# Thread: BTU value before or after AFUE?

1. Regular Guest
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Nov 2005
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If I get a furnace rated at 90K BTU and 90% efficiency, will it put out more heat than a 90K BTU unit with 80% efficiency? In other words, is the BTU rating given for the gas used or is it given for the actual heat production?

Any help appreciated as always...

2. Because furnace are tested with one BTU of natural gas and the BTU's in natural gas are different all over the country furnaces don't necessarily put out what they are rated. A 90% should put out more than an 80%. Depending on the gas and who sets it up they could actually put out 20% less than they are rated.

3. Gas furnaces are usually rated in terms of their gross capacity (fuel burned), not the net amount of heat they deliver to the house.

So, a 100,000 btu 80% furnace delivers 80,000 btus of usable heat. (20% wasted)

A 100,000 btu 90% furnace delivers 90,000 btu's of usable heat.(10% wasted)

AFUE is determined in lab conditions, strictly as a means of comparison between models.

(Sorry if I've oversimplified. Somebody will come along later to show the error of my logic.)

4. Regular Guest
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But in general, all other things being equal, could I expect 9/8 more heat out of a 90% furnace vs. an 80% of a similar model with same BTU rating?

5. Originally posted by krusty1
But in general, all other things being equal, could I expect 9/8 more heat out of a 90% furnace vs. an 80% of a similar model with same BTU rating?
I think you can safely make that assumption.

Please be advised, a 90% will cost considerably more than an 80%.
Be sure to calculate ROI (return on investment) when choosing.
Your local contractor can help with the options.
Another source is your utility company.

6. ## eff ratings

The mfrs. stated eff. rating is based upon input vs. output BTUs in a snapshot.
AFUE is a time averaged study which takes into acct. standing pilots, fire up and standby efficiency variances, etc. to a degree. That degree is because this is lab testing and not in vivo. Also, it does not take into acct. the ductwork. You can have a 95% rated furnace that performs considerably worse than an 80% with tight, properly sized and balanced ducts. I'd start with great ducts then the apppliance. Also, insist on hard ducted returns and duct sealing whereever possible.
As Jim and others stated, the grade of test gas can vary from your local supply. However, as the sp. gr. goes up on the gas, so too can the complications from improper combustion. Combustion testing and commissioning of the equipment should be in the contract.
HTH,

7. Well said, hearthman!

8. Yes, and if I may add another note. The AFUE is a matter of wasted btuhs. What you may also consider is a 90+ with combustion air brought in, does not pull its air through the home sending precious conditioned air out the chimney. Consider the CFM of combustion air burned as a reduction in infiltaration air.

9. Might want to read the comments of the homeowner in the post "Any Answers" in regards to his 90% AFUE furnace that isn't saving him anything noticeably over his old furnace.

10. Professional Member
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midwest
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Originally posted by docholiday
Yes, and if I may add another note. The AFUE is a matter of wasted btuhs. What you may also consider is a 90+ with combustion air brought in, does not pull its air through the home sending precious conditioned air out the chimney. Consider the CFM of combustion air burned as a reduction in infiltaration air.

11. Professional Member
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Oct 2005
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## Efficiency

Efficiency is a measure of how well energy is transferred from one form or place to another form or place. Energy output of a device is compared with energy input to the device.

Combustion efficiency: Oil burner is quit high 98-100 percent almost all chemical energy is changed to heat energy.

Steady state efficiency: Effectiveness of the device in extracting the heat from the chemical energy and transferring it to air /water?

The tag ratings are helpful but there is so much more to take into consideration. It is like comparing mileage in a car. You should ask around and find out what are the most popular units being used in your area and then ask why?

Good luck,

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