It's so easy to get caught up in the heat of the negotiation and to just want to win.

Or the house is advertised as central air/heat. 

I would start by looking at ducted mini splits, heat pump water heaters and induction cooktops.

This was achieved by converting a weird storage room into a "three seasons room", we added additional value by putting in a heat run, making it a 4 seasons room and adding that sqft to the house.

My dad lives in CT (I’m in CA), so I’m not able to drive around searching, and I think he feels out of his element trying to get someone to lend some space for a volunteer opportunity.

Any suggestions as to 1.) how to go about installing more vents or getting more heat/AC (I don’t want window units) 2.) what do do about the mystery intake vent/hole in the floor?

Every decade has it's own points of concern including heat, electrical, plumbing, insulation and foundation.

AC units should have return air also, not just the supply ducts like many heat only systems. 

Thanks All, @Marcus Auerbach I was too heated at the time.

Tenant is responsible for making sure filters in the heating and AC unit are changed regularly; and to make sure heat is not below 60F during the winter.Here's the listing: https://www.realtor.com/reales...