Bad smell when running heat

mr_andyj wrote:
dead mouse on your plenum.
I really don't know what a plenum is but assume it is the housing enclosing where the flame is, and gets really hot, and if the mouse died on top of it, the mouse gets hot, doesnt burn, but roast and smells. The smell will be blown through the vents.
Is your smell from the vents?
Eventually the smell will burn itself out, like a scented air freshner stick that has expired.
If it is a mouse then you likely have gross mouse droppings and you need to clean it all out. Prepare to take things apart to get to the dropping. Droppings are a health hazard.

"Dust burn" smell wears off after one on cycle.

"Mouse smell" however is a life time issue that will never go away with out replacing the entire ductwork system and possibly the furnace.

While you may be able to sweep out the droppings, it is the urine they spray everywhere is the long term issue.. Add a little moisture or heat and the smell just comes back.. You have to fully eliminate the urine from every surface that could have been possibly sprayed while they lived in there. Sometimes that requires just replacing everything that they touched like the ductwork.

Yes, that sounds "extreme" but it is the truth and what it takes to fix it. Otherwise the smell just keeps coming back.

Have been there, done this.

Op could try perhaps treat it as a "pet odor" and try one of those "pet odor" sprays designed to clean up pet urine messes on the ductwork but I suspect they will have difficult time reaching every sq inch with that spray..

Per HERE do not use ammonia, vinegar, steam as those can increase the odor.. However other websites seem to contradict this..

Although one website mentions using mix of vinegar and baking soda since urine can contain both acid and alkaline substances..

Most seem to agree on trying pet urine/odor removers for dogs and cats might work.

The biggest downside is gaining access to every square inch of ductwork..