High Efficiency gas furnace - return air temperature

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The unit in question is Goodman GMV9509050XBA gas furnace (95% eff., 90,000 BTU.) 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 and why?
Thanks
P.S. I'm in the Denver, CO area - 5,300 ft altitude, if that matters.
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snipped-for-privacy@yahoo.com wrote:

Hi, I am just guessing. If return air temp. is to low it may not produce warm enough air. Air is passing thru the heat exchanger at constant speed and think law of physics.
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Ertttttttt! Wrong answer Tony. Stick to what you do because it sure isnt this. Problem is possibility of condensation in the furnace.......................in the PRIMARY! Bubba
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and runs out as waste water
wrote:

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Tony Hwang posted for all of us...

That is ALL you are capable of; the subject does not matter. Go guess back in a.h.r you might find a sucker there.
--
Tekkie Don\'t bother to thank me, I do this as a public service.

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On 18 Jan 2007 21:12:17 -0800, snipped-for-privacy@yahoo.com wrote:

Now are you reading that right? Is that a statement of general operation, or an actual requirement?
tom @ www.YourMoneySavingTips.com
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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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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

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So now you're telling us that if the air entering the furnace is too cold, it can crack the exchanger! WOW!! 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?
--
Respectfully, Bob

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BOB??? You are to stupid for any one to respond to you postings Dido
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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:

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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.

Oy.
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mm wrote:

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.

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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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snipped-for-privacy@ece.villanova.edu wrote:

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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snipped-for-privacy@optonline.net writes:

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 the house.
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.
    Dave
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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?
Nick
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snipped-for-privacy@ece.villanova.edu wrote:

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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It's 300-year-old physics :-) What's the answer to this simple problem?
Nick
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Still no clue? Rewrite the steel conductivity as 50 Btu-ft/h-ft^2-F...
Nick
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