The units had combo central air & heating which is not common in this class of rental. 

Added Central A/C Coil to existing gas forced hot air unit.Replaced 60A with 200A service/panelFinished the attic into a large 450SF bonus room.

Added Central A/C Coil to existing gas forced hot air unit.Replaced 60A with 200A service/panelFinished the attic into a large 450SF bonus room.

Add - Central A/C Coil to existing gas forced hot air unit.Replaced 60A with 200A service/panelFinished attic into a large 450SF bonus room.

Add - Central A/C Coil to existing gas forced hot air unit.

Our initial intention was to live there for years and slowly fix it up, as it needs quite a bit of work; think central heat and air, windows, fencing, as well as updates (among other things).

Here's an article on the subject: https://www.washingtonpost.com/lifestyle/home/considering-having-your-air-ducts-cleaned-think-again/2019/01/28/cb5c14b4-1393-11e9-b6ad-9cfd62dbb0a8_story.html

For instance I needed a wall air conditioner picked up and installed while I was on vacation he picked it up and installed it.

That is an $8400 per year increase in gross income, or $84,000 in 10 years and that is how you need to do the math.I don't have to remodel the unit, but it has a 1960's kitchen and bathroom and I like to re-wire the units with an upgraded electric panel, 110-volt smoke detectors and I always install an 18,000 btu air conditioner through a wall (not in a window.

Seems that the new tenants may just be really sensitive to the air in the house.