If not, think about installing landlord friendly thermostats that max out at a certain temperature then drop down over a few hours.

Most experts agree that anything below 120 degrees creates a risk for bacteria to develop inside your water heater from stagnant water, such as legionella that causes Legionnaire’s disease.How to choose a temperature:  https://www.gopreferred.com/blog/what-temperature-should-my-water-heater-be-set-to/

For example if your thermostat is in a cold hallway, you can tell it to use the master bedroom room sensor to set the house temperature.

Do you leave it unlocked so guests can set it at any temperature?

The temperature would be at 50 degrees, I would kick on the heat when I leave my house, and the rental would be warm by the time I arrived.Then one day I was doing some work and noticed the fan was running constantly.

Just a simple unit where you have to be present to push the button and set the temperature will be just fine for them.

I've heard of thermostats that only allow temperatures within a certain range or that have useful Wi-Fi components for managing utility usage.

Hello, I have a 3 unit property, and I pay the heating on the property I do want to upgrade the thermostat before the colder weather which is recommended heat only I have radiant heat I looking for thermostat that I can lock the temperature tenants cannot raise the temperature.

I haven't had a problem despite the area experiencing temperatures down to zero. 

You can create a rule that says if the indoor temperature reaches “x” degrees, then turn the heat on, AC off, etc.