Heat pumps work really well and are very efficient until the temperature gets below 25-20 degrees.  

Do you just write in the lease agreement tenant to keep AC unit at a certain temperature ?

Another thing we do is have materials delivered early (something like flooring or trim) and have them sit in the house to get to the same temperature as the inside.

It is simply a thermometer that reads the market temperature. 

What is required  varies by state so the best process is to put the request in writing with documented temperatures that the HVAC is heating to. 

The outdoor temperature during this period was around 55 degrees at coldest.   

I would also time the replacement in the spring when temperatures make it unnecessary to run the HVAC.

Now that we are in February and approaching the warmer spring months (both for temperature and demand), you should see the rentals get snatched up much quicker.

Could be those past bills were estimates and you now have an actual reading that “catches up” on past bills that were too low.Another thing is tenants running extension cords from hallway receptacles to run space heaters inside their units.If you have radiators, you want to keep heat on so that they do not have any potential for freezing up, but you don’t have to keep the temperature higher than say 60F.

You would have to calculate based on square footage, then consider variables like insulation, outdoor temperatures, etc.