I didn't replace the thermostat, because it was only about a year old at the time... and it failed last week, requiring a Saturday-night trip to replace it.The bathroom had been redone in the past couple of years, so I mostly left it alone. 

You can use a vacuum against the outside to get a little bit of dirt, leaves, bugs, etc out of it, which will help some.If it has a cord and plug - it's not hard-wired - crank down the thermostat so it comes on and stays on for a while. 

Currently the thermostat/utilities all go under unit A but I am going to price out installing a new meter for unit B and getting them on a separate utility plan..

Now we have renters moving into those rooms, (we moved into the low portion of the house) and I am nervous about how I should heat them, I want to add electric wall heaters with thermostats to maintain 72F but I am afraid the tenants will get crazy with the heat and I will be stuck with the bill.

You can also puchase remote locks and thermostats that you can control from your phone.

Like I said earlier it's got an old gravity steam or hot water boiler from the 1940s which works beautifully, and even only has one thermostat in one apartment.

Those specifics and also sprinkler heads, batteries on thermostats, and  unclog a garbage disposal, . 

I had him send me a photo of the thermostat.

If the blower wheel is plugged up full of hair, get an HVAC tech out to clean it.Go around the house to all of the air vents, and make sure the flaps in them are open (use the little handle/lever/knob that sticks out of the grille), and that they don't have a rug or furniture or something blocking them off.Sometimes people will turn the thermostat down and then close the vents to try to regulate the temperature - if this goes on long enough, the evaporator (A-coil, inside coil) can get ice on it, which really blocks the air flow. 

If it is past the lowest number on the thermostat the intention seems to be to turn it off.