25
SERVICE — MAINTENANCE
TROUBLESHOOTING DEFROST CYCLE AND DEFROST HEATERS
Observation
Possible Cause
Corrective Action
Coil Blocked with Frost /
Ice
Excessive moisture infiltration into cool-
er / freezer (fan guard, motor / fan assem-
bly, unit housing, wall / ceiling panels may
also be covered with heavy frost
accumulation)
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
Fan motor out
See motor troubleshooting chart
Defrost cycle not being initiated by timer
Check / replace defective defrost timer
Defrost heater(s) not energized during
defrost
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).
Replace if defective
Box temperature too low (air defrost unit)
Set room thermostat to min +34 degrees
Install off-cycle “air” defrost timer
Install defrost heaters / replace with electric defrost unit
Heater restraining clips broken / missing
(heaters shifted)
Reposition heaters / replace restraining clips
Incomplete Defrost Cycle
(All Heaters Energized)
Excessive moisture infiltration into cool-
er / freezer
See above
Too few defrost cycles / day
Increase number of defrost cycles per day
Defrost cycle too short
(terminates on time)
Set defrost timer to minimum 30 min per cycle
Defrost cycle too short (terminates on
temp)
Check defrost termination control (closes at +55 deg).
Replace defective control
Ice build-up on fan guard,
fan / motor assembly, mo-
tor mount, fan panel (also
ice droplets on ceiling pan-
el in front of fans and be-
hind coil / on upper back
wall and on product. Also
ice on floor around unit
Incoming voltage too high
Check voltage at term block. Maximum 10% of nameplate
Too many defrost cycles / day
Reduce number of defrost cycles / day
Defrost cycle too long
Check defrost termination control (closes at +55 deg)
Replace defective control
Coil not building frost uniformly
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
Fans start immediately following defrost
cycle
Check fan delay control (closes at +25 deg).
Replace defective control
Heater restraining clips broken / missing
(heaters shifted)
Reposition heaters / replace restraining clips
Drain Pan Filled With Ice
Ice build-up on floor
Unit not mounted level
Check / adjust level of unit as required
Insufficient / improper drain line pitch
Pitch drain line a minimum of 1/4" / foot
Drain line too small
Drain line must be no smaller than outlet connection on pan
Drain line blocked / plugged with debris
Clear blockage / manually clear ice build-up from drain pan
P-trap located outdoors
(un-heated in winter)
Re-locate trap / add heat / insulate
Defective drain pan heater
Check heater resistance / replace if defective
Defective / inadequate drain line heater
Replace defective heater / add additional heat if required.
Insulate drain line piping
See detailed instructions for replacing defrost heaters on page 27
