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.

You also mentioned Smart locks in your article.I also use Digital Watchdog camera/security system to keep an eye on the property when no one is supposed to be there or to make sure the amount of people isn't double what they said or if they are sneaking in pets without a pet fee.Honeywell 9000 color touchscreen wifi thermostats.

(larger number pad on the thermostat) How about 65%?

We have successfully bought  rundown properties off the MLS,  upgraded them (tile bathrooms, newer fixtures & appliances, new windows,  extra insulation, programmable thermostats, etc.)  

Now I'm not an expert, but the scope of work seems to be something like the following:A-CoilCondensers + concrete pads + copper lines220 electrical ran to said condensers + breakers (my panel does have room in it)Charge system with coolant (needs to be done by licensed professional, understood)Run wire for thermostats (no new thermostats needed - have been upgraded and are compatible)I'm getting quotes upwards of $6k.