middle of the winter and although they have not paid for a long time I like occupancy to keep the place heated before disrupting things. 

She left but I had to pay to remove garbage/debris she left, I had some cleaning/wall repair/painting to do, she didn’t match the heating fuel, she ran up the water/sewer bill, and she broke the lease so I filed in civil court for under $160 and we will have a hearing on 1/4/19 (she elected to defend against the judgement so I am anxious to see what she has to present).

We had our home inspector and also had our U&O Inspection’s (heat, chimney and electric).

I suspect your condensation problem relates to heat escaping through the old roof and meeting the cold air. 

In winters I am positive that the heat cranked up higher than normal.

I have been living in one of the unit and renting out other 2 units which covers the mortgage and I’m just paying utilities, heat and water bill out of pocket.

For example, if it is a couple middle aged guys then you could let them sweat a bit, but if it is a little old lady that is sensitive to heat exhaustion, I might replace it sooner than later. 

We hardly ever open windows down here in the deep South because of the heat, but if you have a ripped window screen, you will fail the inspection and the property will be disqualified i.e. you don't get paid anymore from Section 8 if you don't fix before the re-inspection.

Would you rather get infront of it, or would it be better to wait and hope it lasts until its warmer to not shut off heat for a day?

@Pierce Conway I just converted a 3 unit building from a single gas boiler (with cast iron radiators) to 3 separate mini-split AC/Heat systems.