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Sequences of Operation
The following sequences of operation are for a typical “C”
vintage applied rooftop unit that is equipped with an econo-
mizer, a return air fan, a MicroTech controller, an external
time clock, and a Remote Monitor Panel. These sequences
describe the ladder wiring diagram logic in detail; refer to the
schematics in the “Wiring Diagrams” section as you read
them. Note that your unit’s sequences of operation may vary
from those described here. Refer to the wiring diagrams
supplied with the unit for exact information.
For a detailed description of operation information relat-
ing to the MicroTech controller’s software, refer to the
appropriate operation manual (see Table 1). These manuals
describe the various set points, parameters, operating states,
and control algorithms that affect rooftop unit operation.
Power-up
When the outdoor air is suitable for free cooling, the switch
in enthalpy sensor OAE is in position “3” (line 254), and thus
digital input D0 (terminal DH1-0) on the ADI board is ener-
gized. When DH-1 is energized, the economizer is enabled.
If cooling is required, the economizer dampers (ACT3) are
modulated to maintain the discharge air temperature set
point. When energized, output relay OBA8 opens the out-
door air dampers, and OBA7 closes them (line 338). If the
outdoor air dampers are wide open and more cooling is
required, the dampers will hold their positions and mechani-
cal cooling will be activated (see below).
When the outdoor air is not suitable for free cooling, the
switch in enthalpy sensor OAE is in position “1”, and thus
digital input D0 (terminal DH1-0) on the ADI board is de-
energized. When DH1-0 is de-energized, the economizer is
disabled and the dampers are then held at their minimum
position.
Economizer Operation
Fan Operation
When the supply and return fans are commanded to start by
the microprocessor control board MCB1, the unit enters the
Start-up Initial operating state. As a result, a 3-minute timer
is set, solid-state output relay OBA15 energizes, relay R26
energizes (line 328), and the occupied output contacts close
(line 395). On VAV units, output relays OBA10 and OBA12
energize (line 343), causing the supply and return fan inlet
vanes to open. When the vanes open to their minimum
positions, OBA10 and OBA12 are de-energized, causing the
inlet vanes to hold.
After the 3-minute timer expires, the unit enters the
Recirculate operating state. As a result, output relay OBA13
energizes (line 301), causing supply fan contactors M9 and
M10 to energize. Four seconds after OBA13 is energized,
output relay OBA14 energizes (line 303), causing return fan
contactors M19 and M20 to energize. Overload relays OL9,
OL10, OL19, and OL20 (line 301) protect the fans from
excessive current draw. If either the supply or return fan is
drawing excessive current, one of the relays will open its
contacts and cause both fans to stop.
Within 30 seconds after the fans start, the controller
expects airflow switch PC7 (line 263) to close and thus
energize digital input D8 (terminal DH1-8) on the ADI board.
(If DH2-8 does not energize, the controller will assume the
fans did not start. It will then shut down the unit and generate
an alarm.)
During the Recirculate operating state, the outside air
damper is held closed. The controller does this by energizing
output relay OBA7 (line 338). On VAV units, output relay
OBA6, the VAV box output, is also energized (line 395) during
the Recirculate state.
On VAV units, the supply fan vanes (ACT1) are modulated
to maintain the duct static pressure set point. When ener-
gized, output relay OBA10 opens them, and OBA9 closes
them (line 343). On VAV units or CAV units equipped with
return fan inlet vanes, the return fan vanes (ACT2) are
modulated to maintain an acceptable building static pres-
sure. When energized, output relay OBA12 opens them, and
OBA11 closes them (line 343). (Switch S6 on line 342 is
provided for supply/return fan airflow balancing which is part
of the unit check, test, and start procedure.)
Note: The “Typical Actuator Control Circuit” schematic
shows the modulating hot water or steam heat (VM1) option
that is not discussed here.
• external exhaust fan status contacts (line 257)
• Remote Monitor Panel on-off switch (line 259)
• Remote Monitor Panel heat-auto-cool switch (lines 259
and 262)
• airflow interlock switch PC7 (line 263)
• dirty filter switches PC5 and PC6 (lines 264 and 265)
• compressor safety relays R5 and R6 (lines 266-270)
• duct high limit switch DHL (line 272, VAV and CAV-DTC
only)
• gas furnace alarm relay R24 (line 275)
• freezestat switch FS1 (line 278)
• smoke detectors SD1 and SD2 (line 281)
When the fan switch on the Remote Monitor Panel is in the
“off” position, field wiring terminals 105 and 106 (lines 259
and 262) are de-energized. These respective terminals are
connected to the cool enable digital input D3 (terminal DH1-
3) and the heat enable digital input D4 (terminal DH1-4) on the
ADI board. If terminals DH1-3 and DH1-4 are both de-
energized, the MicroTech controller disables fan operation
because it assumes the fan switch is off. If either terminal
DH1-3 or DH1-4 is energized, the MicroTech controller enables
fan operation because it assumes the fan switch is on.
When primary power is connected to the unit, 115 VAC power
is fed through contact circuit transformer T1 and control
circuit fuse F1 (line 200) to compressor crankcase heaters
HTR-1 and HTR-2 (lines 805 and 855).
When system switch S1 (line 205) is closed, low voltage
transformers T2 (line 203) and T3 (line 208) are energized,
and 115 VAC power is supplied to the following:
• economizer actuator ACT3 (line 336)
• supply fan vane actuator ACT1 (line 340, VAV only)
• return fan vane actuator ACT2 (line 346, VAV only)
• compressor circuit switches CS1 and CS2 (lines 807 and
857)
• unit manual pumpdown switch PS3 (line 328)
Transformer T3 supplies 18 VAC center-tapped power to
the power in terminals 1, 2 and 3 on the microprocessor
control board MCB1 (line 210). By way of terminal TB6-47
(lines 207 and 251), transformer T2 supplies 24 VAC power
to the following:
• static pressure sensor SPS1 (line 230, VAV only)
• static pressure sensors SPS2, SPS5, SPS6 (lines 232-236)
• enthalpy sensor OAE (line 254)
• external time clock contacts (line 256)
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