That's an interesting definition of "efficiency". In this context, they
must mean that the heat transfer is higher *per unit area* or *per unit
volume* of heat exchanger.
That's unrelated to the efficiency of a furnace, which is a measure of
how much of the theoretical heat energy in the fuel gets transferred to
You can have two furnaces, one with a thin-wall heat exchanger and the
other with a thick-wall heat exchanger that is somewhat larger, such
that both furnaces have the same amount of heat transferred with the
same air and flue gas inlet and exhaust temperatures. Both *furnaces*
will have the same efficiency at heating the house, but the thin-walled
heat exchanger is more "efficient" because it's smaller.
Not much, I'd ween, if the dimensions of a forced air furnace heat exchanger
mostly depend on the air passages. With less metal, it would weigh less and
cost less, but those are different concerns.
And if the metal is a good conductor, eg steel with 50 Btu/h-ft-F, with poor
airfilm conductances on both sides, eg 5 Btu/h-F-ft^2, thinner steel won't
help much. How much, in this case, starting with 0.050" steel?
Heh, don't try to obfuscate the facts by spewing a bunch of calcs as
usual, trying to cover up. Just admit that you were wrong when you
claimed that "making the metal thinner won't help transfer the heat
more effectively." I showed you that:
1 - By the laws of physics, the heat transfered by conduction is
inversely proportional to the thickness of the metal. Despite your
well known love of spewing equations, you just completely ignored the
equation I provided, complete with reference, that says you are wrong.
2 - A manufacturer of air heat exchangers states in their heat
exchanger data sheet that they offer a metal thickness of .024 for high
efficiency applications and an increase to .050 thickeness for
applications where durability is more important.
And what's the crap about poor air film conductance on both sides of a
heat exchanger in a modern high efficiency furnace. If it's so damn
poor, how come these furnaces are 93%+ efficient? Could it be that
manufacturers know how to make heat exchangers that are efficient,
including using thinner metal and proper air flow techniques?
On Jan 24, 4:00 am, email@example.com wrote:
Screw you college boy. You claimed making heat exchangers thinner in
high efficiency furnaces wasn't a significant factor in improving heat
transfer. Actually, it's inversely proportional, per the equation
backed by reference I provided you. Yet you go on spewing, like some
kind of self proclaimed energy expert, chocked full of formulas and
calculations, when you don't even understand the most basic concepts.
On Jan 25, 3:54 pm, firstname.lastname@example.org wrote:
You've established yourself in the group long ago as someone who likes
to try to impress folks by spouting numbers and equations, but having
no common sense when it comes to practical home repair subjects. In
this thread, you claimed I was wrong when I stated that the thickness
of a furnace heat exchanger does directly affect the heat transfer and
efficiency. You posted:
"But metals are such good conductors that making the metal thinner
help much, given high resistance air layers on both sides, and thicker
metal will spread out hot spots and increase efficiency. "
Clearly you are the clueless one, as I provided both physics as well as
practical references that you are wrong:
Conduction is heat transfer by means of molecular agitation within a
material without any motion of the material as a whole. If one end of a
metal rod is at a higher temperature, then energy will be transferred
down the rod toward the colder end because the higher speed particles
will collide with the slower ones with a net transfer of energy to the
slower ones. For heat transfer between two plane surfaces, such as heat
loss through the wall of a house, the rate of conduction heat transfer
Q/t = kA(Thot-Tcold)/d
Q = heat transferred in time = t
k = thermal conductivity of the barrier
A = area
T = temperature
d = thickness of barrier
Clearly from the above, the conducted heat transfer is inversely
the thickness of the heat exchanger.
And second, from an industrial company that actually makes air to air
In the spec sheet for their air heat exchanger product it says:
"Plate thickness ranges from .024" for high efficiency to a heavy-duty
and durable .050" thick plate"
So, just fess up and admit you were wrong, instead of trying to
obfuscate with one liners and leave people with misinformation. It
must be embarrassing to have been caught in such a blatant lack of
knowledge in your self professed field of expertise. I mean, if you
don't realize that thickness of a material directly affects heat
transfer, which you should have learned in basic physics, what good are
any of your other theoretical pontifications?
while I don't have the math to truly follow along, it would seem, while
ther are valid points fer and agin, the manufacturers would not bother
with potential warranty issues if there were not an advantage, but that
the advantage is relatively small, what with the enormous amount of
square feet in the heat exchanger and the large tmeperature differential
To avoid the aforementioned warranty issues, they probably have to make
the heat exchanger out of more corrosion resistant stuff, ie add nickel
or chrome, which I would assume negates the advantage to a point, since
IIRC stainless steel is less efficient a conductor than plain steel.
Anyway, cantcha jus git along?
Add them. So the "less efficient" heat exchanger would have
a thermal resistance of 1/5+1/12K+1/5 = 0.4000833 h-F-ft^2/Btu
vs the "more efficient" 1/5+1/24K+1/5 = 0.4000417 h-F-ft^2/Btu,
with 0.01% less thermal resistance :-)
Scrubbing surfaces for better heat transfer IS the industries
proverbial _Let's Build a Better Mouse Trap_. Combustion efficiency
design and integration is already well established and has many choices
to meat a criteria.
What was interesting to watch is the Discovery Channel's Lance Armstrong
saga. Specifically, detailing the interaction of air to the surface of his
It went from researching golf ball dimples to mother natures design of a
ISTR, dimpled and scaly surfaces were the focal points.
As relating to heat transfer, a couple of years ago there was a program
showing the advances of ancient peoples, and how their levels of
achievement ranked to modern times.
How interesting that a properly hammered Wok was shown to have the best
heat transfer of all other kinds of modern designed woks.
My point is there's room for improvement.
Well, the verdict is in. I called a Goodman authorized HVAC
dealer/contractor (in Denver Metro), and their technician told me that
the reason that return air temp must not be less than 55F is because of
the possibility of excess condensation.
BTW, I first called the Goodman hotline, but they told me that, for
liability reasons, they didn't provide tech support directly to
individuals. However, they told me to call one of their authorized
dealers with any questions. They gave me three names, and I called one
On Jan 18, 10:12 pm, email@example.com wrote:
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