Thomas & Betts LDAP 1200 - Manual (Page 19)

Form RZ-NA-I-LDAP, P/N 207733 (Rev 2), Page 19
Abnormal Heat
Cycle Functions
Normal Heat
Cycle Operating
Sequence 1) Call for Heat - The thermostat calls for heat by energizing the “W” terminal. The
control checks to see that the limit and high limit switches are closed and the heat section
pressure switch is open and the main combustion air pressure switch (Sizes 800 and
1200) is open. If the limit or high limit switch is open, the control responds as defined in
the “Abnormal Heat Cycle, Limit Switch Operation”. If the heat section pressure switch
or main combustion air pressure switch are closed, the control will do four flashes on the
green LED and wait indefinitely for the heat section pressure switch or main combustion
air pressure switch to open. If the heat section pressure switch and main combustion air
pressure switch are open, the control proceeds to prepurge.
2) Prepurge - The control energizes the heat section venter motor and main venter motor
(Sizes 800 and 1200) and waits for the heat section pressure switch and main combustion
air pressure switch to close. If the heat section pressure switch or main combustion air
pressure switch does not close within 30 seconds of the heat section venter motor and
main venter motor energizing, the control will do two flashes on the green LED. The
control will leave the heat section venter motor and main venter motor energized indefi-
nitely as long as the call for heat remains and the heat section pressure switch or main
combustion air pressure switch is open.
When the heat section pressure switch and main combustion air pressure switch are 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 con-
trol responds as defined in “Fault Modes, Undesired Flame”.
The control runs the heat section venter motor and main 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 16 seconds, the spark is de-ener-
gized and the control proceeds to heat fan on delay. If flame has not been sensed during
the first 16 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 igni-
tion re-tries as specified in “Abnormal Heat Cycle, Ignition Re-try”. If flame is present,
the control proceeds to the fan on delay.
4) Fan On Delay - The control waits for 30 seconds from the time the gas valve opened
and then energizes the fan motor. The gas valve, heat section venter motor, and main
venter motor remain energized. The control proceeds to steady heat mode.
5) Steady Heat - Control inputs are continuously monitored to ensure limit, high limit,
heat section pressure switch, and main combustion air pressure switch 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 off
delay timing.
6) Post Purge - The heat section venter motor and main venter motor outputs remain on
for a 45 second post-purge period after the thermostat is satisfied.
7) Fan Off Delay - The fan motor is de-energized after a fan off delay as selected by the
dip switches. Timing begins when the thermostat is satisfied. Delay times are as in the
table on the left. The factory default when shipped is 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 heat section venter motor and
main 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 heat section venter motor and main venter motor through
post purge, and run the fan motor on heat speed for the selected delay off time.
NOTE: All LED Flash
Codes are on page 38.
Ignition Retry - If flame is not established on the first trial for ignition period, the control
de-energizes the gas valve and the heat section venter motor and main venter motor
TIME SW1 SW2
90 sec OFF ON
120 sec OFF OFF
160 sec ON OFF
45 sec ON ON
Switch Settng Chart for Fan
OFF Delay (See SW1 on W.D.)