Carrier 58DXT Operating Instructions Manual

Also see for 58DXT: User's information manual Service and maintenance instructions

Contents

Carrier 58DXT Operating Instructions Manual Manual pdf 13 page image

This furnace can operate as a 2-stage furnace with a single-stage thermostat because furnace control CPU includes aprogrammed adaptive sequence of controlled operation whichselects low-gas-heat or high-gas-heat operation. This selectionis based upon stored history of the length of previous gasheating on/off periods of single-stage thermostat.The furnace starts up in either low- or high-gas heat. If furnacestarts up in low-gas heat, control CPU determines low-gasheat on time (from 0 to 16 minutes) which is permitted beforeswitching to high-gas heat.If power is interrupted, stored history is erased, and controlCPU selects low-gas heat for up to 16 minutes and thenswitches to high-gas heat as long as thermostat continues to"call for heat." Subsequent selection is based on stored historyof thermostat cycle times.When the wall thermostat "calls for heat," R-W1 circuitcloses. The furnace control performs a self-check, verifieslow-heat and high-heat pressure switch contacts LPS and HPSare open, and starts inducer motor IDM in low speed or highspeed as appropriate.a. Inducer prepurge period—As inducer motor IDM comes upto low speed or high speed, the low-heat pressure switchcontacts LPS (or LPS and HPS) close to begin a 15-secprepurge period.b. Ignitor warm-up—At the end of prepurge period, hotsurface ignitor HSI is energized for a 17-sec ignitorwarm-up period.c. Trial-for-ignition sequence—When ignitor warm-up periodis completed, main gas valve relay contacts MGVR-1 and-2 close to energize low-heat gas valve solenoid GV, gasvalve opens, and 24-v power is supplied for a field-installedhumidifier at terminals HUM and COM. Low-heat gas valvesolenoid GV permits gas flow to the burners where it isignited. After 5 sec, ignitor HSI is de-energized, and a 2-secflame-proving period begins.If furnace control CPU selects high-gas-heat operation,high-heat gas valve solenoid GV is also energized afternormally closed high-heat pressure switch relay HPSRcloses and after inducer motor IDM goes to high speed andprovides sufficient pressure to close high-heat pressureswitch HPS. HPSR is open while furnace is powered instandby mode. If high-heat pressure switch HPS fails toclose and low-heat pressure switch LPS closes, furnaceoperates at low-heat gas flow rate until high-heat pressureswitch closes.d. Flame-proving—When burner flame is proved at flame-proving sensor electrode FSE, control CPU begins bloweron delay period and continues to hold gas valve GV open.If burner flame is not proved within 2 sec, control CPUcloses gas valve GV, and control CPU repeats ignitionsequence for up to 3 more trials-for-ignition before goingto ignition lockout. LOCKOUT IS RESET AUTOMATI-CALLY after 3 hr, or by momentarily interrupting 115-vpower to furnace, or by interrupting 24-v power at SEC1 orSEC2 to control CPU (not at W/W1, G, R, etc.). Openingthermostat R-W circuit will not reset ignition lockout.If flame is proved when flame should not be present,control CPU locks out of gas heating mode and operatesinducer motor IDM on high speed until flame is no longerproved.e. Blower on delay—If burner flame is proven, 45 sec aftergas valve GV is opened blower motor BLWM is energizedon appropriate heating speed, low-gas-heat or high-gas-heat speed. Simultaneously, EAC terminals EAC-1 andEAC-2 are energized with 115v and remain energized aslong as blower motor BLWM is energized.f. Switching from low- to high-gas heat—If furnace controlCPU switches from low-gas heat to high-gas heat, controlCPU switches inducer motor IDM speed from low to high.The high-heat pressure switch relay HPSR closes. Wheninducer motor IDM provides sufficient pressure to closehigh-heat pressure switch HPS, high-heat gas valve sole-noid GV is energized. Blower motor BLWM switchesspeed for high-gas heat 5 sec after control CPU switchesfrom low-gas heat to high-gas heat.g. Switching from high- to low-gas heat—Control CPU willnot switch from high-gas heat to low-gas heat whilethermostat R-W circuit is closed when a single-stagethermostat is used.h. Blower off delay—When thermostat is satisfied, R-Wcircuit is opened, de-energizing gas valve GV, stopping gasflow to burners, and de-energizing humidifier terminalsHUM and C OM. Inducer motor IDM remains energized fora 5-sec post-purge period. Blower motor BLWM and EACterminals EAC-1 and EAC-2 remain energized for 90, 135,180, or 225 sec (depending on selection at blower off delayswitches SW-3 and SW-4). Furnace control CPU is factoryset for a 135-sec blower off delay.2. Non-Adaptive Heating Mode—Two-Stage Thermostat and2-Stage Heating(See Fig. 12 for thermostat connections.)NOTE: The low-heat-only switch SW-2 ON selects low-heat-only operation mode in response to closing thermostat R-W/W1circuit. When high-heat-only switch SW-1 is off, closing thermo-stat R to W1-and-W2 circuits is required for high-gas-heatoperation. When high-heat-only switch SW-1 is on, it alwayscauses high-gas-heat operation when R-W/W1 circuit is closed,regardless of setting of low-heat-only switch SW-2 and regardlessof whether R-W2 circuit is closed or open.The start-up and shutdown functions and delays described initem 1 above apply to 2-stage heating mode as well, except forswitching from low- to high-gas heat and vice versa.a. When wall thermostat "calls for heat," R-W/W1 circuitcloses for low-gas heat or R to W1-and-W2 circuits closefor high-gas heat. The furnace control performs a self-check, verifies low-heat and high-heat pressure switchcontacts LPS and HPS are open, and starts inducer motorIDM in low speed or high speed as appropriate.b. Switching from low- to high-gas heat—If thermostatR-W/W1 circuit for low-gas heat is closed and R-W2circuit for high-gas heat closes, control CPU switchesinducer motor IDM speed from low to high. The high-heatpressure switch relay HPSR closes. When inducer motorIDM provides sufficient pressure to close high-heat pres-sure switch HPS, high-heat gas valve solenoid GV isenergized. Blower motor BLWM switches speed for high-gas heat 5 sec after R-W2 circuit closes.c. Switching from high- to low-gas heat—If thermostat R-W2circuit for high-gas heat opens and R-W/W1 circuit forlow-gas heat remains closed, control CPU switches inducermotor IDM speed from high to low. The high-heat pressureswitch relay HPSR opens to de-energize high-heat gasvalve solenoid GV. When inducer motor IDM reduces13