And is the thermostat on the tank controlling the circulator just like if it was a regular indirect? 

It is probably possible, with the heating arrangement, to separate the three apartments into separately heated zones with their own thermostats -- it may even have been done already.

There're thermostats in every unit when the heat is being paid for by the owner, it could be regulated better.

Once they moved in I got complaint  after complaint first they said the ac wasn’t getting cold I went showed them how to use the thermostat and had the tenent put his hand by the vents.

I’ve looked into putting in smart thermostats but I think tenants will call when they can’t figure them out.

In my county a tenant must have both thermostatic control of their space (meaning the landlady can't have the thermostat in her part of the house).

But with the VRBO renters, it doesn't appear that I have that sort of flexibility to write in the utility contingency.I'm considering putting in a NEST thermostat in the house and a remote pool pump monitor so I can keep an eye on where renters have set the temps?  

What you could do is have it set to a particular temperature with a lock box over the thermostat.  

I would plan to pay for majority of upgrades with a line of credit.I have a few additional reasons beyond monthly cash savings to sub-meter the electric: 1) There is one thermostat in the house, controlled only by tenants in the largest unit.

It is run with wall thermostats.