I've got a REALY old gas furnace - heat only style - it may be 40 years old. Yes, the heat exchanger is cracked, yes it will be replaced sometime soon. I've talked to this forum before about this unit - I was looking at what it would take to run this unit in "Fan Only" mode. The answer was that a "Fan Center" would have to be added. I even got a lead on where to get one. Never followed up on that yet.
Funny thing though... I noticed that the furnace it running - and it is only running the fan! I never noticed this last year as I has it unplugged throughout the summer. It seems that now on a hot day it will kick on by itself. It only has two wires to the thermostat, and it is turned off while it is running.
I'm thinking that it is hot enough up there to fool it into thinking that gas is heating up the exchanger. Could this be a feature? How hot does it have to be to trip a temprature sensor normally?