We have a single HVAC system that heats/cools the entire home.

New floors, appliances, heating/cooling, etc.

The property does have heated radiators through out each unit. everything is separated for each unit electricity, gas, and window units in each property.

Windows would be my next check-point (less than R1) and then wall insulation: I would not be surprised if they have no insulation inside, but you can do blow-in, which only requires a small hole.A home looses heat in two ways: heat conduction (which you can slow down with insulation) and drafts, which in old houses is usually the biggest culprit.

The first house that I bought was 17 years old, 3 br, 1.5 bath, brick ranch house with hardwood floors, Andersen windows, ceramic tile baths, gas heat and a brand new roof. 

yeah the tenant pushing back is my concern. it just so happened that this happened with the tenant that seems to not like paying. and the heating is electric and this winter has been extremely cold which caused the electric bill to jump up to $800!

Redfin shows that properties in this area are at risk of at least Heat and Wind advisories, some also seem to be highly susceptible to flooding.

This is the simplest method, but it also means your tenants are more likely to abuse the utilities by leaving windows open with the heat or A/C running, leaving lights on, ignoring the toilet that constantly flushes on its own, etc.3.

The wrong chemistry, well, see what happens if you put a few drops of brake fluid into dry pool chlorine (actually, do not do this!).

Installing thermostats with regulated heat control (we pay heat) 3.