Form I-UDB, P/N 202658 R10, Page 30
5) Steady Heat
- Control inputs are continuously monitored to ensure limit and pres-
sure 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:
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 blower 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 present
(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.
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 blower off delay.
The control will return to normal operation after blower off delay is completed.
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 recognition
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 blower off delay. When the blower off delay is over, the fan/blower motor
is de-energized, and a heat cycle is begun if the call for heat still exists.
Abnormal Heat Cycle
Functions
8.0 Controls and
Operation
(cont’d)
8.3 Ignition System
(cont’d)
Normal Heat Cycle Operating Sequence (cont’d)
