Install Instructions

30
I-UBX-UBZ-UDX-UDZ (04-21) 1034344-0
Table 19. Operating Sequence (Abnormal Heat Cycle)
Abnormal
Function
Condition Action
Interrupted
thermostat
call for heat
Thermostat demand for heat is
removed before flame is recognized
Control runs venter motor for post-purge period
All outputs are deenergized
Thermostat demand for heat is
removed after successful ignition
Control deenergizes gas valve
Control runs venter motor through post-purge period
Control runs fan/blower motor on heat speed for selected delay OFF time
Ignition retry
Flame is not established on first trial
for ignition period
Control deenergizes gas valve
Venter motor remains energized for 10-second inter-purge period
Spark and gas valve are re-energized
Control initiates another trial for ignition
Flame is not established on second
trial for ignition
Control deenergizes gas valve
Control runs fan/blower motor on heat speed
Venter motor remains energized
Fan/blower motor deenergizes after selected delay OFF period and spark and gas
valve are re-energized
Control initiates another trial for ignition (this fan delay is self-healing feature for open
auxiliary limit switch)
Flame is not established on third trial
for ignition
Control deenergizes gas valve
Venter motor remains energized for 10-second inter-purge period
Spark and gas valve are re-energized
Control initiates another trial for ignition
Flame is not established on fourth trial
for ignition (initial try plus three re-tries)
Control deenergizes gas valve and proceeds to lockout
SSD displays “L” to indicate ignition failure lockout
Limit switch*
Limit switch is open and call for heat is
present
Control deenergizes gas valve
Control runs venter motor and runs fan/blower motor on heat speed
Control is in soft lockout (SSD displays “L”) before returning to normal operation
Limit switch re-closes or call for heat is
not present
Control runs venter motor through post-purge period
Control runs fan/blower motor on heat speed through selected delay OFF period
Pressure
switch
operation
Pressure switch opens before trial for
ignition period
Venter motor runs through 2-second pressure switch recognition delay
Control deenergizes gas valve
Control runs venter motor through post-purge period
Control restarts heat cycle at pressure switch proving state if call for heat still exists
Pressure switch opens for less than 2
seconds during trial for ignition period
(shall not interrupt heat cycle)
Control deenergizes gas valve while pressure switch is open
Pressure switch opens after successful
ignition
Control deenergizes gas valve
Flame is lost before end of 2-second
pressure switch recognition delay
Control responds to loss of flame
Pressure switch remains open for 2
seconds and flame remains
Control deenergizes gas valve
Control runs venter motor through post-purge period
Control runs fan/blower motor on heat speed through selected delay OFF period
When fan OFF delay ends, fan/blower motor is deenergized, and heat cycle begins if
call for heat still exists
Continuous
fan operation
Thermostat calls for continuous fan (G)
without call for heat
Fan motor is energized after 0.25-second delay (this brief ON delay allows terminal
G to energize slightly before terminal Y and allows external changeover relay to
switch from terminal G to terminal W without causing momentary glitches in fan/
blower output
Fan remains energized as long as call for fan remains without call for heat
Thermostat calls for heat (W) during
continuous fan operation
Fan/blower is deenergized
Call for fan is ignored during lockout
*The limit switch is ignored unless a call for heat is present (terminal W energized).
OPERATION—CONTINUED
Startup—Continued
Operating Sequences—Continued