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Posted by Edward R. Voytovich on January 22, 2007, 9:23 am
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 +/-.
AKS wrote:
show/hide quoted text
> Any metal will expand and contract with change of temperature
> but I do not believe that this problem occurs in low temp.
> residential furnaces however anything is possible
> Dido
> > It is my understanding that if the return air temperature is too low
> > the
> > heat exchanger can be "shocked"--that is to say it may expand and
> > contract
> > beyond design limits and fail.
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Posted by mm on January 22, 2007, 3:20 pm
On 22 Jan 2007 06:23:03 -0800, "Edward R. Voytovich"
show/hide quoted text
>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
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.
show/hide quoted text
> 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 +/-.
Oy.
show/hide quoted text
>AKS wrote:
>> Any metal will expand and contract with change of temperature
>> but I do not believe that this problem occurs in low temp.
>> residential furnaces however anything is possible
>> Dido
>> > It is my understanding that if the return air temperature is too low
>> > the
>> > heat exchanger can be "shocked"--that is to say it may expand and
>> > contract
>> > beyond design limits and fail.
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Posted by trader4 on January 22, 2007, 8:06 pm
mm wrote:
show/hide quoted text
> On 22 Jan 2007 06:23:03 -0800, "Edward R. Voytovich"
> >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
> 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.
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.
show/hide quoted text
> 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 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 +/-.
> Oy.
> >AKS wrote:
> >> Any metal will expand and contract with change of temperature
> >> but I do not believe that this problem occurs in low temp.
> >> residential furnaces however anything is possible
> >> Dido
> >> > It is my understanding that if the return air temperature is too low
> >> > the
> >> > heat exchanger can be "shocked"--that is to say it may expand and
> >> > contract
> >> > beyond design limits and fail.
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Posted by nicksanspam on January 23, 2007, 3:38 am
>mm wrote:
>> >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
>> This seems like one of the false efficiencies...
>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.
Nick
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Posted by trader4 on January 23, 2007, 9:22 am
nicksanspam@ece.villanova.edu wrote:
show/hide quoted text
> >mm wrote:
> >> >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
> >> This seems like one of the false efficiencies...
> >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.
> Nick
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
recognize:
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/heatra.html
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
is:
Calculation
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
heat exchangers:
http://www.anguil.com/downloads/Heat-Exchanger-Plate-Anguil.pdf
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.
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> but I do not believe that this problem occurs in low temp.
> residential furnaces however anything is possible
> Dido
> > It is my understanding that if the return air temperature is too low
> > the
> > heat exchanger can be "shocked"--that is to say it may expand and
> > contract
> > beyond design limits and fail.