Hey there Sean, I may be able to shed some light on this subject for you. If you occasionally get "frost" blowing out of the vents, it could be due to a couple of reasons. First off let me explain the conditions that your indoor (evaporator) coil go through. The coil should ALWAYS be "sweating" and never "frosting". Frost on the coil generally is due to a couple reasons. Lack of airflow is the biggest reason. This could be due to a dirty cabin air filter, dirty evaporator coil (if the filter was removed for several years and operated that way), too slow of a blower speed, damper stuck on "MAX AC" where it is constantly recirculating the indoor cabin air. On the refrigeration side of things, frost would be due to a slight UNDERCHARGED condition. If/when this happens you'd likely notice your low pressure cycling switch going nuts (on/off repeatedly every 5 seconds or so). During each "off" cycle, the coil could have a moment to defrost itself. Hope this helps a bit and thanks for bringing up this topic!