The unit that has the dirty filter might have a broken supply air joint or broken ductwork causing the inside to go negative pressure. 

Duct work can be full of smoke or water. 

He was an accountant and wanted to know where every last penny of our budget was going; fair enough.He thought that our numbers for managing of the selective demolition were too high so he decided that this was an easy enough thing to manage on his own so we let him do it and eliminated the $1000 management fee and markup from our contract.His demo guy that he was apparently managing ended up ripping $20k worth of piping and ductwork out of the house that he wasn’t supposed to.Client ended up paying for that all to be replaced, including our mark-up and project management time of the subs (none of the demo work was our responsibility of course, so our hands were clean of this mistake).

@Carlos Pelegrina my advice is always to do what the numbers dictate, installing central heat/air can be expensive especially if you have to expand the electric panel and install duct work.

NEC Article 300.8 states that no raceway can have elements of any other system besides electrical in it; no plumbing, no fuel gas, no ductwork.

@Billy SmithOur office is located in North Kansas City and we have a broad range for work performed that is best described as follows:Various styles of split system replacements and installation with in-house ductwork fabricationComplete roof sweep HVAC RTU replacement projects for school districts with in-house ductwork fabrication 5-105+ Tonnage RTU HVAC installation and replacement for larger commercial buildingsChiller, boiler, cooling tower and custom air handling unit installation and replacement for manufacturing and industrial facilitiesPlan and spec new construction HVAC projects with in-house ductwork fabricationVRF/Mini-Split plan and spec/replacement projectsIn-house ductwork fabricationMedical Marijuana Facility design build HVAC projectsDesign build projectsLarge Refrigeration(no ammonia, no grocery stores)Hope that helps!

Siding of residential home, duct work, electrical panels and dry wall. 

It looked good, and someone I trust checked it out for me - I'm out of state, but being there would not have changed anything, as I wouldn't have been able to identify duct work that's not to code, etc. by sight.

I'm not sure where you are (laws differ from place to place) or what's in your lease... but ripping up three year old destroyed carpet and cleaning pet urine out of the ductwork sounds like textbook usage of a security deposit to me.

It needs a new 5 ton unit downstairs and a new 3 ton unit upstairs, along with a fair amount of replacement and expanded flex ductwork.