Reznor UEAS Unit Installation Manual - Manual (Page 31)

1) Call for Heat - The thermostat calls for heat by energizing the “W” terminal. The
control checks to see that the limit switches are closed and the pressure switch is
open. If one of the limit switches is open, the control responds as defined in the
“Abnormal Heat Cycle, Limit Switch Operation”. If the pressure switch is closed, the
control will do four flashes on the green LED and wait indefinitely for the pressure
switch to open. If the pressure switch is open, the control proceeds to prepurge.
2) Prepurge - The control energizes the venter motor and waits for the pressure
switch to close. If the pressure switch does not close within 240 seconds of the
venter motor energizing, the control will do two flashes on the green LED. The
control will leave the venter motor energized indefinitely as long as the call for heat
remains and the pressure switch is open.
When the pressure switch is proven closed, the control begins the prepurge time. If
flame is present any time while in prepurge, the prepurge time is restarted. If flame
is present long enough to cause lockout, the control responds as defined in “Fault
Modes, Undesired Flame”.
The control runs the venter motor for a 20 second prepurge time, then proceeds to
the ignition trial period.
3) Ignition Trial Period - The control energizes the spark and main gas valve.
The venter remains energized. If flame is sensed during the first 6 seconds, the
spark is de-energized and the control proceeds to heat fan/blower on delay. If
flame has not been sensed during the first 6 seconds, the control de-energizes
the spark output and keeps the gas valve energized for an additional one second
flame proving period. If flame is not present after the flame proving period, the
control de-energizes the gas valve and proceeds with ignition re-tries as specified
in “Abnormal Heat Cycle, Ignition Retry”. If flame is present, the control proceeds
to the fan/blower on delay.
4) Fan/Blower On Delay - The control waits for 30 seconds from the time the gas
valve opened and then energizes the fan/blower motor. The gas valve and venter
motor remain energized. The control proceeds to steady heat mode.
5) Steady Heat - Control inputs are continuously monitored to ensure limit and
pressure switches are closed, flame is established, and the thermostat call for heat
remains. When the thermostat call for heat is removed, the control de-energizes
the gas valve and begins post-purge and fan/blower off delay timing.
6) Post Purge - The venter motor output remains on for a 45-second post-purge
period after the thermostat is satisfied.
7) Fan/Blower Off Delay - The fan/blower motor is de-energized after a blower off
delay of 120 seconds.:
Normal Heat Cycle
Operating Sequence
Abnormal Heat Cycle
Functions
Interrupted Thermostat Call for Heat - If the thermostat demand for heat is
removed before the flame is recognized, the control will run the venter motor for
the post purge period and de-energize all outputs.
If the thermostat demand for heat is removed after successful ignition, the control
will de-energize the gas valve, run the venter motor through post purge, and run
the fan/blower motor on heat speed for the selected delay off time.
Ignition Retry - If flame is not established on the first trial for ignition period, the
control de-energizes the gas valve and the venter motor remains energized for an
inter-purge period of 20 seconds. The spark and gas valve are then re-energized,
and the control initiates another trial for ignition.
If flame is not established on the second trial for ignition, the control de-energizes
the gas valve, energizes the fan/blower motor on heat speed, and venter motor
remains energized. The fan/blower motor is shut off after 120 seconds. When the
fan/blower motor de-energizes, the spark and gas valve are re-energized and the
control initiates another trial for ignition. (This fan delay is self-healing feature for
an open auxiliary limit switch).
If flame is not established on the third trial for ignition period, the control de-ener-
gizes the gas valve, and the venter motor remains energized for an inter-purge
period of 20 seconds. The control then re-energizes the gas valve and spark and
initiates another trial for ignition.
Form I-UEAS, P/N 221232 R13, Page 31