Midco Economite RE6850B - Manual (Page 13)

Part 2 Service
Cleaning of the blower wheel is usually the only service required. Need for cleaning is
indicated if the character of the flame indicates a deficiency of air. Motor cooling air vents (if
present) should also be cleaned at this time.
The motor and wheel are removed as an assembly. Disconnect the motor wires from the
motor terminal strip. Disconnect the motor conduit from the motor and remove the mounting
bolts.
The burner is equipped with a diaphragm type air pressure switch so as to prevent the
burner from firing if the blower wheel is not running at its operating speed. When the motor
is off there is no air pressure present to activate the pressure switch and in turn there is no
power flow to the Ignition Control System.
To make a specific test of the air pressure interlock switch circuit:
1. Turn burner power OFF.
2. Turn Manual Gas Cock OFF.
3. Disconnect the motor wire from the terminal strip to keep the motor off.
4. Turn burner power ON and set the operating control to ON or thermostat to call for
heat. Verify there is 120VAC line voltage by using a multimeter. Check for 24V
between the Ignition Control Module 24V and 24V GND terminals.
A. No voltage: Interlock circuit OK.
B. Voltage present: check that the switch is wired properly or check switch
operation. Replace if switch tests bad.
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Should replacement or service be required, valve manufacturer's instructions must be
followed as outlined in their information sheet.
Outlet pressure settings must be checked while the gas is flowing.
To adjust outlet pressure, remove the seal cap for access to the adjusting screw. Turning
the screw clockwise will increase outlet pressure, counter clockwise will decrease outlet
pressure.
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Prepurge S8670J
When power is applied to L1 and L2 through a safety and operating control, the
combustion air blower starts, once the air proving switch closes and proves blower
operation, energizes the S8670J, and initiates a 30 second delay to allow system prepurge.
After a 30 second prepurge, the 15 second trial for pilot ignition sequence begins.
Trial for Pilot Ignition
On a “call for heat” and after 30 second prepurge, the module energizes the spark source
and the pilot valve relay simultaneously. The pilot valve opens, allowing gas to flow to the
pilot burner during the 15 second ignition trial time. The spark lights the pilot flame when
pilot gas is present. A flame rectification circuit confirms the presence of the pilot flame,
shuts off the spark source, and energizes the main valve relay.
Main Burner Operation
When the main valve opens, gas flows to the main burner where it is lit by the pilot flame.
There is a 2 second flame stabilization period as the main valve opens. The system is now
in the run mode with the presence of the pilot flame continuously monitored by the flame
rectification circuit. If the pilot flame goes out, or pilot flame signal drops below 1.1micro
amps, the control senses loss of pilot flame and shuts off both the pilot valve relay and the
main valve relay.
The control has two LEDs; one for flame sensing and one for system status:
. Flame LED (Yellow)
. Indicates flame presence and strength.
. Status LED (Green)
. Indicates system operation status and error conditions.
FAILED TRIAL FOR PILOT IGNITION S8670J
If pilot flame is not lit and sensed before the end of the15 second trial for ignition time, the
control shuts off the pilot valve (100% shutoff) and goes to lockout. Or if proof of flame is
lost during burner run cycle the system will retry once, if pilot flame is not proven within the
15 second trial for ignition time, the control shuts off the pilot valve relay and lockout. The
control remains in lockout until power is cycled by the system thermostat or removing and
restoring system power.
For operation characteristics, maintenance, service procedures, and LED codes refer to
manufacturer’s literature provided.
WARNING: Explosion hazard. Do not use any electronic device if it gets
wet. It can malfunction and cause serious injury or death. Replace any device that
has been wet.
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XI Valve Train
XII Sequence of
Operation
Part 2
Service
Continued
13
X Motor Blower
Interlock
Continued