Legitimately removing pet urine odor in heating vents?

[MBot]

We own an older home with a brand-new furnace (it was broken and we had replaced with a new high-efficiency one). Two of the vents have a pet urine smell coming from them that persists 2 years after we bought the place.

The wood floors were scratched from large dogs, and we tore out the stairway carpet that stank, but the persistent smell is only from the vents. We think a dog or two took a few chances to urinate in them.

Does anyone have experience with finding a company that legitimately can remove the smell from the ducts/vents? Most duct cleaning I've seen is akin to a fancy shop-vac.  Are there companies or ways to get the smell out at all? If so, what chemicals or process would that entail?

And of course a Mustachian DIY solution would be even better. We have tried spraying an enzyme cleaner (Nature's Miracle Advanced down the vents but it did not significantly change the smell. Plus it's not recommended for that use.

[Another Reader]

One word: rust.  I have cats and this has been a recurring problem.  Two vents have rusted out (house is now around 25 years old).  If your house is typical, the round ductwork fits into a metal box leading to the floor vent.  The duct comes into the box above the bottom and perpendicular to the box.  When urine gets in the vent, it flows to the bottom of the box and rusts it out.  The insulation surrounding the box becomes soaked with urine.  The other problem is urination next to the vent, which will run down the outside of the sheet metal into the insulation and may soak into the subfloor and carpet padding or wood flooring.

Get a bright light and a stick and/or a long screwdriver, remove the louvered vent cover, and start poking around until you find the damage.  You may have multiple problems.  Usually replacing the vent box helps a lot.  Subfloor and carpet padding may have to be treated or replaced.

[Greg]

Another Reader has good advice.  If it were me, I'd consider replacing the vents.  It's just sheet metal and fasteners, not to hard to do. Unless the ducts are hidden in the walls/floor/ceilings of finished space.

If the ducts are below the floor in a crawlspace, the smell could also be from rodents.  They can get into the vents to make nests (and tend to pee constantly) or do the same on top of the duct especially if the ducts are insulated.

[MBot]

Thanks both. The house is 80-85 years old, plaster walls. and very old ducts with likely asbestos around them. We may just have to bite the bullet.