I have a customer with a very erratic problem with thier furnace in attic. Its a Lennox G43UF-60C-110-08. The problem happens on only extreme cold days. I think something is freezing or slushing up in the furnace itself, maybe one of the vac. hoses or drain tubes that lead to the trap. There were some problems with the furnace drain that I corrected. They had it draining down an outside wall, uninsulated in some sections, no heat tape, and they used schedule 80 pipe. I thought for sure when we replaced it with schedule 40, rerouted it down an inside wall, reinsulated it and put heat tape on the pipe and trap, that that would solve the problem. But of course it acted up again, ball buster!!
This attic gets very cold. The water in the bucket I used to drain the pipe we replaced actually started to freeze. This is one of those green houses where everything is tight, and very little heat escapes through the ceiling that would help in this case. I think the problem is definately somewhere in the furnace with water freezing. If anyone is familiar with this furnace and this particular problem, I would appreciate some input. The temp here in NJ has gone up a little bit in the last couple of days, and the furnace is working perfect.