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ULN Res. D Series
Resettable Thermal Switch Continuity Testing
Remove outer jacket door.
Step 1.
Disconnect black wire leads from the resettable
Step 2.
thermal switch.
Using a multimeter capable of measuring
Step 3.
continuity (Ohms), place one probe of the meter
on one of the brass connection tabs of the
resettable thermal switch, and the remaining
probe on the other connection tab.
If continuity is indicated, the switch is closed,
Step 4.
allowing millivolt current to pass.
If continuity is not indicated, the switch is open,
Step 5.
possibly due to an overheating condition. The
switch is designed to open at predetermined
temperatures. An open switch can be reset by
depressing the red colored button located at the
center of the switch. The overheating condition
must be determined prior to putting the water
heater back into service.
Resettable Thermal Switch
Color Code Reference
Approximate switch
Color code
activation temperature
Blue
Yellow
Red
Probable Cause
(open)
240°
270°
Insufficient combustion
290°
1. Weak switch or
switch is out of
calibration.
2. Incorrect switch.
Flammable vapor incident
Probable Cause for
Resettable Thermal Switch Activation
Corrective Action
1. Verify adequate
combustion air supply is
available.
2. Clear jacket slot openings
of any dirt, dust,
restrictions or other
air
obstructions.
3. Inspect flame arrestor
plate and clean with a stiff
brush, compressed air
and/or vacuum to remove
scale deposits and debris.
1. Replace resettable
thermal switch.
2. Verify switch color code
and approximate
temperature.
Replace water heater
35
35
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