The Tenants could be cutting the thermostat down when there not there and turning it up more than a couple degrees at a time.

We buy units that have digital thermostats, and have sufficient cooling capacity for the space, so they work well. 

So we just use the VRBO and AirBNB apps to manage our lake house.We have a Schlage Encode lock, Sensi Smart thermostat and a home monitor.

Even if you do things like install lockable thermostats,  etc people can and will plug in their baseboard heaters if they like it at 90 degrees.     

The previous owner set up the thermostat in the common area of the basement.Now I'll have three flats all with shared heating and one thermostat.

Past history:1 Month ago: Tenant had complained about water not being hot enough and service guy had come and turned up the thermostat.

Make sure they are not running any grow lamps or the neighbors have not tapped into an outlet and buried a cable underground.Make sure they don't leave the doors open too.They might be trying to scam you or put the thermostat too high.

My roommate agreement specified things like: furnished or unfurnished, maximum occupancy- limiting number of overnight guests as it's bills paid roommate living, if it's co-ed or single sex, no smoking vaping e-cigs or substances on the property, cleaning chores/expectations, no underage drinking, pet rules, what utilities are included up to what limits and what utilities aren't included, etc.Require a security deposit from each roommate, even if small.Get a locking thermostat cover; set utility allowances so people won't take advantage of "all bills paid" rooms and have the daughter/master tenant or whoever is paying the utility bills be the person who sets the utilities.

She is now sending me a bill that she is keeping my $1400 deposit due to a thermostat issue, door knob replacement and window damage.  

I read we could add TRVs (Thermostatic Radiator Values) to each radiator so the tenants can control the temperature and we could track how much hot water is flowing through the radiator.