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Economizer Enthalpy Control
The economizer can be ordered with the optional differential
enthalpy control. With this option a solid-state humidity and
temperature sensing device is located in the return and
outdoor airstreams. These devices are labeled RAE and OAE
respectively. When the outdoor enthalpy is lower than the return
air enthalpy, the economizer operation will be initiated. If the
outdoor air enthalpy is higher than the return air, the outdoor air
damper position will be at the minimum setpoint. See
for
further information on the economizer operation.
External Time Clock
You can use an external time clock as an alternative to (or in
addition to) the MicroTech III controller’s internal scheduling
function. The external timing mechanism is set up to open and
close the circuit between field terminals 101 and 102. When
the circuit is open, power is not supplied to binary input ID1.
This is the normal condition where the controller follows the
programmable internal schedule. When the circuit is closed,
power is fed to ID1. The MicroTech III controller responds
by placing the unit in the occupied mode, overriding any set
internal schedule.
Exhaust Fan Option
Economizer units may include exhaust fan options. For units
with CAV applications, the exhaust fans can be ordered as
staged control or they may be ordered with building pressure
control. The building pressure control option has a VFD that
runs the exhaust fan motors and is controlled by the static
pressure sensor number 2 (SPS2). Refer to
for setting
up the unit controller with these two options. The units are only
available with building pressure control on VAV units.
The exhaust fan motors are permanently lubricated and do not
require any additional periodic lubrication.
Figure 49: Pressure Tubing Diagram
Proof-of-Airflow and Dirty Filter
Switch
The proof-of-airflow switch (PC7) and the dirty filter switch
(PC5) are supplied on all CAV units. The tubing is installed
to the switches per
. The proof of airflow switches
senses the pressure difference between the positive pressure
in the supply air fan compartment and the suction pressure on
the leaving air side of the filters. The differential pressure is
factory set at 0.25" for this switch. The dirty filter switch senses
the pressure difference across the filter; from the entering air
side of the filter to the leaving air side of the filters. The switch
is factory set at 1.0". When the pressure difference across the
filters is sensed at this value, the dirty filter alarm will appear
on the DDC controller.
All VAV units also have the PC7 and PC5 switches as standard
(see Figure 40). These switches are tied into the Duct High
Limit switch (DHL) as shown in
The DHL is factory set at 4.0". When this differential pressure
is sensed the normally closed contacts will open on the switch
giving the DHL alarm at the unit controller.
Duct High Pressure Limit
The duct high pressure limit control (DHL) is provided on all
VAV units. The DHL protects the duct work, terminal boxes,
and the unit from over pressurization, which could be caused
by, for example, tripped fire dampers or control failure.
The DHL control opens when the discharge plenum pressure
rises to 3.5" wc (872 Pa). This setting should be correct for
most applications and should not be adjusted.
If the DHL switch opens, digital input ID9 on the Unit Control
Board will be de-energized. The MicroTech III controller then
shuts down the unit and enters the Off-Alarm state. The alarm
must be manually cleared before the unit can start again.
Refer to the operation manual supplied with your unit for more
information on clearing alarms (refer to
TO SUPPLY
FAN PLENUM
HI
LO
HI
LO
HI
LO
PC7
PC5
DHL
TO ENTERING
AIR SIDE OF
THE FILTERS
TO LEAVING
AIR SIDE OF
THE FILTERS
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53
IM 1058-8 • MAVERICK II ROOFTOP SYSTEMS
