"Jim Redelfs" wrote in message
...
Imagine all the added elbow room if the adjoining water heater was GONE
during
a furnace retrofit. Well, that's what I'm going to be doing (See Downsize
Water Heater ) in
a
little less than two weeks.

My contractor wants to DOWNSIZE to 75k from the 100k size of my
14-year-old,
conventional (spec) gas furnace. Of course, this will be done with a "90"
(or
better) model. He claims that the improved efficiency of the new furnace
will
compensate for the difference. Do you agree?

Ratings are based on input. Assuming a 100,000 Btu unit of 80% efficiency,
20,000 is going up the flue, not into your house. With a 100,000 Btu unit
of 90%, only 10,000 is going away, so you have an extra 10,000 going to heat
your house. Thus, a reduction in capacity will give the same amount of
heat.

Other factors can come into play, this is meant as an example, not exact
calculations or your system.