Payback? how long will you live there. If you are very lucky you are at
80%. and a 94.5% unit will save 18%. So say you are at 75% a 94.5% unit
will save apx 26% and that is at todays gas prices, in ten years gas
will probably be double


I believe it's (old afue - new afue) / new afue lowering the savings to 15%
and 21%.


http://www.aceee.org/consumerguide/reheat.htm