I connect it to thermostats, water sensors, door sensors, and more.  

Most likely it is the thermostat.

He asked me about what the issue might be and I suggested thermostat if it hadn't ever been changed (it hadn't).

I chose number 2, and found 2 problems....ground wire fell off the shut off switch plus thermostat brokent.

Does anyone have any recommendations for a thermostat?

Having the smart thermostats are nice because you can keep the tenants from keeping the temps too high in the winter and too low in the summer (we pay the utilities).

NEST thermostat that is web-enabled and capable of learning your daily and weekly (and hourly) settings so it adjusts when you wake, when you leave...wait thats NOW!

There are many ways you can do this by touching up the appearance of the exterior & interior:• A new coat of paint throughout the house (bright colors)• Smart locks• Smart thermostats• A little TLC to the landscaping

If you are in a cold climate, disconnect outside hoses and store them for the winter, cover the outside water spigots, and inform tenants not to use these during the winter.Tune up the heating system, including changing furnace filters and checking the operation of the thermostat.

This house isnt zoned for apartments so there is one electric meter and thermostat.