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8.5.2 Low Water Cut Off Testing and Probe Inspection
How often?
Test Annually. Inspect self cleaning probe every 5 years, clean if necessary. Replace if
damaged. Regardless of condition, replace the probe every 10 years, and the control every 15
years. Note: These instructions apply to the McDonnell and Miller Series 750 Cut-Off. Refer to
the control
manufacturer’s instructions for more details.
Tools Required:
Standard screwdriver, pliers, 1 3/8” open end wrench or adjustable wrench, soft clean cloth,
safety goggles.
Personally verify that the main power is turned off and locked
out at the circuit breaker when inspecting and servicing.
Procedure - All Models (refer to figures):
1. Wear all safety equipment.
2. Test using the test button according to the instructions provided with the control. Manual
test: Slowly drain the boiler of water. When water drops off the probe, the Green light
will remain "ON". The Red light will turn “ON” and the burner will turn “OFF”, if water is
off the probe. If control malfunctions, replace with new part.
Inspection / cleaning (steps 3
– 11):
3. Turn off and lock out power at breaker. Allow the vessel, control and surrounding area
to cool below 80°F.
4. Remove the low water cut-off control cover screws. Remove the cover.
5. Remove the wing nut and the lead wire from the probe. Loosen the probe screws.
Rotate, and lift the low water cut-off control from probe and set aside.
6. Remove the low water cut-off probe. Probe will turn counter-clockwise.
7. Inspect the low water cut-off probe. The sleeve should not be worn, cracked or
damaged in any way. Probe should not be loose. Replace the probe with a new one if
there is damage.
8. Carefully clean the probe. Sleeve and rod should be free of deposits and scale. Remove
deposits and scale with by wiping with a soft, clean cloth and finish rinsing with water.
Replace the probe with a new one if scale or deposits cannot be removed in this
manner, or if there is damage.
9. Reinstall probe into the boiler vessel. Apply a small amount of pipe dope to the first
external threads and thread the probe into the vessel. Do not use Teflon Tape or
Thread Sealant. If all threads are totally covered, the probe will not have metal to metal
contact, which is necessary for proper operation. Tighten probe to 47 ft-lbs.
10. Reinstall the low water cut-off control. Install in reverse order of disassembly. Replace
cover and tighten screws. Turn power to boiler back on.
