Midco DS24A - Manual (Page 8)

■ ■ ■ ■ Cleaning of the blower wheel is usually the only service required. Need for cleaning is indicated
if the air inlet slots in the burner housing show an accumulation of dust and lint, or if the charac-
ter of the flame—long, hazy and yellow (sooty)—indicates a deficiency of air. Motor air cooling
vents if present, should also be cleaned at this time.
■ ■ ■ ■ If the motor must be replaced, remove the blower wheel first and remove the retainer clips at
the rubber motor grommets, pull the motor out of keyhole brackets. Remount in reverse order,
noting that brass flanges of rubber motor mounts are located against motor. Do not omit the
three clips. Position blower wheel on motor shaft to provide
1 / 8 inch clearance between blower
inlet opening and blower wheel ring.
■ ■ ■ ■ Shelf gaskets are installed on the inside of the back cover and the inside the front of the burn-
er cabinet. Be sure they are in place when re-installing the shelf assembly. Otherwise, blower air
will be lost and combustion could be affected.
The 24 volt Combination Valve serves three functions:
1. Main Manual Gas Shut-off
2. Manifold gas pressure regulation
3. Automatic electric redundant (double seated) gas valve
■ ■ ■ ■ For manual control the Manual Gas Cock Knob is turned full ON or full OFF.
■ ■ ■ ■ The Combination Valve Gas Regulator has an outlet pressure setting range of 2.0" to 4.0"
W.C. and is factory set for 2.0" W.C. Manifold Gas Pressure Tap is located on the outlet end of
the Combination Valve body.
■ ■ ■ ■ If pressure adjustment is required for setting capacity, remove regulator cap for access to slot-
ted adjustment screw. Turning of adjustment screw counterclockwise reduces pressure; clockwise
increases pressure. Do not adjust past point where no change in pressure is noted.
Note: Pressure setting can only be made with burner running and gas on.
CAUTION: If gas supply pressure is below its specified range during adjustment, an
overfire condition could result when pressure returns to normal, particularly if the regu-
lator 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.
■ ■ ■ ■ If the 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 a steady 14.0" W.C.
maximum during standby, and a steady 5.0" W.C. minimum with the flame on, the regulator por-
tion of the valve is defective and the entire valve must be replaced.
CAUTION: If the Combination Valve has been moved or replaced, soap bubble test
for leaks with the burner running.
CAUTION: If leakage through the valve occurs on standby, as evidenced by the pres-
ence of any flame, the entire valve should be replaced.
The electronic control is a 24 volt AC, solid state electronic device that automatically ignites and
monitors the main flame (in Canada, a 30 second prepurge is required). It has an integral high
voltage transformer and upon a call for heat, applies high voltage to the spark electrode and 24
volts to the Combination Valve. When the flame is proven, the spark is terminated and the burner
run continues.
■ ■ ■ ■ If the flame is not proven within 6 seconds (36 seconds for burners with prepurge) or if the
proof of flame is lost during the burner run and is not re-established within 6 seconds (36 sec-
onds for burners with prepurge), the control will shut off the Combination Valve and LOCK OUT.
To RESET the system, set operating control to OFF or thermostat below room temperature for at
least 30 seconds.
■ ■ ■ ■ If the electronic control is changed, the replacement must be identical to the make and model
number, or it must be an authorized substitute.
WARNING: Explosion hazard. Can cause serious injury or death. This device can mal-
function if it gets wet. Never try to use a device that has been wet-replace it.
XI Combination Gas Valve
XII Electronic Control
8
X Blower Shelf Assembly
Continued