I guess the main question is, how efficient is my existing furnace? if it's 80% I won't bother, but it's possible it's not as good as that, in which case it may be worth the change. I'd like to sell the house in a year or so....on the one hand, a new furnace might be an unnecessary expense, but on the other it may be a selling feature.

Thanks for all the feedback, everyone.

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