Tyler ice cream case has slow ice build-up

[rdabs54]

Greetings all, I have a recurring problem with a Tyler frozen food case. (model L5FG5). Here is the set-up: It is one of two connected together, common suction, liquid feed, etc, except for separate thermostat/solenoid controls. Each is a 5 glass door used to display ice cream products. Starting from a fully warm and defrosted condition, the temperatures pull down at approximately the same rate, and the air-flow from the upper vents is approximately 420FPM (just what the "book" asks for). The holding temperature is about 0 to -10F, which is about as cold as it will go no matter where the thermostat is set (the evap temp is about -25F, clue?). All is well for three weeks. Then one unit (always the same one) begins a slow temperature climb over several days, and the airflow is reduced to approx 200 FPM. Ice has formed in the air duct in the back, and eventually down to the coils. Customer usage, I believe, is approximately the same for both units, I replaced defrost termination thermostat, both heaters are good, gaskets are good, fan thermostat operates (not yet replaced), defrosts set twice a day, the weather has been stable here in Northern California (always is). It's a long winded question looking for a short answer. Any ideas would be greatly appreciated.

[install monkey]

more defrosts?

[al]

Do both units have independent defrost termination? It sounds like the fan on the offending unit is either on too early or way too much heat is being generated on this coil leading to steam up the duct.

al

[rdabs54]

Maybe, though Tyler suggests two. Certainly worth a try. I have been reluctant to do that because the matching case works fine, and it has been two defrosts since it was installed 12 years ago. It is possible that things change, like the fin-to-tube heat transfer, insulation quality, etc. Thanks for the reply, and I will consider it.

[rdabs54]

I didn't think of the fan delay, but it makes sense that it could come on too early after defrost. I'll check that. Also, they do share a common defrost timer, but have separate fan delays (and separate terminations for heater contactors only). Probably best to change out all the little bi-metal thermostats to eliminate them as a possibility. Thanks for the opinion.

[rdabs54]

Problem solved... I think. You were on the mark, I believe. Unbeknown to me was that a energy saving "black box" was installed six months ago, about the same time this problem started to occur. It was supposed to read humidity and ambient temperature and control the door heaters accordingly. The installer it wired it to the fan circuit, so the fans cycled more-or-less randomly. Naturally, they were on every time I was there previously. Today I observed the fans switching on and off every few minutes, and after bypassing the box, the fans ran fine. Why the two cases reacted differently to the failed condition is a mystery, perhaps they had significant usage differences. Although I am a fan of saving energy, I have yet to see any of these retrofit solutions do anything other than ramp up repairs costs far more than the energy they save. Just one man's opinion. Thanks for your response earlier.

[al]

A lot of cases cycle the door heaters, great idea but should have no impact on evaporator fans, though i'm open to correction?

al