I'm in the same debate. will be looking to move possibly within the next 5-10 years. House was built in 82', with the furnace and AC done in 94', furnace is a 100,000btu unit with 80% efficiency rating, my gas bill this winter averaged pretty steady around $120 a month for 1,800sq.ft and will drop down to about $25-30 this summer for just the water heater. looking at a new system, parents just put a new AC/furnace combo in at a cost of around $6,500 I think after rebates from MidAm and the manufacturer, and one of my coworkers is looking at about the same cost for his house. I won't make the investment back in my bill over the amount of time we live here, and it won't be that big of an improvement to the house compared to us adding a 4th bedroom and a deck among other smaller details when we go to sell. the AC is what I am most concerned about over the furnace as it looks in rough shape but it works. right now I just plan to keep both older units until they need to be replaced due to breakdown.