Freeze Protection Thermostat Logic

Freeze-Stats must be provided on any air-side HVAC system that:

Has a water coil within the main HVAC unit or in the downstream ductwork that can be subject to entering air temperatures below 40°F. Guidance: Consider the worst-case scenario for determining coil entering air temperatures. For example, a VAV system with a fixed minimum outside air flow will have a much lower mixed air temperature when the VAV boxes are at minimum flow.

Has a water coil within the main HVAC unit or in the downstream ductwork that has an air side economizer cycle. Guidance: Although air side economizers are designed to prevent mix air temperatures below freezing, the failure of an economizer damper or actuator can result in temperatures well below freezing. A freeze –stat provides an addition layer of protection for water coils.

Serves an area with temperature sensitive equipment or materials that could be damaged by freezing should the HVAC system fail and introduce excessively low supply air temperatures.

For systems that have a low mixed air temperature due to large quantities of outside air, with zone temperature controlled heating, it may be necessary to provide a more complex freeze protection control system than the standard freeze-stat to prevent nuisance tripping of the freezestat.

Freeze-stats shall be located in such a way as to minimize nuisance tripping while maximizing protection of coils. Guidance: In general heating coils should be place first in the air stream with the freeze-stat located downstream of the coil. This allows the hot water coil to temper the air and to protect a downstream cooling coil. Water heating coils subject to entering air temperatures below 40°F shall be equipped with a local recirculation pump. The correct location should always be shown on design documents and not left to the choice of the contractor.

Freeze-stats shall be hardwired to perform the following functions automatically upon tripping. All functions shall not require any control signal or power from the BAS system to perform the required action. Valves and damper actuators, upon loss of power from the freeze-stat hardwired connection shall spring return15 to the required position:

a. Hot water valves shall move to provide a maximum heated flow the coil.

b. Chilled water valves shall move to provide the maximum flow to the coil. This shall occur independently of whether the chilled water pumps are operating.

c. Outside air and exhaust air dampers shall close and return air dampers shall open

d. Supply, return and exhaust fans in the HVAC unit shall stop. This shall occur, independent of the position of any local hand-off-auto switch.

The Building Automation control system (BAS) shall have a dedicated freeze-stat input to detect the status of the freeze-stat. When the BAS system detects a freeze-stat trip alarms shall be generated and the BAS shall position valve, damper and fan signals to match the hardwired shutdown positions.

Guidance: The BAS is to position the valve, damper and fan signals to match the hardwired positions as a redundant backup to the required hardwired fail positions. This also makes the computer based user interface more closely display the actual status of the system. On systems that require multiple freeze-stats to cover a large coil area, the output of all commonly located freeze-stats may be combined into one single BAS input provided the individual freeze-stats have some sort of local indicator to identify which freeze-stat has tripped.

On makeup air systems using a heating source that requires air flow prior to actuation (gas, electric) or on units located outside where the freeze-stat might be subject to low temperatures when the fan is off, provide an auto-reset freeze stat with a timed bypass switch with a maximum bypass of 10 minutes. An operator accessible momentary bypass button shall be used to initiate the bypass timer and shall be clearly marked with its purpose.

Guidance: On units where the fan must start before the heat can be started, the unit cannot be restarted on a cold day. The freeze-stat will trip before the heating can be started (a gas unit may need to purge for upwards of a minutes before igniting and providing heat). A manually activated bypass timer allows an operator to restart the equipment, but assures the equipment will shut down should the heat fail to operate within the time delay. The manual actuation of the bypass timer at the HVAC unit assures the operator is present and can monitor the restart. This is an exception to the normal manual-reset Freeze-stat requirement.

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