The controller on your glycol chiller has been setup to operate between 26 and 31 degrees. This provides the optimum efficiency range for the compressor and other components.
In some cases a user will operate their chiller on the lower end of this range, and then set their FTSs controller from fermenting at ale temps in the high 60's or low 70 degree range to 32 degrees to crash their beer. The controller will then recirculate 26 degree glycol endlessly until the set temp is reached.
The problem arises when the coil begins to ice up inside the fermenter. Ice is an insulator, and insulates the beer from the colder glycol inside the chiller coil, and the crash will stall out usually in the high 30's to low 40's.
To overcome this problem you can do one of two things:
1. Raise your glycol temp up to at least 28 degrees. Then depending on ambient temps and other efficiency related factors, you should be able to reach the mid 30 degree range on your beer.
2. "Walk" your beer down to crash temps. Meaning, set the temp 5-10 degrees lower than the current beer temp, then once that temp has been reached, allow a "defrost" cycle to commence, where the pump only runs to maintain the set temp. This intermittent duty cycle will prevent ice from building on the chiller coil.