Midco 400B-02 - 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.
When the pilot gas pressure is in the proper range (see Table 4) lint, dust or corrosion is then
the usual cause for most pilot problems.
The Venturi Casting and Pilot Assembly are part of the drawer assembly which can be
removed as a unit. Remove the two back plate screws, two elbow screws. Disconnect thermo-
couple and pilot tube at Pilot Pressure Tap. Swing away Combination Valve and pull out drawer
assembly.
When servicing, clean the Venturi Casting Ports and Pilot Assembly, including the pilot ori-
fice, and the spark electrode. Also, examine the thermocouple. If there is any serious corrosion
or loss of metal at the tip replace the thermocouple.
Check that the pilot orifice diameter is
correct (see Table 4).
Make sure that the burner tube is prop-
erly positioned in the combustion chamber
entry. It must be set 1" short of the inside
of the combustion chamber as shown in
Figure 1,2, or 7. It must not protrude into
the chamber, because high combustion chamber temperatures will cause premature thermo-
couple and pilot deterioration.
If cleaning does not eliminate the lighting problem, further checks are required.
CAUTION: Do not indiscriminately increase pilot orifice diameter. Pilot trou-
bles are rarely cured in this manner and new troubles may be introduced by
causing the pilot flame to float and lose contact with the thermocouple.
CAUTION: If there is more than a 1.0" W.C. differential between STANDBY
inlet pressure when all appliances are OFF, and the inlet pressure when the
burner and all other appliances are ON, a
1 / 8 inch Maxitrol RV12-Series (or
equal) pressure regulator must be installed in the pilot gas line. With the adjust-
ment screw wide open, set the pilot flame gas pressure at 4.0" W.C. for NATUR-
AL gas, 8.0" W.C. for PROPANE.
1. Standby gas pressure-inches W.C.
2. The efficiency of the thermocouple and the pilotstat safety control can be determined by
timing the safety shut-off. With the burner power switch off and with the combustion chamber
cool, allow the pilot to operate at
least five minutes. Then turn off
the pilot and time the period until
a click is heard from the
Combination Valve. If less than
30 seconds, the thermocouple,
pilot flame adjustment, or the
pilotstat safety control of the
main valve is faulty.
3. An accurate check of the thermocouple and pilot flame adjustment can be made with a mil-
livoltmeter. The thermocouple, when heated by the pilot, should develop at least 15 millivolts
minimum to 25 millivolts maximum open circuit (thermocouple disconnected from valve). If milli-
volt reading is above maximum, readjust pilot gas input slightly to reduce voltage. Check results
with blower motor off and on.
4.
Tests to isolate thermocouple and/or valve pilotstat problems can be conducted under
closed circuit conditions (thermocouple connected to valve through an adaptor that provides for
connecting the millivoltmeter). If the thermocouple does not develop at least 7 millivolts when
the pilot flame is properly
adjusted, it is defective. If the
valve does not hold in with a
7 millivolts, the pilotstat is
defective and entire valve
must be replaced.
5.
If the thermocouple
meets conditions of para-
graph 3 or 4, but timing test
of paragraph 2 remains
under 30 seconds, the pilot-
stat safety control can be
suspected of being defective
and entire valve must be
replaced.
8
Nominal BTU/cu. ft. Value
NATURAL
over 14.0"
below 4.0"
4.0 to 7.0"
7.0 to 14.0"
over 14.0"
below 8.0"
8.0 to 11.0"
11.0 to 14.0"
PROPANE
See Section VI
Set pilot adjustment screw
wide open
Set pilot gas pressure at 6.5"
NATURAL, 7.5" PROPANE
REMARKS
Figure 7: General Assembly
Orifice Diameter
Pilot Gas Pressure
Approximate Capacity .021
3.5-7"
PROP-2500
.012
7.5-11"
1000 BTU/Hr.
NAT-1000
Table 4: Pilot Specifications
IX Venturi Casting
and Pilot
Part 2 Service