Several weeks ago, a very obnoxious odor was detected from around/under our inside HVAC unit, which of course then spreads through the vents & cold air return. I can't decide if it's a sewer smell, or more like a dead rodent, but it's really nasty & gives me headaches. Our house is on a slab (no basement) and we are all electric (so that rules out propane or gas fumes). I recently determined this odor only occurs after running the hot water (not cold). Husband has checked all P traps, drains, clean-outs, toilet seals and ran a snake through vent stack on the roof, but smell still persits. Any suggestions before we spend the money on a plumber? Many thanks! JTS

Dead animals smell unlike sewer gas, which generally smells like rotten eggs, sewage, or decayed food.
Dead animal smell is indescribable. It's worse than any sewer smell. I suspect that you are dealing with one, and it can be anywhere, even way down a vent or in the sewer, but I would start outside.