Midco DS45B - Manual (Page 9)

A. No voltage: Interlock circuit OK.
B. Voltage present: check that switch bracket is screwed down tight (see Figure 7).
If so, without disconnecting switch wires, remove switch bracket and manually
depress switch plunger. If voltage is still present, or if the plunger has to be
depressed to where dimension "A" of Figure 7 is less than 9/32" when switch
clicks over, replace the switch.
5. If the switch tests OK, check dimension "A" of Figure 8 as follows:
Switch plunger free...................9/32 to 11/32"
Disk free....................................7/32 to 1/4"
Disk held all the way in.............15/32" min.
6. If plunger dimension is wrong, replace switch. If disk dimension is wrong, check that
the actuator operates freely with a minimum movement of 1/4" from the free position.
If movement is OK, reposition blower wheel on motor shaft. If not, replace wheel.
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The 24 volt combination valve serves three functions:
1) Manual Gas Shut-off, 2) Manifold gas pressure regulation, 3) Automatic electric redun-
dant (double seated) gas valve.
For manual control the Manual Gas Cock Knob is turned full ON or full OFF. The dial has
to be depressed to be turned in one direction, but it depends on the make of the valve
whether it is to the ON or OFF position.
The main gas pressure regulator, which has an outlet pressure setting range of
approximately 2.0" to 4.0" W.C., is factory set for a manifold gas pressure of 3.5" W.C. for
NATURAL. If pressure adjustment is required for setting capacity, remove regulator cap for
access to slotted adjustment screw. Turning of adjustment screw counter-clockwise reduces
pressure; clockwise increases pressure. Do not adjust past the point where no change in
pressure is noted.
NOTE: Pressure setting must be made with burner running and main gas
ON.
CAUTION: When adjusting the regulator, if gas supply pressure is below its
specified range, an overfire condition could result as pressure returns to normal,
particularly if the regulator adjustment screw is bottomed out.
ALWAYS confirm that at least the minimum rated gas pressure is being supplied to the
burner during regulator adjustments, and NEVER bottom out regulator screw.
To set to low outlet pressure for initial startup, turn the adjusting screw counter-clockwise
2.5 turns to raise it to about 1/4" below the top.
The tap for manifold gas pressure measurement is located in the burner spud holder (see
Figure 5 or 8). Remove plug for access.
If the gas pressure regulator fails to maintain a constant manifold gas pressure within +
0.1" W.C., and it is confirmed that the inlet gas pressure to the combination valve is 14.0"
W.C. maximum during standby, and 5.0" W.C. minimum for NATURAL gas (11.0" W.C. for
PROPANE gas), with the main flame on, the regulator portion of the valve is defective and
the entire valve must be replaced.
9
Part 2 Service
Part 2
Service
Continued
Figure 7: Motor, Blower Wheel and Interlock Assembly
XI Combination Gas Valve
IX Blower Assembly
Continued