MGD Gas Furnace
Service Manual
HEAT EXCHANGER REMOVAL/REPLACEMENT
Removal of the heat exchanger from the unit is only required if the heat exchanger has failed. The
causes of heat exchanger failures are normally NOT due to defects in material and workmanship
in the heat exchanger itself.
Before replacing any heat exchanger, the cause of its failure must be determined and corrected.
Failure to do so will cause the replacement heat exchanger to also fail for the same reasons.
Some possible causes of heat exchanger failure are as follows:
1. Temperature rise too high, causing heat exchanger to exceed design-operating
temperature. Operating outside allowable temperature rise range will eventually cause
cracks to appear in the heat exchanger. Filter may be dirty, registers may be closed off,
blower speed may be too low, or unit may be over—fired as a result of manifold pressure
being too high or incorrect orifice being used.
2. Temperature rise too low, causing heat exchanger to never achieve design-operating
temperature. Operating outside allowable temperature rise range can allow condensation
to form inside the heat exchanger, eventually causing corrosion to appear in the heat
exchanger. Blower speed may be too high, return air duct may be open and leak ambient
air or unit may be under—fired as a result of manifold pressure being too low or incorrect
orifice being used.
DIAGNOSTIC
FEATURES
This section will describe the diagnostic features incorporated into the control. The furnace
electronic controls utilize an on-board LED to transmit a diagnostic flash code, which may be used
in troubleshooting system problems. The table below describes these codes.
Table of Diagnostic Codes
FLASHES FAULT
CONDITION
CHECK
STEADY ON
NORMAL OPERATION
1
IGNITION FAILURE (ONE HOUR LOCKOUT) GAS FLOW, GAS PRESSURE, GAS VALVE, FLAME SENSOR
2
CLOSED PRESSURE SWITCH
PRESSURE SWITCH STUCK CLOSED
3
PRESSURE SWITCH FAILED TO CLOSE
VENTER, PRESSURE SWITCH, VENT BLOCKAGE
4
LIMIT SWITCH OPEN
MAIN LIMIT, AUX. LIMIT
5
GAS VALVE ENERGIZED WITH (W) OFF
GAS VALVE
RAPID FLASH
INCORRECT LINE VOLTAGE PHASING
L1 AND L2 CONNECTIONS REVERSED
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