Midco EC200 & EC300 - Manual (Page 10)

10
Midco International Inc.
8471 38
- Cleaning of the blower wheel is usually the only service required. Need for cleaning is indicated
if the air inlet of the burner housing shows an accumulation of dust and lint, or if the character of
the fl ame—long, hazy and yellow (sooty)—indicates a defi ciency of air. Motor air cooling vents, if
present, should also be cleaned at this time.
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The 24 volt Combination Gas Valve serves three functions:
1. 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 is factory set for 4.1″ W.C. for the EC 200 and 4.0″ for
the EC 300. Manifold Gas Pressure Tap is located on the outlet end of the Combination Valve body.
-
If pressure adjustment is required to change settings, remove regulator cap for access to slotted
adjustment screw. Turning of adjustment screw counterclockwise reduces pressure; clockwise
increases pressure. Do not adjust past the point where no change in pressure is noted.
Note: Pressure setting can only be made with burner running and gas on.
CAUTION: If the gas supply pressure is below its specifi ed range during adjustment,
an overfi re condition could result when normal pressure returns, particularly if the regulator
adjustment screw is bottomed out. ALWAYS confi rm 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 confi rmed that the inlet gas pressure to the Combination Gas Valve is a steady 14.0″ W.C.
maximum during standby, and a steady 6.0″ W.C. minimum with the fl ame on, the regulator portion
of the gas valve is defective and the entire gas valve must be replaced.
CAUTION: If the Combination Gas Valve has been moved or replaced, perform test
for leaks with the burner running.
CAUTION: If leakage through the gas valve occurs on standby, as evidenced by the
presence of any fl ame, the entire gas valve needs to be replaced.
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- Operation - Power Up/Standby: Upon applying power (24 volts) to 24VAC, the control will reset,
perform a self check routine, initiate full time fl ame sensing, fl ash the diagnostic LED for up to four
seconds, and enter the thermostat scan state.
-
Heat Mode: when a call for heat is received from the thermostat, the control will check the
internal blower motor interlock switch for normally open contracts. The combustion blower is then
energized and once the internal blower motor interlock switch contacts close, a pre-purge delay
begins. Following the pre-purge period the gas valve is energized and spark will commence for the
trial for ignition period.
- When
fl ame is detected during the trial for ignition, spark should shutoff immediately while
the gas valve and combustion blower remain energized. The thermostat, internal blower motor
interlock switch, and main burner fl ame are constantly monitored. When the thermostat is satisfi ed
and the call for heat ends, the main valve is de-energized immediately, the control senses the loss
of fl ame signal and initiates an (optional) post-purge period before de-energizing the combustion
blower.
-
Failure to Light - Lockout: Should the main burner fail to light, or fl ame is not detected during
the trial for ignition period, the control will go into lockout. The valve will be turned off immediately,
and the combustion blower will be turned off following an optional post purge period.
WARNING: Explosion hazard. Can cause serious injury or death. This device can
malfunction if it gets wet. Never try to use a device that has been wet or submerged.
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XIV - Electronic Control
Part 1 Installation
XIII - Combination
Gas Valve
XII - Blower Assembly