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High Efficiency gas furnace - return air temperature
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. |
High Efficiency gas furnace - return air temperature
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High Efficiency gas furnace - return air temperature
On Fri, 19 Jan 2007 05:59:45 GMT, Tony Hwang wrote:
wrote: 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. 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. Blow me. -- Click here every day to feed an animal that needs you today !!! http://www.theanimalrescuesite.com/ Paul ( pjm @ pobox . com ) - remove spaces to email me 'Some days, it's just not worth chewing through the restraints.' 'With sufficient thrust, pigs fly just fine.' HVAC/R program for Palm PDA's Free demo now available online http://pmilligan.net/palm/ |
High Efficiency gas furnace - return air temperature
well lewts say the home is 40 degrees, and moisture is accumulating.
pretty quick the temp should rise, and the moisture go away perhaps he should call the manufacturer? long term would more likely be a problem |
High Efficiency gas furnace - return air temperature
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Tony hunglow guessing - who wudda thunk?
Tony Hwang posted for all of us...
I am just guessing. 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. |
High Efficiency gas furnace - return air temperature
On 18 Jan 2007 21:12:17 -0800, wrote:
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. Now are you reading that right? Is that a statement of general operation, or an actual requirement? tom @ www.YourMoneySavingTips.com |
High Efficiency gas furnace - return air temperature
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. |
High Efficiency gas furnace - return air temperature
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 "Redcrosse" wrote in message ups.com... 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. |
High Efficiency gas furnace - return air temperature
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: 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 "Redcrosse" wrote in message ups.com... 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. |
High Efficiency gas furnace - return air temperature
On 22 Jan 2007 06:23:03 -0800, "Edward R. Voytovich"
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, 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 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 "Redcrosse" wrote in message ups.com... 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. |
High Efficiency gas furnace - return air temperature
mm wrote: On 22 Jan 2007 06:23:03 -0800, "Edward R. Voytovich" 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, 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. 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 "Redcrosse" wrote in message ups.com... 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. |
High Efficiency gas furnace - return air temperature
wrote:
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 |
High Efficiency gas furnace - return air temperature
wrote: wrote: 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...mo/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...ate-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. |
High Efficiency gas furnace - return air temperature
and runs out as waste water
"Bubba" wrote in message ... n Fri, 19 Jan 2007 05:59:45 GMT, Tony Hwang wrote: wrote: 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. 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. 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 |
High Efficiency gas furnace - return air temperature
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High Efficiency gas furnace - return air temperature
Dave Martindale wrote:
... 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? Nick |
High Efficiency gas furnace - return air temperature
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High Efficiency gas furnace - return air temperature
wrote:
wrote: Dave Martindale wrote: ... 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... It's 300-year-old physics :-) What's the answer to this simple problem? Nick |
High Efficiency gas furnace - return air temperature
wrote in message oups.com... wrote: Dave Martindale wrote: 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? 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 clothing. It went from researching golf ball dimples to mother natures design of a Tuna! 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. -zero |
High Efficiency gas furnace - return air temperature
wrote:
... 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... It's 300-year-old physics :-) What's the answer to this simple problem? Still no clue? Rewrite the steel conductivity as 50 Btu-ft/h-ft^2-F... Nick |
High Efficiency gas furnace - return air temperature
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 of them. Cheers. On Jan 18, 10:12 pm, wrote: 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. |
High Efficiency gas furnace - return air temperature
wrote:
... 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... It's 300-year-old physics :-) What's the answer to this simple problem? Still no clue? Rewrite the steel conductivity as 50 Btu-ft/h-ft^2-F... Still stuck? Try 600 Btu-inch/h-ft^2-F. This is an extremely simple heatflow problem :-) Nick |
High Efficiency gas furnace - return air temp
wrote:
... 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... It's 300-year-old physics :-) What's the answer to this simple problem? Still no clue? Rewrite the steel conductivity as 50 Btu-ft/h-ft^2-F... Still stuck? Try 600 Btu-inch/h-ft^2-F. This is an extremely simple heatflow problem :-) Another clue: the thick steel conductance is 600/0.050 = 12,000 Btu/h-F-ft^2. Beginning to understand the basics yet? :-) Nick |
High Efficiency gas furnace...
wrote:
... 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... It's 300-year-old physics :-) What's the answer to this simple problem? Still no clue? Rewrite the steel conductivity as 50 Btu-ft/h-ft^2-F... Still stuck? Try 600 Btu-inch/h-ft^2-F. This is an extremely simple heatflow problem :-) Another clue: the thick steel conductance is 600/0.050 = 12,000 Btu/h-F-ft^2. And if we halve the thickness, it becomes 24,000 Btu/h-F-ft^2. Wow! Got a clue yet? :-) Nick |
High Efficiency gas furnace..
wrote:
... 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... It's 300-year-old physics :-) What's the answer to this simple problem? Still no clue? Rewrite the steel conductivity as 50 Btu-ft/h-ft^2-F... Still stuck? Try 600 Btu-inch/h-ft^2-F. Another clue: the thick steel conductance is 600/0.050 = 12,000 Btu/h-F-ft^2. And if we halve the thickness, it becomes 24,000 Btu/h-F-ft^2. Wow! So the steel thermal resistances are 1/12K and 1/24K h-F-ft^2/Btu. Now what do we do with resistors in series? Nick |
High Efficiency gas furnace..
wrote:
... 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... ... 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... It's 300-year-old physics :-) What's the answer to this simple problem? Still no clue? Rewrite the steel conductivity as 50 Btu-ft/h-ft^2-F... Still stuck? Try 600 Btu-inch/h-ft^2-F. ... the thick steel conductance is 600/0.050 = 12,000 Btu/h-F-ft^2. And if we halve the thickness, it becomes 24,000 Btu/h-F-ft^2. Wow! So the steel thermal resistances are 1/12K and 1/24K h-F-ft^2/Btu. Now what do we do with resistors in series? 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 :-) Nick |
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