58
Rev. 2
SYSTEM OPERATION
SEQUENCE OF OPERATION
(GUIV Variable Speed Two Stage Models with White-
Rodgers 50A51-255 Integrated Ignition Control)
NORMAL SEQUENCE OF OPERATION
Normal Heating Sequence
1. Thermostat calls for heat (high heat or low heat).
2. The induced draft blower is energized on high speed for
a 10 second prepurge.
3. The induce draft blower is energized on low speed.
4. The ignitor is energized and is allowed to preheat for 17
seconds.
5. The gas valve is energized delivering gas to the burners
at the low stage heat flow rate.
6. The control checks for a signal from the flame sensor
within seven seconds after the gas valve is energized
(opens). Gas will only continue to flow if a signal is
present.
7. The control checks the thermostat to see whether the
call for heat is for low stage heat or high stage heat. If
the call is for high stage heat, the induced blower is
switched to high speed and the gas valve is energized
on high flow rate.
8. The control waits 30 seconds and turns on the air circu-
lation blower fan to the appropriate speed (high heat speed
for high heat or low heat speed for low heat). The circula-
tion blower fan will take 30 seconds to ramp up to full
speed.
9. The furnace is now operating on the specified stage called
out by the two-stage thermostat.
10. If the two-stage thermostat changes the call from low
heat to high heat, the control will immediately switch the
induced draft blower motor, gas valve, and air circulating
speed to their high stage setting.
11. If the two-stage thermostat changes the call from high
heat to low heat, the control will immediately switch the
induced draft blower motor to low speed and switch the
flow rate on the gas valve to low. The air circulator motor
will remain on high heating speed for thirty seconds be-
fore switching to the low heat circulating speed. The 30
second delay feature on the circulator speed eliminates
the possibility of tripping the high (primary) limit when
the high stage flue products are traveling through the
heat exchanger upon a call for low stage heat.
12. The two-stage thermostat is satisfied and opens.
13. The control turns the gas valve off.
14. After a five second delay while flue products are purged
from the heat exchanger, the induced draft blower motor
is turned off (the induced draft blower is on low speed
during the first five seconds post purge).
15. The air circulation blower fan has an adjustable delay-off
timing of 60, 90, 120, or 180 seconds (starting from the
time the gas valve closes). The speeds run during this
period depend on the last heat call seen by the thermo-
stat.
If the last call for heat was a call for low heat, the air circula-
tor motor will run on the low heat speed for the duration of
the adjusted delay-off timing (60, 90, 120, or 180 seconds).
If the last call for heat was a call for high heat, the air circu-
lating motor run on the high heating speed for thirty seconds
and then switch to the low heating speed for the balance of
the adjusted delay-off timing (30, 60, 90, or 150 seconds).
Example: A GUIV090DX50 has the heating blower off delay
set to 120 seconds by the installer. When the thermostat is
satisfied after a call for high heat, the air circulation blower
fan will run on high speed for 30 seconds and then switch to
low circulator speed for 120 - 30 = 90 seconds.
The adjustable delay-off timing allows for more heat trans-
ferred to the conditioned space from the furnace. After the
delay time has elapsed, the air circulating motor is de-
energized. The adjustable delay-off timing feature allows the
installer to customize the comfort level based on the pre-
dominant staging requirements of the living space.
16. The air circulating blower fan will ramp down for another
30 seconds and shut down.
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