Midco DS24A - Manual (Page 10)

Make sure the thermostat and operating controls are calling for heat. Defective wiring or loose
connections can simulate the component defects outlined below. Check associated wiring before
replacing a component.
ELECTRICAL AND FLAME CHECKS MUST BE MADE IN ORDER LISTED.
A. Confirm 120V between strip terminals 1 and 2 and verify the circuit polarity and electrical ground, between
strip terminal 1 and burner chassis metal.
B. Check 24V* operating control circuit;
1. Between left strip terminal T and GND.
a. No voltage, transformer defective.
b. Very low voltage,* circuit overloaded or transformer defective.
2. Between right strip terminal T and GND. No voltage, circuit between T and T is open.
C. Confirm 120V between strip terminal 2 and motor relay terminal 4:
1. No voltage, motor relay is defective.
2. Voltage present, motor is defective.
A. Confirm that both Main Manual Shut-Off Valve and Manual Gas Cock Knob on Combination Gas Valve
are in the ON position.
B. Whenever the burner fails to light during the 6-second trial for ignition, or if the flame is lost during the
burner run and not re-established within 6 seconds (36 seconds in Canada) the Electronic Control will shut
off the Combination Valve and LOCK OUT. To RESET the Control for restart, set the operating control to
OFF or thermostat below room temperature for at least 30 seconds.
1.
Check for 24V* between strip terminal GND and Electronic Control 25V terminal. No voltage, blower
interlock circuit is defective.
2. If Electronic Control has a fuse, test for 24V from each end of fuse to strip terminal GND.
C. For each of the following tests, reset the Electronic Control per step II.B. TESTS ARE VALID ONLY DURING
THE 6 SECOND TRIAL FOR IGNITION.
1.
Turn Manual Gas Cock Knob to OFF. Check for 24V between the Electronic Control 25V
terminal and strip terminal GND. No voltage, defective Electronic Control.
2. Check for 24V* between valve MV terminal and valve body:
a. With voltage, RESET Electronic Control and listen for audible CLICK as valve operators open. No
CLICK, replace valve.
3. Check for ignition spark (spark length approximately 1 / 8 "). Since this is a capacitor discharge system,
observe closely when visually checking the spark as it is faint and thread-like and may be overlooked in
bright light.
a. Between Electronic Control high voltage terminal and strip terminal GND. No spark, defective
Electronic Control.
b. Between Electronic Control high voltage terminal and Spark Electrode Wire (insert head of #8 x
3/4" or longer round head screw into snap terminal inside right angle rubber boot). No spark,
broken wire, cracked insulator, or "spark gap" too wide, see Figure 6.
c. Between spark electrode tip and top of ground rod, see Figure 6. If spark is not visible and/or can
not be heard, remove nozzle assembly (see Section IX Nozzle and Electrodes) and ground it
solidly to burner metal. No spark, cracked insulator and spark traveling to ground along crack.
4. Turn Manual Gas Cock Knob to ON. Connect manometer to the manifold gas pressure tap and
during trial for ignition, check the gas pressure:
a. Pressure should be between 2.0" and 4.0" W.C. for NATURAL and PROPANE and steady. Verify per
Section VII Main Gas Spud that the spud orifice size and manifold pressure are correct.
b. Zero, erratic low or high pressure: confirm that the inlet pressure to the Combination Valve is
between 5" and 14" W.C. for NATURAL and PROPANE during standby and during trial for ignition.
c. If manifold pressure is zero, below 2.0" or above 4.0" W.C. for NATURAL and PROPANE or erratic,
the regulator section of the Combination Valve is defective and the entire valve must be replaced.
A. With motor running check burner line voltage terminals for 120V as follows:
1.
Between strip terminals 1 and 2-120V; voltage OK.
2. Between strip terminals 1 and GND-120V: ground OK.
3. Between strip terminals 2 and GND-"0"V: no backfeed, OK
B. Follow RESET procedure as specified in step II.B.
1.
Check sense electrode position per Figure 6.
2. Check sense wire for continuity.
3. Connect DC microameter in series with Electronic Control SENSE terminal and sensor wire.
With flame on, flame signal should be at least 2 microamps.
A. Low manifold gas pressure.
B. Main gas spud orifice too small.
C. Primary air adjustment open too far.
A. High manifold gas pressure.
B. Main gas spud orifice too large.
C. Primary air adjustment closed too far.
D. Dirty blower wheel.
A. Automatic electric (redundant) valve section of Combination Valve is defective and entire valve must be
replaced.
* Normal low voltage:
Motor running-24V minimum.
Combination Valve energized-21V minimum.
** CAUTION: If changes are made in the Main Gas Spud Orifice size, Manifold Gas
Pressure or Primary Air Adjustment, change the installation data tag accordingly.
10
Trouble Chart
I. Motor Will Not Run
or Motor Runs
in Repeated Cycles.
II. Motor Runs Continuously,
But No Flame.
III. Flame on Only During 6-
Second Trial for Ignition.
IV. Short Flame.**
V. Long Hazy Flame.**
VI. Gas Fails to Shut Off.