Reznor UDAS Unit Installation Manual - Manual (Page 24)

Page 24
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 fan/blower off
delay (120 seconds). Timing begins when the thermostat is satisfied.
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 10 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-energizes
the gas valve, and the venter motor remains energized for an inter-purge period of 10
seconds. The control then re-energizes the gas valve and spark and initiates another
trial for ignition.
If flame is not established on the fourth trial for ignition (initial try plus 3 re-tries), the
control de-energizes the gas valve and goes into lockout. The control goes to one flash
on the green LED to indicate ignition failure lockout.
Limit Switch Operation - The limit switch is ignored unless a call for heat is pres-
ent (W energized). If the limit switch is open and a call for heat is present, the control
de-energizes the gas valve and turns the fan/blower motor on heat speed and runs
the venter motor. The control will be in soft lockout and flashing fault code “3” before
returning to normal operation.
When the switch re-closes or the call for heat is lost, the control runs the venter motor
through post purge and runs the blower/fan motor through the selected fan off delay.
Pressure Switch - If the pressure switch opens before the trial for ignition period, the
venter motor will run through the pressure switch recognition delay (2 seconds), the
gas valve will be de-energized, and the venter motor will run through the postpurge
time. The control will re-start the heat cycle at the pressure switch proving state if the
call for heat still exists.
Pressure switch opening for less than 2 seconds during the trial for ignition period shall
not interrupt the heat cycle. (Gas valve will de-energize while the pressure switch is
open.)
If the pressure switch opens after a successful ignition, the control will de-energize
the gas valve. If flame is lost before the end of the 2 second pressure switch recogni-
tion delay, the control will respond to the loss of flame. If the pressure switch remains
open for 2 seconds and the flame remains, the control de-energizes the gas valve, the
venter motor runs through post-purge, and the fan/blower motor runs on heat speed
through the selected fan off delay. When the fan off delay is over, the fan/blower motor
is de-energized, and a heat cycle is begun if the call for heat still exists.
When the thermostat calls for continuous fan (G) without a call for heat, the fan motor is
energized after a .25 second delay. NOTE: This brief on delay is to allow the “G” termi-
nal to energize slightly before “Y” and an external changeover relay to switch from “G”
to “W” without causing momentary glitches in the fan/blower output. The fan remains
energized as long as the call for fan remains without a call for heat.
Abnormal Heat Cycle
Functions
8.3 Ignition System
(cont’d)
8.0 Controls and
Operation
(cont’d)
Normal Heat Cycle Operating Sequence (cont’d)