Thomas & Betts UDAS, UDAP Ignition System cont’d, Normal Heat Cycle Operating Sequence cont’d

20.Ignition System (cont’d)

Normal Heat

Cycle Operating

Sequence (cont’d)

Abnormal Heat Cycle Functions

When the pressure switch is 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 control responds as defined in “Fault Modes, Undesired Flame”.

The control runs the 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-energized and the control proceeds to heat fan/blower 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 ignition re-tries as specified in “Ab- normal Heat Cycle, Ignition Re-try”. If flame is present, the control proceeds to the fan/blower on delay.

4)Fan/Blower On Delay - The control waits for 30 seconds from the time the gas valve opened and then energizes the fan/blower motor. The gas valve and venter motor remain energized. The control proceeds to steady heat mode.

5)Steady Heat - Control inputs are continuously monitored to ensure limit and pressure 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 as selected by the dip switches. Timing begins when the thermostat is satisfied. Delay times are as follows:

SW1 SW2 Time

OFF	OFF	120 seconds (default when shipped)
OFF	ON	90 seconds
ON	OFF	160 seconds
ON	ON	180 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 the longest selectable fan delay time (180 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.

Form RZ-NA I-UDA, P/N 195673 Rev 5, Page 20