Installation manual

IM-487 Page 47
drives the outdoor air dampers towards the open position,
and MCB-BO5 towards the closed (line 338). If the outdoor
air dampers are wide open and more cooling is required, the
dampers will hold their positions and mechanical 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", de-energiz-
ing binary input MCB-BI11 (Alternatively, the outdoor air
temperature is above the changeover setpoint plus the econo-
mizer changeover differential). When the economizer is dis-
abled, the dampers are held at their minimum position.
Heating
Gas Furnace, Modulating Burner (3 to 1 turn down)
Refer to “Typical Gas Furnace Control Circuit (Modulating
Burner, Mixed Air Intake)” on page 58 as you read this
sequence of operation. Note that the gas furnace wiring dia-
grams supplied 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 sup-
plied to the furnace control circuit. If burner switch S3 and
safeties HL22, HL23 are closed, terminal 5 (line 615) on the
flame safeguard control (FSG) will be energized as will the
modulating gas valve VM1. If heating is enabled (MCB-BI4
is energized) and heating is required, the MCB-BO11 will
energize (line 605), 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 6 (line 631) on the flame
safeguard control (FSG) will be energized. FSG will ener-
gize terminal 4 to start the blower motor (BM) through con-
tactor M29 (line 615). If the blower is operational, air switch
AS (line 633) will close and make FSG terminal 6 to 7. After
a 90-second prepurge period, FSG terminals 8 (line 621) and
10 (line 630) are energized. As a result, ignition transformer
IT and pilot gas valve GVI are energized. The pilot flame
will ignite and be detected by FSG through flame rod FD
(line 622). Upon detection of pilot flame after the 10 second
trial for ignition period, the FSG will de-energize terminal 10
and energize terminal 9 to energize main gas valves GV2 and
GV3 (lines 625, 626) and low fire start relay R23 (line 628).
The R23 contacts (lines 642 and 643) allow the MCB to
modulate gas valve actuator VM1 as required to satisfy the
heating demand.
Whenever the burner is operating, its firing rate will be deter-
mined by the position of gas valve actuator VM1. This actua-
tor 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 MCB-
BO10 energizes (line 645), VM1 modulates toward open and
the firing rate increases. When MCB-BO9 energizes (line
644), VM1 modulates toward closed and the firing rate
decreases. When both MCB-BO10 and MCB-BO9 are open,
VM1 holds its position and the firing rate remains constant.
When heating is no longer necessary, MCB-BO11 opens, 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 609) will override
supply fan start/stop output MCB-BO1 through R67 (line
607) 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 II control of
VM1 via MCB-BO10 and MCB-BO9. Because relay R23 is
de-energized whenever GV2 is de-energized, the burner will
always start at low fire.
Flame Control 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 "lockout" state. FSG terminals 4, 8,9
and 10 will be de-energized and the burner will be shut
down. FSG terminal 3 will energize relay R24 (line 633). The
R24 contacts (line 244) signal the controller that the problem
exists by energizing the input to MCB-BI5. If a lockout
occurs, the flame safeguard control must be manually reset.