Maybe a smart thermostat or something.Do not let tenants dictate your upgrades - they sound like a PIA, honestly.

Unless you get the utilities separated along with separate heating for each unit, you'll never find a 'fair*' way to distribute costs IF the tenants themselves have access to the thermostat, a/c, hot water etc.

So of course if the tenant set the thermostat to an unachievable temperature it may cause the unit to freeze up.

The thermostat is on the 2nd floor and is set to 72.

Then you meet them at the property, hand them the key and give expectations such as when and how to replace air filters, properly use the thermostat including making sure the fan is on 'auto' and never 'on', where the trash/recycling cans are kept and when to put them at the curb, etc. 

We replaced the thermostats with thermostats that max out at 72 degrees.  

I haven't yet targeted specific commuting professionals, but heard a podcast where the investor advertised offering tech packages to attract young professionals: usb outlets, smart doorbell, lighting and thermostat.

They have claimed their thermostat changes temperature (ex. they set it at 72 before bed and wake up and it's 65 degrees in the morning.) 

@Svend W. the most practical solution to be 99.9% sure is to have a small electric heater on a thermostat.

Boilers don't run continuously--when the thermostat calls for heat, the boiler fires up.