This is an interesting way of tackling pet stains in carpet, but I wouldn't recommend it. Cleaning carpets in Tampa FL I run into a lot of customers who have problems with pet stains in the carpet. They ask how to remove pet stains from carpet after it has dried and turned yellow. This one of the most difficult stains to tackle because it happens with regular frequency.
Let me start by explaining why I wouldn't recommend using ammonia, water, and an iron to remove pet stains from carpet. Water is the universal solvent and is an essential part of any carpet clean up but ammonia is carpet friendly. Funny enough ammonia is off gassed from urine as it breaks down over time. Which explains the strong ammonia smell you get from stains in carpet.
The best thing you can do to remove pet urine in carpet is use a lot of water to remove the left over yellowing. When I say a lot of water I mean a gallon or two of water. Using vinegar, ammonia, or heat can easily set the stains and make the odors worse. Water will break down and wash away the urea in the urine and lighten the stain.
If the stain is not removed your best bet is to add some Hydrogen Peroxide to the water. Peroxide will help oxygenate the stain and break it down further. Make sure to test any peroxide mixture in an inconspicuous area of the carpet to make sure the carpet will not be bleached. You will have to let the water and peroxide mixture stand for about 5 minutes before toweling it dry.
Ultimately your best solution is to hire a professional carpet cleaner that has a commercial strength urine remover like we do.