Quality of water has alot to do with it as well as location of machine. A machine in the back room will not grow mold as fast as one in the prep area. Minimum is 2x a year and more often than not on service calls
Local 597 Service Fitter
Metal Trade Journeyman
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It was working when I left...
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depends on your water and air quality. Get one that's easy to clean, which rules out two major brands right away, if you have a choice. Avid Scotsman prodigys like the plague, and I would also avoid Hoshizaki. Ice-O-Matic is cheap, but its lack of computerized controls means that it lacks all but the most rudimentary operational safeties. Manitowoc Indigos are at the other extreme, tending to lock out if you look at them funny.
I serviced an ice machine last week that was installed by me in 2004 and it had never been cleaned, not even by the staff at the cinema where it is installed. Looked pretty good, just minor scale in a few places. The inlet valve was bad. Switched it out and cleaned out the float boot (Hoshi) and left. Owner didn't want it cleaned. It really didn't need it.
We have another account, a doughnut making factory, where the Scotsman they have needs to be cleaned every 6 weeks and it looks like it hasn't been cleaned in 10 years from one visit to the next.
I have noticed that having well-fitting panels around the evap, and SEALING THE EVAP AREA AGAINST AIR FLOW FROM CONDENSER make a huge difference in places where there is yeast in the air.