You are attempting to optimize and balance the following:
- Comfort & convenience
- Capital outlay (purchase price)
- Future maintenance costs
- Future operating costs
To do this properly, you need to evaluate 3 separate items
and determine how to spread your capital among the three:
- Furnace unit
- AC unit
- Passive systems
Your passive systems include attic & wall insulation, caulking
(including the attic), attic venting, windows, storm windows, etc.
I just want to point this out, because this part of the equation
is often overlooked.
For example, many homeowners fail to consider issues such as
the fact that the difference in cost between a high efficiency & a
super high efficiency furnace could be more cost-effective if put
into additional attic insulation. Or into storm windows.
This is all especially true since many of the upgrades to passive
systems are easy to do yourself.