Thomas & Betts UDAP, UDAS dimensions Continuous Fan Operation Fault Modes

Continuous Fan Operation

Fault Modes

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.

Form RZ-NA I-UDA, Mfg #195673 Rev 5, Page 21