The unit in question is Goodman GMV9509050XBA gas furnace (95% eff.,
It says in the Installation Instructions (page 7, Location Requirements
& Considerations) that the following must be observed:
"The temperature of the return air entering the furnace is between 55F
and 100F when the furnace is heating."
I am curious why the 55F requirement. I mean, when I'm not in the
house, I would like to set the temp as low as possible in order to save
on my heating bill. I think I could otherwise set it as low as 45-50F
and still keep the water pipes from freezing. But I wonder why I'm not
supposed to go below 55F. What could happen? Could the unit get damaged
P.S. I'm in the Denver, CO area - 5,300 ft altitude, if that matters.
So now you're telling us that if the air entering the furnace is too cold,
it can crack the exchanger!
And all this time , I thought furnaces were designed to take cold air and
warm it up!
How many times have you experienced this, Dildo, or is experience the wrong
word to use with you?
I am told by experts in the field that one of several ways that
high-efficiency furnaces squeeze more BTU's from their fuels is by
using thinner and thinner heat exchangers that naturally have tighter
tolerances for expansion and contraction. Old coal fired furnaces had
cast iron heat exchangers that could and did last for a very long time.
Most "cracked heat exchangers" in those appliances seem to have been a
salesman's way of getting a customer to buy . . . not an actual crack.
Modern furnaces with crimped stainless steel heat exchangers are often
projected to have a life of 15 years +/-.
On 22 Jan 2007 06:23:03 -0800, "Edward R. Voytovich"
This seems like one of the false efficiencies, for the most part.
Although it would take a few seconds, even a minute maybe longer to
heat a thick heat exchanger wall, once it was heated all the way
through, it would be just as efficient as a thin one.
Then at the end of the cycle, there would be more heat left over,
which would disperse, some warming the circulating air which would
continue to be circulated by the fan (until the low-limit thermostat
switched the fan off) and the rest would eventually heat the basement
a little bit, or wherever the furnace was. In the case of my
basement, I need a bit of heat there in the winter, and there is a
heating duct, and the furnace radiates is a small amount but probably
needed for my comfort.
If the furnace were in the garage, well one normally goes to a garage
even less than a basement, but doesn;t the whole furnace radiate heat,
not just the rather small amount in even a thick heat exchanger wall.
That simply isn't true. The heat is going to transfer more
effectively across the thinner material. The thickness of the metal
provides a resistance to heat flow, just as thicker insulation, wood,
or anything else would.
But metals are such good conductors that making the metal thinner won't
help much, given high resistance air layers on both sides, and thicker
metal will spread out hot spots and increase efficiency.
Wrong. Making the metal thinner does have a direct and significant
impact on the heat transfer. Here's two references for you:
Theoretical, which from experience is the only type of source you
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 proportional to
the thickness of the heat exchanger.
And second, from an industrial company that acutally makes air to air
In the spec sheet for their product it says:
"Plate thickness ranges from .024" for high efficiency to a heavy-duty
and durable .050" thick plate"
Cearly they agree cutting the thickness in half makes a significant
difference in efficiency.
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?
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