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What are the major design differences between a 90% and 95% furnace?
How do they squeeze out that extra 5% ?
Professional Member*
Pencil whip it.
AFUE is not the same as thermal efficiency (TE). TE is what really impacts your gas bill.
For example, did you know that doing nothing but changing the temperature that the high limit trips at can gain a manufacturer points on AFUE? There are a lot of other equally meaningless things they can do to get the numbers up.
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Usually a higher fan speed.Originally Posted by 54regcab
Global Moderator
Enlarge the secondary, tweak the inducer and yes, higher fan speed. That's why oversizing a 95% is such a problem, the amount of air those suckers need to move to stay off limit.
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Am std uses same cfm for 90% vs 95%. Lennox actually uses less cfm, they allow up to 80 degree temp rise in some models.
Professional Member
Had this same question and asked Allied Air's tech support and was told that it involved messing with the secondary. As Baldlooney said, you can increase the tube size, or you could also change the number of passes.
Global Moderator
A-S/Trane dropped the BTU capacity of their downflow 95 single stage units. Rheem just went to a big blower. The old TDX80 is now a 65K. Rheem just went from a 3 ton drive 17" to a 4 ton drive 21" on the 75K downflow.
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What isn't really fair to the consumer is that a properly tuned 90% will be just as efficient as a properly tuned 95% furnace +/- 1%.
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Tax rebate.
Bunch if stupid crap that is all fudged numbers on the manufactures behalf. Outdoor sensor can help by lowering the operating temperature (but will make the heat not work correctly for the home owner unless the know how the system operates.), variable speed fans can help a little bit aswell. But for the most part it's all bull****. In reality is the difference of 2% more efficient. 95%'ers usually run at about 92 in the real world
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I don't believe AFUE accounts for blower efficiency.
"The only true wisdom is in knowing you know nothing" Socrates
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How so? Are the secondaries not really different as Baldloonie stated?
Professional Member*
Yes, some of them added a row of tubes to the secondary. If a secondary is more efficient, it can only gain about 1% real world efficiency. Any combustion engineer will tell you that dropping flue temp 30 degrees will increase efficiency by 1%, all other readings being equal.
All condensing furnaces run around 140 or lower flue temp, so there isn't more than about 30-40 degrees to gain. Let's say you could make a miracle furnace that ran 80 flue temp at 70 return temp. That would only be 2% higher efficiency than a 90%er.
Professional Member*
Are there differences between TE and AFUE that could cause the difference between a 90% and 95% AFUE furnace? During the warm-up phase is combustion efficiency reduced much when compared to after the heat exchanger warms up?
Running the higher CFM per BTU as previously stated makes sense trying to get that 140 degree discharge temp down a little more. Could there be some extra capacity gained by increasing the latent heat extraction in the secondary, or is it pretty much maxed out once you get flue gas temps down to 140?
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