Looking at a graph of average daily temperatures for the area, it looks like your heating season is Oct through April, with Janurary being the coldest.

Tenant informed me that the temperature would not fall below 79 degrees even at night (they have it set at 72).

In my market (Sacramento) and in a few outlying smaller markets, real estate has been pretty hot with prices climbing ever higher, approaching pre-crash levels. A few months ago I was noticing things cooling off a bit...

I am not sure what you mean by bypassed.All three units were installed the same exact way, used the same exact filter, changed the same time, there is a dramatic difference what they look like after 8 months.The only other difference of course is the temperature settings of individual tenants - some like it cooler than others, but other than that the three units are the same in every which way.When I changed filter every month, when I open the closet door to the air handler, the floor is always black in unit 3, the unit with two dogs. 

My question is, have you ever had to weld anything or hire a welder at one of your properties?

Mini Splits are a great option.1) Yes, mini splits heat and cool.2) Mini splits can handle a very wide range of temperature conditions.3-4) Depends on your personal needs.  

I'm new too, but you're way ahead of me, I'm just dipping my toes in and checking out the temperature :). 

This will definitely do well for those two units, but I am curious as to whether or not it will help the heating/temperature on the second flood apartments.

From the investor-buyer end that would be something I would appreciate and most assuredly give repeat business.I hope to find a someone like you when I get into flipping or just in need of a contractor when I buy to hold but the property needs some work.Thank you Daria, I appreciate the feedback... given the dusting of snow on the ground this morning coupled with sub-teen temperatures, someone like me is thinking about moving somewhere like there!

For example, if you have a rental property in a place where temperatures routinely go below freezing in the winter, does your lease include a requirement that the tenant needs to keep the furnace / heat on and at a minimum temperature?