To date, we have been buying mostly Pentair equipment, and loading the Pentair Screenlogic app onto a tablet that stays at the house, so renters can toggle from Pool to Spa mode, and adjust the Spa temperature. 

In localities having a winter design temperature of 32°F (0°C) or lower as shown in Table R301.2(1) of this code, a water, soil or waste pipe shall not be installed outside of a building, in exterior walls, in attics or crawl spaces, or in any other place subjected to freezing temperature unless adequate provision is made to protect it from freezing by insulation or heat or both.

If you are forced to pay for heat for example then you should of course try to make sure the property is well insulated and it might be beneficial to buy a thermostat with a temperature limiter.

I haven't quite looked around yet but just trying to gauge an initial temperature for the Houston market regarding the single family home.

I've thought of adding a temperature sensor (freeze sensing) and maybe a line current sensor on the furnace. 

Even if the actual temperature is below freezing if it isn't too windy the surface temperature of the roof will melt your shingles together quickly once installed.

See what your obligations are as a Landlord and keep an eye of the items that can create a potential "urgent" or "high" situations.For example: In Chicago Landlords are required to provide tenants with a certain temperature inside the units.

I have temperatures for winter, notification times, deductible (they pay the first $100), etc really helps.

The summers get muggy warm but very rarely does it reach temperatures of 90 or higher.

My A/C guy tells me that it is industry standard that an AC unit is expected to reduce the inside temperature by approximately 30 degrees.