I purchased a home and when the heat goes on it spells of animal urine. When the heat is not on, the house is fine. It is on the first floor only, and in one particular room. The smell comes up through the heat vent and is very pungent. There is no basement, just a concrete slab so I can't replace the duct work. Any ideas as to how to clean the ducts in this one room to get rid of this odor? Thanks.
CLV

We can guess that formerly an animal was peeing down a vent or near the furnace or both. So is it just one vent that the funk comes up from then it was probably that vent.

I'd reach down there with a well soaked rag with soap and water and scrub it out as far back as you can reach... then even try to spray a deodorant back in there further.

... and maybe someone else has another idea to try too. Good luck with it!

Maybe have the vents professionally cleaned.
Crank the heat when the guy is there so he can smell what your talking about. Maybe he can pin point where it is exactly coming from.

Just stumbled on your question and you may have already solved this problem.

Try this remedy:

Baking Soda
White Vinegar
Dishwashing Detergent
3 % Hydrogen Peroxide


Directions:

If the cat urine has dried and you are not sure where the soiled area is you can use a black light to detect it. In a darkened room, try shining it down the duct to see if you notice any urine markings. Hand held black lights can be purchased for between $15 to $25

2. Do as the former writer, RickJ, suggested and wash down the duct with a paper towel and a solution of 50% white vinegar and 50% water. I'd try attaching the paper towel to a "swiffer" for reachability.

The acidity of the vinegar will neutralize the ammonia in the cat urine.

3. Next, and be sure to wear rubber gloves for this step: Make a paste of baking soda (1/2 cup), hydrogen peroxide (1/4 cup) and dishwashing detergent (Cascade, 1 teaspoon) and smear on the sides of the duct. Scrub the sides of the duct with a long handled brush or swiffer.

The vinegar will neutralize the ammonia and hydrogen peroxide is a powerful oxidizer capable of killing bacteria which cause the urine smells. Baking soda is a well known deodorizer which absorbs odors. If you notice improvement, try it again in a month or so.

Important. Never use ammonia or ammonia-based products. One of the ingredients of urine is ammonia and your cat may well be encouraged to re-offend in the same area if it detects the smell of ammonia. Many household cleaner cleaners contain ammonia so be sure to read the label.