I have a grandaire central unit, model# WJH33600K000A1, serial #X103194556. In cold weather, 30 degrees and less, the fan cuts off while the unit steams quite a lot. The fan restarts fine, but the blades of the fan, and the cowling around the fan, are icing up. There is a freezing rain here, and I'm worried the auto defrost isn't working properly. I've covered the exhaust on top (partially- it has a six-inch gap to vent) but am concerned. The tech that looked at it last winter (when it did the same thing) said that was normal. Is it?