I did have one tenant get frustrated with us because we locked the thermostat (put a cap on how low/high he could adjust the temp).

We have thermostatically controlled faucets in our bathrooms.  

I would fix the leaky vents and thermostat and not pay the utility bill.

Originally the purpose was for sensors (humidity/moisture, temperature, electrical consumption, motion), but them we started to add control features (thermostat limits, electronic lock, etc), with the ability to delegate some access to the tenants.While it doesn't increase our rent, it does make looking after our student houses easier and the "coolness" factor does seem to help with word-of-mouth marketing.

I keep one in my bedroom and turn the house thermostat down substantially at night.Since it seems like their bill is MUCH higher than yours was when you lived there, make sure they are turning the thermostat down when they're at work & at night.

Number may be closer to %30   not the %34-%39 shown at the bottom6 plex BUILDING PERMITS   (  $97,200.00  single $16,200.00 UTILITIES $0.00 $500.00 EXCAVATION $24,000.00 $4,000.00 GRAVEL $2,500.00 $416.67 CONCRETE PUMP $1,750.00 $250.00 CONCRETE $18,000.00 $3,000.00 FOUNDATION/CONCRETE-LABOR $9,000.00 $1,500.00 CONCRETE CUTTING $750.00 $125.00 SEWER CONCRETE FLAT WORK $12,000.00 $2,000.00 FRAMING LABOR $21,000.00 $4,947.00 ROOF TRUSSES $14,300.00 $1,831.67 BUILDING MATERIALS $70,000.00 $19,128.40 WINDOWS & SKYLIGHTS $8,500.00 $1,419.38 EXTERIOR DOORS $5,400.00 $1,050.00 ROOF COVER $16,800.00 $2,800.00 GAS FIREPLACE $6,000.00 SIDING LABOR $13,115.00 $2,185.00 GARAGE DOOR $4,914.00 $819.00 GUTTERS $2,400.00 $525.00 PLUMBING $30,000.00 $7,000.00 HEATING $30,000.00 $4,800.00 AIR CONDITIONING $0.00 NEST THERMOSTAT $325.00 GAS BBQ GAS DRYER ELECTRICAL $21,000.00 $4,000.00 MASONRY $3,000.00 $500.00 INSULATION $12,000.00 $2,735.00 SHEET ROCK $21,000.00 $7,100.00 SHEET ROCK GYP BOARD $750.00 INTERIOR TRIM $16,500.00 $2,500.00 TRIM LABOR $12,000.00 $2,500.00 SECURITY SYSTEM $900.00 FINISH HARDWARE $2,400.00 $350.00 UNDERLAY LABOR $1,500.00 $250.00 CABINETS $24,000.00 $4,854.40 COUNTERTOPS $11,100.00 $1,850.00 MANTLE $1,200.00 TILE $9,000.00 $1,597.80 VINYL AND FORMICA $3,000.00 $207.43 HARDWOOD FLOORS $6,600.00 $1,138.10 CARPET $13,000.00 $2,110.10 PAINTING $25,200.00 $5,359.25 WATERPROOF DECK $0.00 $0.00 KITCHEN APPLIANCES $8,400.00 $1,400.00 LIGHTS $3,000.00 $500.00 SHOWER DOOR &MIRRORS $3,000.00 $790.00 FINAL CLEANING $2,400.00 $400.00 PICK UP.

You hate to send someone to fix the heat to find out the tenant doesn't know how to change the battery on the thermostat.... but that is the risk you take. 

I have a wifi thermostat to save electricity when no one is there. 

I'm not sure that central AC is more efficient - the tenant has to cool the whole space based on a temperature from a single thermostat.

We have a nest thermostat,which is awesome because you can limit it to whatever temperature you want.