NightPurge

Function

NightPurge starts and stops ventilation systems to pre-cool rooms when cold outside air is available during non-occupied hours (usually during the night).


To switch on the air conditioning as late as possible, this function permits room temperature to drop below room temperature setpoint during night cooling. NightPurge achieves this action by decreasing the room temperature setpoint.  A minimum outdoor air temperature prevents damage from excessively cold outdoor air.

With an outdoor air temperature at night of 15°C, the plant purges the room air with 100 percent outdoor air to enable the cooling function to start as late as possible the following day.

Fig. 201.   Night purge function

NightPurge and DutyCycle Operation

Using both NightPurge and DutyCycle in a system can result in command conflicts. You should use switching logic to force NightPurge to override DutyCycle commands.


Night Purge Application Example

In this example, the input “In StartStop” controls the dampers and the supply fans. It opens the dampers to 100 percent and switches on the fan with a high signal from NightPurge. This action results in 100 percent outdoor air purging. The following schematics shows the wire-sheet programming:


Fig. 202.   Night purge application example


The input “In StartStop” can also be set by a time schedule. The time schedule must set it to 1 a few hours before the occupied period (for night cooling on) and to zero in the morning (night cooling off). Because the NightPurge has no built-in delay, the application engineer has to delay the fan signal if has plant has fresh air dampers instead of mixing dampers.


Fig. 203.   Wire-sheet Icon

Fig. 204.   Property Sheet

Fig. 205.   Slot Sheet

Inputs and Outputs

Table 126.   Inputs

Inputs

Type

Functional description

In RoomTemp

numeric

Room temperature

In FiltOATemp

numeric

outdoor air temperature

In StartStop

numeric

Night cooling on/ off (digital input) This input switches night cooling on and off

0 = false (off)

1 = true (on)

In RoomSpShift

numeric

room temperature setpoint shift

In RoomTempSp

numeric

room temperature setpoint

In Enabled

boolean

If the input “In Enabled” is set to false disabled in the slot sheet, the output “Out” will be disabled and is set to the value “0”.


Table 127.   Outputs

Outputs

Type

Functional description

Out Active

boolean

One digital output “”. This output switches to 1 when all the following conditions are met:

  1. Room temperature minus outdoor air temperature is greater than P2 Minimum Diff. RoomTemp/ OATemp
  2. Room temperature “In RoomTemp” is greater than room the temperature setpoint “In RoomTempSp” plus room temperature setpoint shift “In RoomSpShift”. The value of the shift must be negative.
  3. Outdoor air temperature “In FiltOATemp” must be greater than the P1 Minimum OATemp.
  4. Input “In StartStop” must equal 1 (night cooling is on).

Out Condition

numeric

Analog output displaying the NightPurge conditions:

1 - if “In RoomTemp” minus “In FiltOATemp” > P2 Minimum Diff. Room Temp/ OATemp.

2 - if “In RoomTemp” > “In RoomTempSp” plus “In RoomSpShift”.

4 - if “In FiltOATemp” > P1 Minimum OATemp.

When more than one condition is, true the output displays the sum of all values.
E.g. if all three conditions are fulfilled the output value will be 7.


Parameter List

Table 128.   Parameter List

Description

Value Range

Default Value

Unit

P1 Minimum OATemp

This value is the minimum outdoor air temperature at which Night Purge continues to function. If the outdoor air temperature drops below this value, the supply of 100 percent outdoor air stops immediately.

-25 …30

15

°C

P2 Minimum Diff. Room Temp/ OATemp

This value specifies how many degrees K the outdoor temperature must be lower than the room temperature so that Night Purge operation can start.

0…20

null

K

P3 Switching Hysteresis

This value specifies how many degrees K the outdoor temperature must be lower than the room temperature so that Night Purge operation can start.

0…10

null

K