Beat me to it. I had the same issue last night at 1 am. Went to the condensate drain opening hit it with a hair dryer and after 5 min it was clear and the furnace kicked on. I knew what it was because it happened to me once last year too. Glad you got it figured out.
I have a normal gas furnace. About the only really interesting thing if I were to go high efficiency furnace is that I would have a spare chimney to maybe use for a wood or pellet furnace.
Don't know why the condensate drain would freeze up? Must be installed in an unconditioned space?? The installer would have to plan for that.
The unit is installed in the attic. The temp was below 0 that night. The condensation pipe drains into the main drain pipe for the bathroom. The condensation pipe running across the attic is wrapped with insulation and heat tape,but there is a 2 inch piece right at the furnace connection that is not wrapped. I am going to wrap it later today as it froze again last night.
