I just replaced an AC for some folks that had a kitchen/attic fire in the past. They mentioned that there was always a light smell of "burning house" coming from the duct work. Now that we have dropped in the new AC, they are complaining that the smoke smell is 10X worse.

I was over there today, and they are correct. When I replaced the AC there was some mild smoke/heat dmg on the plenum, but everything else was in more than reasonable shape (unit was from 1987).

Since we don't do duct cleaning (and the house is two stories w/ inwall ducting, so replacing it would be $$), I need to send them off to the local competition.

Anyone have some advice I should offer them in terms of:
-type/method of cleaning they need?
-Chemical do/don't?