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TROUBLESHOOTING DEFROST CYCLE AND DEFROST HEATERS
Observation
Coil Blocked with Frost / Ice
Incomplete Defrost Cycle
(All Heaters Energized)
Ice build-up on fan guard,
fan / motor assembly, motor
mount, fan panel (also ice
droplets on ceiling panel in
front of fans and behind
coil / on upper back wall
and on product. Also ice on
floor around unit
Drain Pan Filled With Ice
Ice build-up on floor
SERVICE — MAINTENANCE
Possible Cause
Excessive moisture infiltration into cooler /
freezer (fan guard, motor / fan assembly,
unit housing, wall / ceiling panels may
also be covered with heavy frost
accumulation)
Fan motor out
Defrost cycle not being initiated by timer
Defrost heater(s) not energized during
defrost
Box temperature too low (air defrost unit)
Heater restraining clips broken / missing
(heaters shifted)
Excessive moisture infiltration into cooler /
freezer
Too few defrost cycles / day
Defrost cycle too short
(terminates on time)
Defrost cycle too short (terminates on
temp)
Incoming voltage too high
Too many defrost cycles / day
Defrost cycle too long
Coil not building frost uniformly
Fans start immediately following defrost
cycle
Heater restraining clips broken / missing
(heaters shifted)
Unit not mounted level
Insufficient / improper drain line pitch
Drain line too small
Drain line blocked / plugged with debris
P-trap located outdoors
(un-heated in winter)
Defective drain pan heater
Defective / inadequate drain line heater
See detailed instructions for replacing defrost heaters on page 24
Relocate unit away from doors / other openings
Seal / repair all sources of air leaks into walk-in box
Install strip curtains on access doors
Limit door openings / close door when entering / exiting walk-in
Install door switch (interlock fans and liquid solenoid valve)
Install P-trap in drain line at unit cooler
See motor troubleshooting chart
Check / replace defective defrost timer
Check voltage at term block. Must be +/- 10% of nameplate
Check for loose or burned wires / connections at terminal block.
Repair or replace as necessary
Check heater safety (opens at +70). Replace defective control
Heaters burned out. Check resistance across heater leads.
Replace any heaters reading "open" (infinite resistance)
Check external heater contactor (when supplied).
Set room thermostat to min +34 degrees
Install off-cycle "air" defrost timer
Install defrost heaters / replace with electric defrost unit
Reposition heaters / replace restraining clips
Increase number of defrost cycles per day
Set defrost timer to minimum 30 min per cycle
Check defrost termination control (closes at +55 deg).
Replace defective control
Check voltage at term block. Maximum 10% of nameplate
Reduce number of defrost cycles / day
Check defrost termination control (closes at +55 deg)
Replace defective control
No P-trap installed in drain line at unit cooler. Install trap
Superheat too high. Adjust TXV / lower superheat
Poor refrigerant distribution in coil (liquid temp too low).
Insulate liquid line / increase liquid temp to TXV
Check fan delay control (closes at +25 deg).
Replace defective control
Reposition heaters / replace restraining clips
Check / adjust level of unit as required
Pitch drain line a minimum of 1/4" / foot
Drain line must be no smaller than outlet connection on pan
Clear blockage / manually clear ice build-up from drain pan
Re-locate trap / add heat / insulate
Check heater resistance / replace if defective
Replace defective heater / add additional heat if required.
Insulate drain line piping
22
Corrective Action
Replace if defective
See above
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