I think you mean a 2 stage valve with variable speed motor. It will make since to upgrade a little bit. I always but a variable speed motor in. My customers actually feel the difference and told me they actually set the temp down 3 degrees from 71 to 68 degrees in the house. They raise it to 71 and keep the fan going when they have parties. Which is like once every other month. Also with an updated a/c unit it should feel cooler longer...........
Wise :)

Good for you for doing the math.

Let me throw a few issues into your calculations either for you or others that might be reading:

1. Depending on your climate and other factors, such as hot water use, the cost to heat your house is a significant portion of your bill but not the entire gas bill. Be sure you try to isolate the heating portion of the bill.

2. Your gas bill may include a "Delivery and services" charge that is constant, regardless of the amount of gas you use. If that's the case, remove that from your calculation.

3. Although there is some fluctuation now, the trend in energy prices will be up over the life of the furnace.

4. Take into account financing costs and opportunity costs. The money that you are talking about may have better payback if it was used to insulate or seal the duct work, for example.

5. In your case, the insulation in the attic will reduce your total gas bill and the break even will be lower than the $1200 that you estimate.

Bottom line is that it usually doesn't make sense to go from 92% to 95% unless you can capture the tax credits. There can be a comfort benefit with multi-stage furnaces and variable speed blowers.

The 'BUILDER' of a 20 year old home is going to replace
the furnaces for FREE??? WOW!!!!!