Look at your heating bill and the additional cost of the high-eff furnace. If your heating bill is $1000 per year, you can expect to save $100 or so each year. If the additional cost is $700 to $1000, it may be worth it. If it is less than $600, it is very likely worth it to upgrade on a straight cost/benefit calculation. What you also have to consider is any additional cost you will incur with high-eff furnaces (higher servicing costs and higher parts costs). As well, if the fuel price is likely to increase, it will be more worth your while to chose the high-eff furnace.