Should I replace a 9 year old furnace?

[mark Ransley]

90% vs 50 = 40% savings, . One gas one is 96.6% , many are 94 and 95%. And a variable speed blower may save 75% on your old units blower KWH cost. Not knowing how much it runs It cant be figured , but it could easily save 15$ a month in electric costs and boost SEER for central air by 1 seer. Plus not pullng inside air for combustion helps. Your unit wasnt designed for efficency. Nothing was then.

[Ermalina]

If H is the amount of heat needed in a given situation, for example, to heat your house this Winter:

Let x = the amount of fuel it takes to produce H by a 50% efficient furnace y = the amount of fuel it takes to produce H by a 90% efficient furnace

Therefore:

.50x = .90y

So, y = (5/9)x = .56x

In other words, a 90% efficient furnace uses only 56% of the fuel used by a 50% efficient furnace to produce the same amount of heat. So, a 90% efficient furnace (THEORETICALLY) should save 44% in fuel. However, I suppose YMMV.

[Ermalina]

BTW, we live in the Midwest (COLD winters) and have a 25 year-old Amana Air Command that is going strong and will not be replaced until it dies or develops serious probs. The folks (a family business which has been around for 3 generations) who maintain it, and who will be doing the installation of the new one which replaces it, agree.

[Stormin Mormonn]

About 40%.

[CBHvac]

Give or take your situation, and usage, between 55%% to 62% less..Installed correctly, depending on your fuel costs not, it could, theoretically, have a tremendous payback. Altho, I dont sell on the payback theory, since you have to pay to get it installed, and it does cost every time you turn it on.

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[CBHvac]

The design, and the final AFUE rating.

Both. Todays furnace isnt the one that you had when you grew up...(well..for some it might still be..LOL) but seriously, huge design changes, computer controls, secondary heat exchangers, better blowers...it all adds up.

[CBHvac]

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What formula you use to get that wrong number?