IM 178 / Page 33
Cooling Operation
Chilled water coil, modulating valve
Refer to the “Typical Actuator Control Circuit ” schematic in
the following section, “Wiring Diagrams,” as you read this
sequence of operation.
If cooling is required, the valve motor VM5 modulates to
maintain the discharge air set point. When terminal 4 and 1 are
energized, output relay OBA2 opens the chilled water valve
(line 332). When terminals 4 and 2 are energized, output relay
OBA1 closes the chilled water valve (line 332). If both output
relay OBA1 and OBA2 are closed, VM5 will stall. During periods
of power failure, the chilled water valve is fully opened.
Heating Operation
Gas furnace, modulating burner
Refer to the “Typical Gas Furnace Control Circuit (Modulat-
ing Burner, Mixed Air Intake)” schematic in the following
section, “Wiring Diagrams,” as you read this sequence of
operation. Note that the gas furnace wiring diagrams sup-
plied with the units include a detailed sequence of operation.
Refer to the wiring diagram supplied with the unit for exact
wiring and sequence of operation information.
When system switch S1 is closed, 115 VAC power is
supplied to the furnace control circuit. If heating is enabled
[digital input D4 (terminal DH1-4) on ADI board energized]
and heating is required, the MCB1 controller will energize
solid-state output relay OBA3 (line 602), thus energizing
relay R20. The normally open R20 contacts (line 610) close,
and if manual burner switch S3 and safeties HL22, HL23, FLC
(high limit switch), LP5, and HP5 are closed, terminal 16 (line
631) on the flame safeguard control (FSG) will be energized.
Relay 3K in FSG is energized via normally closed contacts
1K2 (line 628) and SSW (line 631). The flame safeguard then
energizes its terminal 8 (line 623), which energizes combus-
tion air blower motor BM (line 615). If the blower is opera-
tional, air switch AS (line 625) will close and energize FSG
terminal 3. After a 90-second prepurge period, FSG relay 1K
is energized and thus terminals 18 (line 630) and 5 (line 622)
are energized. As a result, ignition transformer IT and pilot
gas valve GV1 are energized. The pilot flame will ignite and
be detected by FSG through flame rod FD (line 635). After the
10-second trial for ignition period, the FSG will energize relay
2K and light an onboard LED (lower left corner). The 2K1
contacts de-energize transformer IT via terminal 18 (line 630)
and energize main gas valves GV2 and GV3 and low fire start
relay R23 via terminal 6 (line 625). The R23 contacts (lines 642
and 643) allow the MicroTech controller to modulate gas valve
actuator VM1 as required to satisfy the heating demand.
Whenever the burner is operating, its firing rate will be
determined by the position of gas valve actuator VM1. This
actuator modulates the butterfly gas valve and combustion
air damper (lines 690 and 692), thus varying the furnace firing
rate between 33% and 100% of full capacity. When the
MicroTech controller closes output relay OBA5, VM1 modu-
lates toward open and the firing rate increases. When the
controller closes output relay OBA4, VM1 modulates toward
closed and the firing rate decreases. When both OBA4 and
OBA5 are open, VM1 holds its position and the firing rate
remains constant.
When heating is no longer necessary, the controller opens
OBA3, de-energizing relay R20 and opening its contacts in
line 610. As a result, the flame safeguard control is de-
energized, all gas valves close, the combustion air blower
motor stops, and gas valve actuator VM1 closes. If the furnace
is warm enough to close it, the FLC fan controls switch (line
606) will override supply fan start/stop output OBA13 (line
603) and keep the supply fan running until the furnace cools
down (this might happen during night setback operation).
If the furnace overheats, the FLC high limit control (line
610) will cycle the burner, preventing the furnace temperature
from exceeding the limit control’s set point. When the fur-
nace is cycled off, low fire start relay R23 de-energizes. The
normally closed R23 contacts (line 643) cause VM1 to drive
to its minimum position, overriding MicroTech control of VM1
via OBA4 and OBA5. Because relay R23 is de-energized
whenever GV2 is de-energized, the burner will always start at
low fire.
Safety lockout
If the pilot flame does not ignite or the flame safeguard fails
to detect its flame within 10 seconds, the flame safeguard
control will enter the “safety lockout” state. FSG terminals 5,
6, 8, and 18 will be de-energized, and thus the burner will be
shut down. The normally open SSW contacts (line 632) will
close and energize relay R24 (line 633). The R24 contacts
(line 275) will energize the Remote Monitor Panel “Heat Fail”
light and signal the controller that the problem exists by
digital input D9 (terminal DH2-8) on the ADI board. If a safety
lockout occurs, the flame safeguard control must be manu-
ally reset.
Multistage electric heat
(CAV-ZTC units only)
Refer to the “Typical Electric Heat Control Circuit (Multi-
stage)” schematic in the following section, “Wiring Dia-
grams,” as you read this sequence of operation.
When system switch S1 is closed, 115 VAC power is
supplied to the electric heat control circuit through terminals
17 and NB2 (line 559). Heating switch HS1 (line 560) is closed
for normal electric heating operation.
If heating is enabled [digital input D4 (terminal DH1-4) on
ADI board energized] and heating is required, the MCB1
controller will energize solid-state output relay OBA3 (line
560), allowing power to flow through the heater of sequenc-
ing relay SR1. After approximately 10 to 30 seconds, con-
tacts in SR1 close. If the high limit temperature switches are
also closed, contactors M31 and M41 will be energized (lines
560 and 562), thus supplying power to heaters 1A, 1B, 2A,
and 2B (lines 511-516). These heater power circuits are
protected by fuseblocks FB31 and FB41 and the high limit
temperature switches. This is stage 1.
When more heat is required, the MicroTech controller
energizes OBA4 for stage 2 and OBA5 for stage 3. When less
heat is required, the controller de-energizes the output relays
in reverse order.
Heating coil, modulating valve
Refer to the “Typical Actuator Control Circuit” schematic in
the following section, “Wiring Diagrams,” as you read this
sequence of operation.
If heating is required, the valve motor VM1 modulates to
maintain the discharge air set point. When OBA5 is ener-
gized, terminals 1 and 4 on the valve motor actuator are
made, which drives the heating valve open (line 363). When
OBA4 is energized, terminals 2 and 4 on the valve motor
actuator are made, which drives the heating valve closed
(line 363). During periods of power failure, the heating valve
is fully opened.
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com
