Even if they do, just get a wifi connected thermostat and home control system and you are set.

You can also install programmable thermostats to try and control things.

Another issue is that the upstairs tenant has control of the thermostat, so if the lower level tenant wants heat, they need to call the upstairs tenant to turn it up or down.  

My maintenance crew visits in the winter to change the furnace filters and explain how to use the thermostat.

I installed smart thermostats in my rentals and they let me monitor humidity in the rental units remotely.

I have a NEST Thermostat, so I can turn down the temperature remotely and even when my long term renters go away, the thermostat knows they are gone and turns down the temperature automatically.

Not to be rude, but I really don't believe you have your thermostat set at 48 degrees.

Dr) 175.00 LOCKS Replace key 5.00 Replace door lock 47.00 Replace passage doorlock 28.00 Replace deadbolt lock 47.00 GENERAL REPAIRS Replace refrigerator shelf 25.00 Replace stove/oven knob 16.00 Repair ceramic tile 150.00 Replace countertop 275.00 Replace cutting board 40.00 Replace kit/bath cabinet knobs 10.00 Replace mirror 45.00 Replace medicine cabinet 85.00 Replace towel bar 22.00 Replace tub/shower enclosure 195.00 Re-grout bath/shower tiles 165.00 Repair porcelain 135.00 Replace thermostat 75.00 Replace fire extinguisher 35.00 Remove junk and debris 250.00 Replace doorbell button 5.00 Replace doorbell unit 50.00 Replace Garage door (each) 525.00 GROUNDS / EXTERIOR Major yard Cleanup 425.00 Minor yard Cleanup 225.00 Mow lawn front and back 50.00 Clean gutters 185.00 Trim bushes 20.00 EXTERMINATING Exterminate for cockroaches 550.00 Exterminate for fleas 375.00 

I keep a few motor and start/run caps at my house, as well as nozzles and electrodes for the oil furnaces, a thermostat, and a spare transformer.

You could always turn the gas on in your name, and then detach their thermostat, and put your own on and lock it at like 40-50 degrees.