j j wrote:

so, let's say current furnace is 50% efficient.
let's say we get a 90% efficient one. how much less fuel is that per year?

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

Therefo

..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.