LAARS Mighty Therm VW-PW (Sizes 175-400) - Installation, Operation and Maintenance Instructions - Manual (Page 29)

Mighty Therm Volume Water Heaters
Page 29
Thermocouple Thermocouple
Adapter
Gas Valve
Millivolt
Meter
Pilot
Figure 41. Testing the pilot thermocouple.
2. Clip one lead of a millivolt meter on the center
post of the tube and the other lead on the copper
pilot thermocouple shield. Turn the gas valve
control knob to the “Pilot” position, depress it,
and light the pilot. You will have to keep the gas
valve control knob depressed throughout this
test.
3. If the millivolt meter reads about 30 millivolts,
go to step 5.
4. If the millivolt meter does not show voltage or
voltage is very low, it could indicate a dirty or
clogged pilot. Clean and recheck the pilot
thermocouple. If voltage still does not show or is
very low, replace the pilot thermocouple.
5. To test the pilot thermocouple under load, screw
the pilot thermocouple into the end of a millivolt
reading adapter (see Figure 41).
6. Clip one lead of a millivolt meter to either side of
the millivolt adapter and clip the other lead to
ground.
7. Light the pilot following the instructions found
on the inside of the heater.
8. Set the temperature control high enough to call
for heat.
9. With the heater firing, take a millivolt reading.
10. If the millivolt meter does not read about 10
millivolts, replace the pilot thermocouple.
5.2.14 Testing for Pilot Burner
(for spark ignition)
When the pilot burner lights, the sparking should
stop and the main burners should fire. If the sparking
continues and the main burners do not fire:
1. Check all wire connections, ignition cable, and
using a mirror, make sure there is a pilot flame.
2. Make sure the wiring agrees with the wiring
diagrams found on the inside of the heater.
3. Check for sparking at the igniter electrode.
4. If there is sparking at the igniter electrode, but
there is no pilot ignition after 10-20 seconds, clip
a lead from the voltmeter between the yellow
wire terminal on the transformer and the orange
wire terminal on the gas valve.
5. If the voltmeter does not show voltage, replace
the ignition control.
6. If the voltmeter reads 24 to 28VAC, but there is
no pilot ignition, check for
a: air in gas lines.
b: restrictions in gas line, valve or pilot tubing.
c: clogged pilot orifice.
If these are correct then replace gas valve (see
Section 4C).
WARNING
Never attempt to repair the gas valve. Such
attempts will void the warranty, and could lead to
dangerous results.
5.2.13 Testing the Pilot Thermocouple
(for standing pilot)
If there is voltage to the gas valve, the pilot is lit,
the thermocouple is properly positioned, the
temperature control is set high enough to call for heat;
then the gas valve or the pilot thermocouple may be
the problem. To test the pilot thermocouple:
1.
Remove the pilot thermocouple fitting from the
gas valve.
Figure 42. Flame sensing current.