Another thing to consider is you can purchase a thermostat controlled plug/switch that can alleviate anyone having to switch on and off the heat source.

Your county building dept will have physical requirements for a separate dwelling unit (such as thermostatic control, accessibility to the breaker box, basement window egress and fire separation.

You can also add wifi lights and thermostat, allowing you to control the lights and temperature at a distance.Only allow them to enter the premises in your absence after they email you a photo of their driver's license and if it is more than one person, get photos of all of their driver's licenses.

-There is a 1200sqft unit on the main floor and 2x 600sqft units upstairs-There are two furnaces and two thermostats, but both thermostats are on the main floor, and the furnaces/ducting are split side to side. 

Rather than starting a new thread I will tag on to this....I have a couple of rentals with a similar situation as original poster, but they are 2 units, under 1 heating system, controlled by 1 thermostat in the 1st unit.  1 property is a gas furnace1 property is an oil furnace (bills have been painful this winter)Gas Furnace Property- I had an HVAC company check it out, specifically with converting 1 of the units to Mini Split system. 

@Jim Cummings One way of setting up automation for HVAC is to have nest thermostat, get periodic data from it, do some processing based on outside temp and humidity.

First we have to understand that I had the thermostat set for 88 degrees (the outside temperature was 110F). 

@Harv Yergin IV My rentals are as high tech as they get ( video doorbells, smart thermostats, water leak detection etc.,) but smart locks weren't good ROI for me. 

I vaguely remember that I thought there was central air in this one because I had glanced at the thermostat and it had a "cool" option (yeah, they all start to blend together) and that I mentioned this on a couple of the tours.I must have mentioned it when the people who became my tenants took the place, because I just got a text saying the central air isn't working.

If you must include utilities consider taking control of the thermostat away from the tenants and measure their water usage.