Midco DS45B - Manual (Page 8)

DANGER: Be sure that the Main Manual Shut-off Valve, Combination Valve
and Burner Power Switch are turned OFF before removing any parts for service.
The venturi casting and electrodes are part of the drawer assembly which can be
removed as a unit. Remove the two back plate screws; then disconnect the elbow and pull
out the drawer assembly.
When servicing, clean venturi casting openings and electrodes. Inspect the wires and
porcelain insulators carefully for hairline cracks which might provide an electrical leak path
that could short out the ignition spark or flame signal.
Examine the electrodes for any serious corrosion or loss of metal at the tip. The
electrode assembly is not adjustable. If the electrodes have been bent out of position, or the
spark gap is too wide (see Figure 6), the assembly is defective and must be replaced. Make
sure that the wires go to the correct electrodes and that the rubber boots are in place.
Make sure that the burner tube is properly positioned in the combustion chamber entry. It
must be set 1" short of the inside face of the combustion chamber as shown in Figures 1,2
or 8.
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NOTE: BEFORE SERVICING, mark with a scribe line or measure opening of primary air
controlling shutter, so that it can be reset to its original position following servicing.
Cleaning of the blower wheel is usually the only service required. Need for cleaning is
indicated if the air cage assembly shows an accumulation of dust and lint, or if the character
of the flame indicates a deficiency of air. Motor cooling air vents should also be cleaned at
this time.
The blower side plate, motor and wheel are removed as an assembly. Disconnect the
motor wires from terminal 2 on the burner terminal strip and 4 on the motor relay.
Disconnect the motor conduit from the control box and remove the side plate screws.
The blower wheel is equipped with a spring loaded centrifugal actuator to operate an
electrical interlock 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, the actuator spring forces the disk
against the switch plunger to push it past its operating point. When running, the actuator
pulls the disk clear of the plunger.
To make a specific test of the interlock circuit:
1. Turn burner power OFF.
2. Turn Manual Gas Cock Knob on Combination Valve to OFF.
3. Disconnect the motor wire from terminal 4 of the motor relay to keep the motor off.
4. Turn burner power ON and set the operating control to ON or thermostat to call for
heat. Check for 24V between the Electronic Control 25V and 25V GND terminals.
8
Part 2 Service
Figure 6: Venturi Casting and Electrode Assembly
X Venturi Casting
and Electrodes
Part 2
Service
IX Blower Assembly