Bill,
It had to be condensation in the air line. The compressor was still running when I noticed the areation hole was frozen over. I had an moisture filter (air drier)on the line coming out of the compressor last year and it still froze up.
Looks like the light bulb idea worked, so far. It heats up the air inside the housing enough to keep the air line open or warms the ground to prevent a freeze up. We'll see if it works this weekend, we are to get some brutely cold weather coming. Highs during the day in the single numbers and lows down to minus 20 below. If the wind is blowing it'll get colder.
The first year I moved to NH, it was 40 below one morning I went ice fishing. It was so cold a liter of soda froze soild in my back pack walking out to the ice shack. When we got there the propane was so cold it wouldn't flow to light the heater. Luckly one of the guys had a cook stove bottle of gas on his person and we heated up the shack with that until we could get the big heater going. I'll never forget that day. I lost feeling in all my fingers for two weeks, and still can't take the cold. I now don't go ice fishing if the weather is below zero.
