York P*XU/G9D-UP Installation Instruction

Contents

035-17480-000 Rev. A (800)28 Unitary Products GroupINTERMITTENT BLOWER - COOLINGOn cooling/heating thermostats with fan switch, when the fanswitch is set in the auto position and the thermostat calls forcooling, a circuit is completed between the R, Y and G termi-nals.The motor is energized through the cool fan terminal andruns on the selected speed. The fan off setting is fixed at 60seconds for SEER enhancement.HEATING OPERATIONAL SEQUENCETwo-Stage ThermostatIn a typical system, a call for first stage heat is initiated byclosing the W1 thermostat contacts. The inducer blower isenergized at high speed and the control waits for the lowpressure switch contacts to close. The inducer blower isenergized at low speed and the 120 VAC silicon carbideigniter is powered.At the end of the igniter warm-up time, the first stage of thetwo-stage gas valve is energized (low fire). Flame must bedetected within 7 seconds. If flame is detected, the delay-to-fan-on period begins. After the delay-to-fan-on period ends,the circulator fan is energized at low heat speed and the aircleaner and humidifier are energized.A call for second stage heat (W1 and W2) after a call for firststage heat will energize the inducer at high speed and ener-gize the circulator at high heat speed. The second stagepressure switch contacts will close and energize the secondstage gas valve (high fire).If a call for second stage heat occurs at the same time as acall for first stage heat, the second stage heat is energized asabove.When the second stage of the thermostat is satisfied, theinducer motor is reduced to low speed and the second stagegas valve is de-energized upon opening of the second stagepressure switch contacts. The circulator remains on high heatspeed for 30 seconds and then is reduced to low heat speed.When the first stage of the thermostat is satisfied, the gasvalve is de-energized and the heat delay to fan off begins tim-ing. The inducer will post-purge for an additional 5 seconds,then the inducer and humidifier will de-energize. Upon com-pletion of the heat delay-to-fan-off, the circulator and aircleaner are de-energized. The control timing is graphicallyshown in Figure 29 on page 27.If flame is not detected during the trial-for-ignition period, thegas valve is de-energized, the ignitor is turned off and the50A51 control goes into the retry sequence.The retry sequence provides a 120-second wait with inducerinter-purge following an unsuccessful ignition attempt (flamenot detected). After this wait, the ignition sequence isrestarted with an additional 10 seconds of ignitor warm-uptime. Two retries will be attempted before the control goesinto system lockout.If flame is detected, then lost, the 50A51 control will repeatthe initial ignition sequence. After 5 attempts to sustain flame,the control will go into system lockout.If flame is established for more than 10 seconds after ignition,the 50A51 controller will clear the ignition attempt (or retry)counter. If flame is lost after 10 seconds, it will restart the igni-tion sequence. This can occur a maximum of 5 times beforesystem lockout.During burner operation, a momentary loss of power of 50milliseconds or longer will de-energize the main gas valve.When power is restored, the gas valve will remain de-ener-gized and a restart of the ignition sequence will begin imme-diately.A momentary loss of gas supply, flame blowout, or a shortedor open condition in the flame probe circuit will be sensedwithin 0.8 seconds. The gas valve will de-energize and thecontrol will restart the ignition sequence.Recycles will begin and the burner will operate normally if thegas supply returns, or the fault condition is corrected, beforethe last ignition attempt. Otherwise, the control will go intosystem lockout.If the control has gone into system lockout, it may be possibleto reset the control by a momentary power interruption of onesecond or longer. Refer to the diagnostics section of thisinstruction for reset information.Single-Stage ThermostatIf a single stage thermostat is used the unit will be controlledas above with the following exception. On a call for heat theunit will operate on low fire as detailed above. If the heatingrequirements cause the cycle to exceed 10 minutes, the highfire timer will energize the second stage heat. The secondstage operation will function as detailed above. The heatingcycle will be completed in high fire operation.Hot Surface Ignition SystemDo not attempt to light this furnace by hand (with amatch or any other means). There may be apotential shock hazard from the components of thehot surface ignition system. The furnace can onlybe lit automatically by its hot surface ignitionsystem.