or other methods should be used other than lowering the trip temperature.
Calibration of the system should be done on an annual basis preferably before the cold weather. Additionally, trip testing should be done at the same time to ensure a proper control sequence for the freeze stat. Always follow the sequence of operation when testing the freeze control system. A busted coil from freezing is expensive to repair.
Air Handler Water Coil Freeze Stat