# Furnace Efficiency

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...
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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.
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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?
wrote:

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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.
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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.
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