Discounting OTHER losses, that's probably an adequate understanding.

Let's go to automobile fuel economy. There are TWO numbers - one for
city driving and one for highway driving. Well, it's the SAME engine
and once it's fully warmed up and driven at a steady speed, it has the
same fuel economy, city OR highway.

Where's the difference? Braking, shifting gears and idling at traffic
signals lower the fuel economy.
How does that relate to a furnace? Well, it does not run all the time
and we don't get the heat directly from the fuel source. A heat
exchanger must be heated before it warms the air; once the thermostat
reaches set temperature, that heat in the exchanger is lost when the
fan is no longer running.

The 90% and 95% AFUE's achieve high efficiency by capturing exhaust
heat that simply goes up the stack in the 80% to 85% units. Of course
that recapture equipment and the acid moisture that results make the
initial unit cost increase.

Here in Texas where it's winter for 2 weeks per year, the 80% unit is
cost-effective once the price of the furnace is considered over its
useful life (before it must be replaced). Why buy a race car if the
trip is typically less than a mile and I only go once per week?


On Wed, 30 Aug 2006 23:32:38 -0500, Al Bundy
wrote:
How does the efficiency rating relate to fuel usage?

I mean if one furnace is rated at 85% and another at 90%, does it take 5%
more fuel to heat the same?

Thanks,
Al...