I had mice build a nest in one of the heat ducts leading from the furnace to the floor vent. It stunk really bad. The ducts are just super-thin foil with a spiral wire through it. The mice chewed a hole through the foil under the floor (we have a underbelly cover) to get access.
Check the smell at each vent to see if its everywhere or just one line. The mouse urine smell is really bad when heated.
I ended up blowing the nest out with a shop vac. Then I repaired the chewed spot with aluminum duct tape. Finally, I blew lysol through the line with the shop vac to disinfect as much as possible. It still took a long time to get rid of the smell.
Steve
