Frost protection/ frost recovery after frost alarm

When the frost protection via the plant mode signal becomes active, the following action occurs:


Frost protection actions are normally performed in the respective output modules.


When the frost failure has been reset, there will be a start-up procedure at low outside air temperatures (START-01). To prevent frost alarm again, the air handling unit with mixed air dampers will start on forced recirculation, followed by a temporarily raised supply air temperature setpoint (START-02), which will be slowly decremented to the value of its normal supply air temperature setpoint. The ramp-down time can be adjusted by parameter START-03.

If during start-up the outside air temperature is below 6°C, defined by parameter START-01, the mixed air dampers starts on forced recirculation during the time of the parameter START-04, when the plant mode changes to "occupied period" (4) or "extended operation"(5). During forced recirculation, the DamperCtrlSig output signal will have a value of “1”, which forces the mixing damper output macro to ignore its own minimum damper position adjustment.


When the frost failure has been reset, there will be a start-up procedure at low outside air temperatures. To prevent frost alarm again, the supply air temperature setpoint will be temporarily raised immediately after the FanEnable input becomes true (=1) and afterwards slowly ramped down to its normal setpoint. The ramp-down time can be adjusted by parameter START-03.


Description

Default value

START-01: limit OAT to start frost protection

6 °C

START-02: supply air temperature setpoint raise

10 K

START-03: SAT control raised setpoint decrease time

600 sec

START-04: period of forced recirculation after start

600 sec