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Walk-In/Electric Defrost Icing Problems

The most common form of defrosting a freezer’s unit cooler is done by using electric heaters embedded in the fin coil and controlling the heaters with a timer.  An electric defrost system is simple, inexpensive, and easy to install.  If not installed properly or set up properly after installation, problems can arise that are often blamed on “poor design” of the unit cooler.  Seldom, if ever, is this the case.

Icing problems, such as icicles forming on the ceiling of a freezer above the unit cooler or around the fan openings of the unit cooler are the result of either too many defrost cycles per day or too long a defrost cycle.  If there is too many defrost cycles per day there will be little frost on the fins.  If there is not much frost, the defrost heaters will quickly melt the frost and the water formed will turn to steam.  The steam rises and condenses on any cold surface, usually the ceiling of the freezer and/or in the fan section of the unit cooler.  A defrost cycle that is too long will also cause steaming and produce the same results.

The normal number of defrost cycles is usually two, maybe three times a day.  The real criterion should be the amount of frost on the fin surfaces of the coil just prior to initiating defrost.  Defrost is only needed when frost has built up enough to impede airflow through the coil.  The length of a defrost cycle is usually 15 to 22 minutes.  Coil manufacturers size the heaters for a coil to supply enough heat so that a fully frosted coil will defrost within the 15 to 22 minute time frame.

Common sense needs to be used when setting the defrost timer for the first time.  After the freezer has been in normal use for three or four days, it should be checked to see if any adjustments need to be made.


A freezer in a restaurant is heavily used from 9:00 A.M. to 9:00 P.M. each day.  People are going in and out three to four times an hour.  The restaurant closes at 9:00 P.M. and the freezer is unused until 9:00 A.M. the next day.  Common sense would tell us that the greatest frost formation would occur from 9:00 A.M. to 9:00 P.M.

It would seem to be a good idea to start the period of heavy usage with a frost-free coil, so a defrost period initiated about 8:30 A.M. would be a good idea.  Frost will accumulate all day, and by 9:00 P.M. it would seem the unit cooler will probably be heavily frosted and require defrosting.  Initiating a defrost cycle at about 9:00 P.M. would seem appropriate.

After operating on this schedule for three or four days, the installer should check the freezer to see if any adjustment to the defrost schedule is needed.  Since each individual box is different in construction, location, load, and usage, the box may need to be checked more than once to determine the optimum defrost schedule.

Demand defrost systems that actually measure frost build up and defrost only when necessary, eliminate coil-steaming problems, and save considerable energy too.  Unfortunately, they are initially more expensive to buy and install than a simple defrost timer.

The Paragon 8145 Series timer is the most used defrost timer.  It is time-initiated, temperature terminated.

This control starts the defrost cycle according to the times set, up to six a day.  There is a minimum of four hours between defrosts.  The duration of defrost fail-safe is adjustable from 4 to 110 minutes in 2 minute increments.

First, the defrost cycle trippers are set on the 24 hour dial.  Next the fail-safe tripper is set on an inner dial.  If the compressor is not brought back on line by an increase in temperature, the fail-safe tripper will terminate defrost.  The fail-safe tripper should not be set for more than 30 minutes.  If the length of a defrost cycle is more than 22 minutes, something is wrong!

To check for too long a defrost time, manually advance the timer to start a defrost cycle and time it.  If the defrost period is over the set period or over 22 minutes, and the fins cleared of frost, chances are one of the following is the problem.

  1. Check to make sure the defrost termination control for the evaporator is wired to terminal “X” on the defrost timer.  If it isn’t, the defrost timer is terminating defrost when it reaches its fail-safe setting, not by temperature.
  2. Another possible cause of long defrost is a defective defrost termination thermostat.  Again, the defrost timer’s fail safe will be the means of termination.  Check the thermostat and if bad, replace it.
  3. Still another reason for too long defrost is the switching solenoid in the timer.  If it is defective, the timer can’t switch to the failsafe mode.

Power outages, obviously, will cause the defrost schedule to change, so it is a good idea to check the time of day dial periodically, and reset it if required.

Many other things can cause unusual coil icing conditions.  If the bottom portion of a coil ices up more than the rest of the coil, it may be due to the lack of a trap in the drain line.  Without a trap, moist air will be sucked through the drain line when the evaporator fans are running.  The excess moisture collects on the bottom of the coil fins.  When in defrost, the bottom of the coil may not completely defrost and thus begins to build excessive ice.  The solution is to install a drain line trap in a heated area outside the freezer.

If refrigerant is not evenly distributed in the coil, it tends to flood the lower circuits and starve the upper circuits.  Most defrost termination thermostats are located in the lower portion or about in the middle of the coil.  Because frost is now much heavier in the lower portion of the coil, it will take longer to defrost.  In the meantime, the upper portion of the coil has cleared and begun to steam.  Correct the cause of unequal refrigerant distribution.

If ice is building up on one end of the coil, the TXV end most likely, the TXV is starving the coil.  Adjust the super heat setting to fully feed the coil.  A starved coil can also be the result of an undercharged system.

These are the most common causes of unusual coil icing conditions.  There can be other causes.  Each job site is different, and may require close scrutiny to discover the solution to the problem.

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