I certainly don't want to be paying those holding costs.I guess the exit strategy of selling the property is only as good as the temperature of the market.A friend of mine just sold his grandparents house in the same town.

You'll need to turn your HVAC system off, so hope it is moderate temperatures.

Installing multi-zones on a furnace so each apartment can control their own temperature can be challenging and more expensive than with a boiler and hot water.  

Last month, or before the temperature dropped below freezing?

There are charts online that show what are acceptable humidity levels in relation to outside temperatures. 

I have been involved in various areas of construction for over 20 years, with the last 5 being primarily spent as a project manager in the temperature control (HVAC) industry.

I would consult with a property manager in that area, or even check the local listings to test the temperature.

You may even want to lower the temperature or change your adjustment times to provide even greater savings.For many people, the impact they’re having on the environment is an important concern, but there are things that homeowners can do to minimize their impact.

The renter has stated two different low temperatures, neither which were experienced by us, or the HVAC guy.  

It's very common for the temperature to be 10-20 degrees below freezing in Colorado at night in the winter.