McQuay MicroTech III RAH, Руководство по эксплуатации и обслуживанию, Страница: 78 / 122

78 McQuay OM 920-1
Operator’s Guide
Post Heat Operation
After leaving the Recirc or Heating operating state and entering either the Fan Only or Min
DAT operating state, the unit performs “post heat” operation if the Post Heat Timer is set to a
non zero value. “Post heat” operation occurs within the Fan Only or MinDAT operating state.
During “post heat” operation, the VAV Box Output remains open (heat) while the discharge
fan capacity is forced to a minimum value (default 25% speed for VFD). By forcing the
discharge fan capacity to a minimum value before the VAV Box Output closes (cool), “post
heat” operation is designed to prevent duct over-pressurization by decreasing the duct pressure
before the VAV boxes can close.
“Post heat” operation remains active until either the discharge fan capacity reaches the
minimum value or until the Post Heat Timer expires, whichever occurs first. When “post heat”
operation ends, normal duct static pressure or position control resumes.
Note – Note: During “post heat” operation and for 120 seconds afterward, the proof of airflow input
is ignored. This is to prevent nuisance Fan Fail fault alarms that may occur if the airflow
switch opens during or following “post heat” operation. The unit cannot leave the Fan Only
or Min DAT operating state while the airflow switch input is being ignored. The parameter is
adjustable via the following path: Extended Menus/Timer Settings/Post Heat.
Dehumidification
In the dehumidification mode, mechanical cooling is used to cool air low enough to wring
moisture out of it. Hot Gas Reheat or the standard unit heating equipment (Gas, SCR Electric
or Steam/Hot water) is used to raise the temperature of this cooled air to a desirable value.
Dehumidification Initiation
An analog sensor is mounted in the return duct, the space, or outdoor to sense Relative
Humidity. The location is selected by setting the Sensor Location value on the keypad to
Return, Space, or OAT.
Note – Note: This setting is used to dictate which temperature sensor is used to calculate the
dewpoint. OAT can only be selected for units with DAT control. The Relative Humidity and
a calculated Dew Point temperature are both displayed on the keypad. The calculated dew
point temperature is based on the Relative Humidity and Return, Space, or OAT value
depending on the location of the humidity sensor. Humidity control is disabled if cooling is
disabled for any reason. Dehumidification operation is initiated when Humidity Control is
set to either Relative Humidity or Dew Point and that value rises above the appropriate
setpoint by more than half its deadband.
Dehumidification Termination
Dehumidification is terminated if the selected variable, Relative Humidity or Dew Point,
drops below the appropriate humidity setpoint by more than half its deadband.
Dehumidification is also terminated if cooling is disabled for any reason. Other ways that
dehumidification may be terminated are if Dehum Method is set to “None” on the keypad, or
if the unit goes to Off Net, Off Sw, Off Alm, or Off Man Ctrl. The Dehum Method parameter
is adjustable via the following path: Extended Menus/Dehumidification Setup/Dehum
Method.
When Dehumidification is terminated, all mechanical cooling is turned off except when the
unit is in the Cooling state. Modulated cooling reverts to normal control when
dehumidification is terminated in the Cooling state. For units with compressors, the number of
cooling stages is reduced by one and control reverts to normal control when dehumidification
is terminated in the Cooling state. Another compressor stage change could then occur after one
Cooling Stage Time has elapsed