Thomas & Betts UDAP - Manual (Page 21)

Form RZ-NA I-UDA, Mfg #195673 Rev 5, Page 21
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 fan
off delay. The control will return to normal operation after fan off delay is com-
pleted.
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 sec-
onds), 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 recog-
nition 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” terminal 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.
If a call for heat (W) occurs during continuous fan, the fan/blower will de-ener-
gize.
A call for fan is ignored while in lockout.
Undesired Flame - If flame is sensed longer than 20 seconds while the gas valve
is de-energized, the control shall energize the venter motor and fan/blower motor
on heat speed. When flame is no longer sensed, the venter motor will run through
postpurge, and the fan/blower motor will run through the selected heat fan off
delay time. The control will do a soft lockout, but will still respond to open limit
and flame. The FLAME (yellow) LED shall flash rapidly when lockout is due to
undesired flame.
Gas Valve Relay Fault - If the control senses the gas valve as energized for more
than one second when the control is not attempting to energize the gas valve, or the
gas valve is sensed as not energized when it is supposed to be energized, then the
control will lockout with green LED off. The control assumes either the contacts
of the relay driving the gas valve have welded shut, or the sensing circuit has
failed. The venter motor is forced off to open the pressure switch to stop gas flow
unless flame is present.
Continuous Fan
Operation
Fault Modes