Lock the thermostat so tenants do not have ability to change temp settings as well.

Also what's the best cheapest digital thermostat I can buy?

As for the furnace you are best to lock the thermostat.

I've replaced these items from supplyhouse.com, & honestly you're probably better off replacing these items from the get-go & it'll be (relatively) problem free.... especially if you run them a 2-line thermostat to make things easy for the tenants.

While this green-friendly venture is DIY, it will still provide much-needed water when you’re washing the car or watering the plants.Install A Programmable ThermostatOne of the best ways to conserve energy in your home and lower your electricity bill each month is to install a programmable thermostat.

I choose this over Bradford White because of the thermostat.

We stayed in touch with the owners over the months, and after meeting up a couple times and some negotiations we were under contract.I'll keep it simple to the details but let me know if you have any questions.4 bed - 3 bath - 3 car garage - SFHPurchase price: $160K1st Loan: $112,0002nd Loan carried by sellers: $49,000Down payment: $2,500Rehab Cost: $13,000Real Estate fees and closing cost: $8,900Basic list of repairs: carpet, vinyl tile floors, vinyl plank flooring, faucets, toilets, light fixtures, thermostat, interior paint, appliances, gutters, backsplashes, resurfaced counter in kitchen with an epoxy countertop, re-stained all cabinets throughout the house, added pulls to all cabinets, new kitchen sink & faucet - finished it off with some light staging for photos and to help it sell quicker!

(Most costly)2) Put a lockbox on the thermostat.

I would look into locks incorporated into home control systems, so your thermostats, locks, lights, and security are linked. 

The air heat was blowing through the vent at 66 degrees and the thermostat was set at 74.