Carrier PG9UAA Troubleshooting Manual

Contents

Carrier PG9UAA Troubleshooting Manual Manual pdf 3 page image

3resistance. This evaluation is then used to determine therequired RPM necessary to operate the inducer motorduring the low--heat prepurge period and low--heatmode.NOTE: The heat cycle can start in either high-- or low--heat. If ahigh--heat cycle is initiated, the furnace control CPU willde--energize the high--heat pressure switch relay HPSR to close theNC contact and continues to increase the inducer motor speed afterthe low--heat pressure switch LPS closes. When the high--heatpressure switch closes, inducer motor RPM is noted by the furnacecontrol CPU, and a 25--second prepurge period begins. The RPMis used to evaluate vent system resistance. This evaluation is thenused to determine the required RPM necessary to operate theinducer motor in high--heat mode.b. Igniter Warm--Up -- At the end of the prepurge period,the Hot--Surface Igniter HSI is energized for a17--second igniter warm--up period.c. Trial--For--Ignition Sequence -- When the igniterwarm--up period is completed the main gas valve relaycontact GVR closes to energize the gas valve solenoidGV--M. The gas valve solenoid GV--M permits gasflow to the burners where it is ignited. After 5 seconds,the igniter HSI is de--energized and a 2--second Flame--Proving period begins.If the furnace control CPU selects high-heat operation, thehigh-heat gas valve solenoid GV-HI is also energized.d. Flame--Proving -- When the burner flame is proved atthe flame--proving sensor electrode FSE, the furnacecontrol CPU begins the blower--ON delay period andcontinues to hold the gas valve GV--M open. If theburner flame is not proved within two seconds, thecontrol CPU will close the gas valve GV--M, and thecontrol CPU will repeat the ignition sequence for up tothree more Trials--For--Ignition before going toIgnition--Lockout. Lockout will be reset automaticallyafter three hours, by momentarily interrupting 115 vacpower to the furnace, or by interrupting 24 vac power atSEC1 or SEC2 to the furnace control CPU (not atW/W1, G, R, etc.).If flame is proved when flame should not be present, thefurnace control CPU will lock out of Gas-Heating modeand operate the inducer motor IDM at full speed untilflame is no longer proved.e. Inducer Speed Change – If the cycle starts in low--heat,the furnace control CPU reduces the inducer speedslightly after flame sense. If the cycle starts in high--heat, the furnace control CPU increases the inducerspeed 15 seconds after flame sense. The reduction inspeed in low--heat is to optimize combustion formaximum efficiency.f. Blower--On delay – If the burner flame is proven theblower--ON delay for low--heat and high--heat are asfollows:Low--heat -- 30 seconds after the gas valve GV-M isopened the blower motor BLWM is turned ON atlow-heat airflow.High-heat – 35 seconds after the gas valve GV-M is opened theBLWM is turned ON at high-heat airflow.Simultaneously, the humidifier terminal HUM and electronic aircleaner terminal EAC--1 are energized and remain energizedthroughout the heating cycle.g. Switching from Low-- to High--Heat -- If the furnacecontrol CPU switches from low-heat to high-heat, thefurnace control CPU will de-energize the high-heatpressure switch relay HPSR to close the NC contact andslowly increase the inducer motor speed until thehigh-heat pressure switch HPS closes. When thehigh-heat pressure switch HPS closes, the high-heat gasvalve solenoid GV-HI is energized and the inducermotor RPM is noted by the furnace control CPU. TheRPM is used to evaluate vent system resistance. Thisevaluation is then used to determine the required RPMnecessary to operate the inducer motor in high-heatmode. The blower motor BLWM will transition tohigh-heat airflow five seconds after the furnace controlCPU switches from low-heat to high-heat.Switching from High- to Low-Heat – The furnacecontrol CPU will not switch from high-heat to low-heatwhile the thermostat R to W circuit is closed when us-ing a single-stage thermostat.h. Blower--Off Delay -- When the thermostat is satisfied, theR to W circuit is opened, de--energizing the gas valve GV--M, stopping gas flow to the burners, and de--energizing thehumidifier terminal HUM. The inducer motor IDM will re-main energized for a 15--second post--purge period. Theblower motor BLWM and air cleaner terminal EAC--1 willremain energized at low--heat airflow or transition to low--heat airflow for 90, 120, 150, or 180 seconds (dependingon selection at blower--OFF delay switches). The furnacecontrol CPU is factory--set for a 120--second blower--OFFdelay.2. TWO--STAGE THERMOSTAT AND TWO--STAGEHEATINGNOTE: In this mode the low--heat only switch SW1--2 must beON to select the low--heat only operation mode in response toclosing the thermostat R to W1 circuit. Closing the thermostat R toW1--and--W2 circuits always causes high--heat operation,regardless of the setting of the low--heat only switch.The wall thermostat ”calls for heat”, closing the R to W1 circuit forlow--heat or closing the R to W1--and--W2 circuits for high--heat.The furnace control performs a self--check, verifies the low--heatand high--heat pressure switch contacts LPS and HPS are open, andstarts the inducer motor IDM in high--speed.The start up and shut down functions and delays described in item1. above apply to the 2--stage heating mode as well, except forswitching from low-- to high--Heat and vice versa.a. Switching from Low-- to High--Heat -- If thethermostat R to W1 circuit is closed and the R to W2circuit closes, the furnace control CPUwill de-energizethe high-heat pressure switch relay HPSR to close theNC contact and slowly increase the inducer motor speeduntil the high-heat pressure switch HPS closes. Whenthe high-heat pressure switch HPS closes, the high-heatgas valve solenoid GV-HI is energized and the inducermotor RPM is noted by the furnace control CPU. TheRPM is used to evaluate vent system resistance. Thisevaluation is then used to determine the required RPMnecessary to operate the inducer motor in high-heatmode. The blower motor BLWM will transition tohigh-heat airflow five seconds after the furnace controlCPU switches from low-heat to high-heat.b. Switching from High-- to Low--Heat -- If thethermostat R to W2 circuit opens, and the R to W1circuit remains closed, the furnace control CPU willenergize the high-heat pressure switch relay HPSR toopen the NC contact and slowly decrease the inducermotor speed to the required low-heat RPM . When thehigh-heat pressure switch HPS opens, the high-heat gasvalve solenoid GV-HI is de-energized. When theinducer motor IDM reduces pressure sufficiently, thehigh-heat pressure switch HPS will open. The gas valvesolenoid GV-M will remain energized as long as thelow-heat pressure switch LPS remains closed. The