MGD Gas Furnace
Service Manual
PRESSURE CONTROL LOCKOUT
Pressure control lockout happens when the differential pressure switch fails to operate properly
during a call for heat. If the pressure switch is continuously closed, even with the draft inducer off,
no further operation of the furnace will occur, except if flame is detected, main valve is detected, or
if the limit opens. If the condition persists for 30 seconds, the control will flash lockout code 3 until
the condition is corrected or the call for heat is removed. If the differential pressure switch will not
close after the draft inducer has been energized for 30 seconds, the control will begin to flash
lockout code 3 until the condition is corrected or the call for heat is
removed. If the differential
pressure switch has operated properly but opens during normal operation with W energized, the
main valve is de-energized immediately and lockout code 3 will flash. If the differential pressure
switch re-closes, normal operation will return but the differential pressure switch failure will be
counted as a recycle. If the differential pressure switch does not re-close, the control will continue
to flash the lockout code until it does close or the call for heat is removed.
FLAME SENSOR INPUTS
The flame sensor input is capable of reliably detecting the presence of a flame through the use of
the flame Rectification Principle. In order to achieve this, the circuit must properly sense a flame or
its absence whenever the AC Line voltage (between L1 and L2) is at 90 Vrms, and flame
rectification current built into the system is recognized. This allows the verification of the circuits
operation if a flame is detected.
ONE HOUR LOCKOUT
A one-hour lockout occurs following: three retries, five recycles, and five limit cycles in a single call
for heat. During the one-hour lockout, flame detection, limit conditions, and main valve are tested.
Any improper results will cause the appropriate action to occur. While in the one-hour lockout, the
cooling operation of the furnace will continue to function fully. Removing the call for heat will reset
the one-hour lockout period.
20 of 22
