Bryant 579F - Manual (Page 53)

OPERATING SEQUENCE (cont)
When first-stage thermostat calls for heat, time-delay relay for
evaporator fan begins timer sequence. Induced-draft relay closes,
and induced-draft motor starts.
Pressure switch closes and pilot valve no. 1 opens, allowing gas to
flow to the first-stage pilot. Spark ignitor ignites pilot flame. Sensor
detects flame and the main gas valve no. 1 opens. Gas flows to
main burners and first-stage burners ignite. Spark ignitor turns off.
When sequence is complete, time-delay relay closes and evapora-
tor fans start.
Second Stage — 579F180-300 Low-Heat and 579F180 High-
Heat Units — With an additional heating call, the second-stage
thermostat closes. (The control relay closes during the first stage of
operation.) Pilot valve no. 2 opens, and the spark ignitor ignites pi-
lot. The sensor detects a flame and energizes main gas valve coil
no. 2, opening main gas valve no. 2. Gas flows to the main burners,
and the second stage burners ignite. The spark ignitor turns off.
When the second-stage thermostat is satisfied, the second-stage
gas valve closes.
When the first-stage thermostat is satisfied, the first-stage gas valve
closes. The induced-draft motor turns off, the time relay opens, and
the timer sequence begins. When the sequence is complete, the
evaporator-fan motor turns off.
Second Stage — 579F240,300 High-Heat Units — With an addi-
tional heating call, the second-stage thermostat closes. (The control
relay closes during the first stage of operation.) The second-stage
induced-draft relay closes, and the second-stage induced-draft mo-
tor starts.
The second-stage pressure switch closes and pilot valve no. 2
opens, allowing gas to flow to the second-stage pilot. Spark ignitor
ignites pilot flame. The sensor detects the flame, and main gas
valve no. 2 opens. Gas flows to main burners and second-stage
burners ignite. Spark ignitor turns off.
When the second-stage thermostat is satisfied, the second-stage
gas valve closes and the second-stage induced-draft motor turns
off.
When the first-stage thermostat is satisfied, the first-stage gas valve
closes. The first-stage induced-draft motor turns off, the time relay
opens, and the timer sequence begins. When the sequence is com-
plete, the evaporator-fan motor turns off.
COOLING, UNITS WITH VARISLIDE™ ECONOMIZER
(580D036-150) — When the outdoor-air temperature is above the
OAT (outdoor-air thermostat) setting and the room thermostat calls
for cooling, compressor contactor no. 1 is energized to start com-
pressor no. 1 and the outdoor (condenser) fan motor(s) (OFM). The
indoor (evaporator) fan motor (IFM) is energized and the econo-
mizer damper moves to the minimum position. Upon a further call
for cooling, compressor contactor no. 2 will be energized, starting
compressor no. 2 (090-150). After the thermostat is satisfied, the
damper moves to the fully closed position when using an auto fan or
to the minimum position when using a continuous fan.
When the outdoor-air temperature is below the OAT setting and the
thermostat calls for cooling, the economizer dampers move to the
minimum position. If the supply-air temperature is above 57 F, the
damper continues to open until it reaches the fully open position or
until the supply-air temperature drops below 52 F.
When the supply-air temperature falls to between 57 F and 52 F,
the damper will remain at an intermediate open position. If the
supply-air temperature falls below 52 F, the damper will modulate
closed until it reaches the minimum position or until the supply-air
temperature is above 52 F. When the thermostat is satisfied, the
damper will move to the fully closed position when using an auto fan
or to the minimum position when using a continuous fan.
If the outdoor air alone cannot satisfy the cooling requirements of
the conditioned space, economizer cooling is integrated with me-
chanical cooling, providing second-stage cooling. Compressor
no. 1 and the condenser fan(s) will be energized and the position of
the economizer damper will be determined by the supply-air tem-
perature. Compressor no. 2 is locked out.
When the second stage of cooling is satisfied, the compressor and
OFM will be deenergized. The damper position will be determined
by the supply-air temperature.
After a 30-second delay, the IFM shuts off. If the thermostat
fan selector switch is in the ON position, the IFM will run
continuously.
COOLING, UNITS WITH PARABLADE ECONOMIZER
(580D036-060) — When the outdoor air is above the enthalpy con-
trol (EC) setting, and the room thermostat calls for cooling, and the
compressor contactor is energized to start the compressor and the
condenser-fan motor. The evaporator-fan motor is energized and
the economizer damper moves to the minimum position. After the
room thermostat is satisfied, the damper will spring return to the
fully-closed position.
When the outdoor air is below the (EC) setting and the thermostat
calls for cooling, the economizer outdoor-air damper is opened pro-
portionally to maintain between 50 and 56 F at the mixed-air sensor.
If outdoor air alone cannot satisfy the cooling requirements, econo-
mizer cooling is integrated with mechanical cooling. When the room
thermostat is satisfied, the damper will spring return to the fully-
closed position.
COOLING, UNITS WITH PARABLADE ECONOMIZER
(580D072-150) — When the outdoor air is above the enthalpy con-
trol (EC) setting, and the room thermostat calls for cooling, and
compressor contactor no. 1 is energized to start compressor no. 1
and the condenser-fan motor. The evaporator-fan motor is ener-
gized and the economizer damper moves to the minimum position.
Upon a further call for cooling, compressor contactor no. 2 is ener-
gized, starting compressor no. 2. After the room thermostat is sat-
isfied, the damper will spring return to the fully-closed position.
When the outdoor-air temperature is below the EC setting and the
thermostat calls for cooling, the economizer outdoor-air damper is
opened proportionally to maintain between 50 and 56 F at the
mixed-air sensor. If outdoor-air alone cannot satisfy the cooling re-
quirements, economizer cooling is integrated with mechanical cool-
ing, and the second compressor is locked out. When the room
thermostat is satisfied, the damper will spring return to the fully-
closed position.
COOLING, UNITS WITH ECONOMIZER (579F180-300) — Upon a
call for cooling, when outdoor ambient is above the enthalpy control
setting, the economizer damper moves to VENT position. The com-
pressor(s) and evaporator and condenser fans energize.
Upon a first call for cooling, when outdoor ambient is below the en-
thalpy control setting, the evaporator fan starts and the economizer
is fully open. The compressor(s) remains off.
Upon a second-stage call for cooling, compressor no. 1 is ener-
gized and mechanical cooling is integrated with economizer cool-
ing. If the outdoor-air temperature drops below 50 F, a cooling
lockout switch prevents the compressors from running.
When supply-air temperature drops below a fixed set point, the
economizer damper modulates to maintain the temperature at the
fixed set point.
A freeze protection thermostat (FPT) is located on the evaporator
coil. It detects frost build-up and turns off the compressors, allowing
the coil to clear. Once frost has melted, the compressors can be re-
energized.
HEATING, UNITS WITH ECONOMIZER (580D036-150) — When
the thermostat calls for heating, terminal W1 is energized. In order
to prevent thermostat short-cycling, the unit is locked into the Heat-
ing mode for at least 1 minute when W1 is energized. The induced-
draft motor is then energized and the burner ignition sequence
begins. The indoor (evaporator) fan motor (IFM) is energized
45 seconds after a flame is ignited and the damper moves to the
minimum position. On units equipped for two stages of heat, when
additional heat is needed, W2 is energized and the high-fire sole-
noid on the main gas valve (MGV) is energized. When the thermo-
stat is satisfied and W1 is deenergized, the IFM stops after a
45 second time-off delay. The economizer damper then moves to
the fully closed position. When using continuous fan, the damper
will remain in the minimum position.
HEATING, UNITS WITH ECONOMIZER (579F180-300) —
Outdoor-air damper stays at VENT position while evaporator fan is
operating. Refer to Heating, Units Without Economizer (579F180-
300) section above for remainder of operating sequence.
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