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...

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

Sorta but as with all percent questions it's percent of what.

If you have two furnaces that put out 100,000 BTU/H and one is 85%
efficient while the other is 90% IIRC that means the first is burning fuel
to make 117,647 while the more efficient one is only burning fuel to make
111,111 BTU/H, the difference being what goes up the stack. (100,000
divided by .85 and .9 respectively.) Presuming fuel burned correlates to
input BTU/H:

117,6647 - 111,111 = 6536

6536 / 117,647 = about .056

6536 / 111,111 = about .059

Different points of reference. Compared to the the 85% furnace the 90%
will burn 5.6% less fuel. Compared to the 90% furnace the 85% furnace will
burn 5.9% more fuel.

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...

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?

Because I like race cars, and I'd rather give the money to the HVAC guy
than the propane guy. I'm betting fuel cost continue to rise, and the
payback time will be trimmed. I have less of a problem with a one time
payment than I do with a monthly bill, while both reduce my stash, the
monthly bill interferes with my idea of budgeting.

Because I like race cars, and I'd rather give the money to the HVAC guy
than the propane guy. I'm betting fuel cost continue to rise, and the
payback time will be trimmed. I have less of a problem with a one time
payment than I do with a monthly bill, while both reduce my stash, the
monthly bill interferes with my idea of budgeting.

Agreed. However, unless you are building a new house, there are some
infrastructure requirements that change at the 90% efficiency level. I
started with a 60% AFUE furnace 15 years ago when NG was cheap. When we
replaced the furnace this year, the minimum had risen to 80% AFUE, but
it was just a simple swapout using the same stack and NG piping. At the
90% AFUE and above, some plumbing has to be added (one-time event) to
drain off the acid condensate as a result of increase heat recapture.
The change in technology does produce an additional one-time cost.
I do agree that the cost of fuel is not likely to decrease any time
soon due to the high demand.