VAV-SVX07B-EN 57
Calibration, Operation Modes, and Control
Freeze Protection (Hot Water Only)
Units with hot water coils installed are susceptible to freezing. It is important to prevent the water
coils from freezing. Freeze protection occurs only when the controller is in the OFF state or during
the unoccupied period when the supply air valve is closed and the reheat is disabled. During
occupied operation, the control algorithm indirectly provides freeze protection. Refer to the
following table for unoccupied VFC control.
Stand-alone Controller Freeze Protection
Controllers operating without communications do not have the source temperature available to
them. In these cases, the discharge air temperature sensor is assumed to provide a reasonable
representation of the coil temperature at conditions with no airflow, if the sensor is close to the coil.
If the discharge air temperature (at no airflow) falls below the configured outdoor air low limit, the
hot water valve is fully opened. The hot water valve remains OPEN until the discharge air
temperature is 10°F above the configured outdoor air low limit or occupied operation begins. If the
communicated source temperature is invalid and the local discharge air temperature sensor has
failed, freeze protection is active and the water valve is open.
Unoccupied Ventilation Flow Control
The controller closes the air valve and the local electric heat and hot water heat are disabled. The
hot water valves OPEN, if needed for freeze protection. The operation of freeze protection for local
hot water heat depends on whether the controller is operating as a network controller or a stand-
alone controller.
Table 9. Unoccupied VFC Control, Freeze Protection for Hot Water Reheat
Controller
Operation
Air Valve
Position Condition
Hot Water Valve
Position
Networked and
valid
communicated
supply air
temperature
Closed no
flow
Communicated supply air temperature greater than configured outdoor air low limit. Closed
Communicated supply air temperature less than or equal to configured outdoor air low limit,
freeze protection active diagnostic (with 10°F hysteresis).
Open to 100%
Stand-alone, coil
temperature
sensed at
discharge air
temperature
(a)
Closed no
flow
Discharge air temperature greater than configured outdoor air low limit. Closed
Discharge air temperature less than or equal to configured outdoor air low limit, freeze
protection active diagnostic (with 10°F hysteresis).
Open to 100%
(a) Place the discharge temperature sensor in close proximity to the hot water coil discharge. The sensor effectively reports coil temperature when there
is no airflow through the coil. Therefore, the coil can be freeze protected by the discharge air temperature sensor.
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