The corroded heat exchanger and a few other worn parts have been replaced. My wallet is lighter but the furnace is working again. Not that I can relax. Winter is four months long here and from December to February we can expect periods of below zero weather that might last for more than a week. But the furnace works again, the propane tank is full, and there is a nice pile of wood right inside the door just in case. And some matches and old newspaper, too.