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.

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.

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.

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/

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/

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

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."

Yep, they spec that so that condensation is unlikely
to occur in the main (pri) heat exchanger, which could
cause early failure.

It can become a warranty issue (if you care).

Jim

wrote:
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


Interesting question. One thing I've observed on my gas forced air
system is that if the house gets very cold, when the furnace first
starts up, the blower will come on for a min or two, then shut off
again. This is because the cold air causes the temp in the furnace to
drop enough that the temp switch turns off the blower. Maybe that is
one consideration. But it shouldn't be harmful, just somewhat
wasteful of electricity. In my case, if the house is down at 45, I
just put the blower on manual for awhile, until the house gets up to 55
or so.

Speedy Jim 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."

Yep, they spec that so that condensation is unlikely
to occur in the main (pri) heat exchanger, which could
cause early failure.

It can become a warranty issue (if you care).

Jim

it becoming a warranty issue - how can they _prove_ I ever allowed
the temp to drop below 55F?

wrote:

Speedy Jim 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."

Yep, they spec that so that condensation is unlikely
to occur in the main (pri) heat exchanger, which could
cause early failure.

It can become a warranty issue (if you care).

Jim


it becoming a warranty issue - how can they _prove_ I ever allowed
the temp to drop below 55F?



Well, I don't rep the co., but I imagine they would say something
like: "Your exchanger rusted out....you must have allowed the
inlet air to get below 55 degrees, else it wouldn't happen."

Jim

wrote:

Speedy Jim 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."

Yep, they spec that so that condensation is unlikely
to occur in the main (pri) heat exchanger, which could
cause early failure.

It can become a warranty issue (if you care).

Jim


it becoming a warranty issue - how can they _prove_ I ever allowed
the temp to drop below 55F?

They probably can't prove it. The spec just says the range over which
it was designed to work correctly. It might also be that it is not
designed to drain condensate from the primary. It may also depend
upon the dew point of the combustion intake air.
Ask the manufacturer what will happen.

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.

wrote:

Speedy Jim wrote:

....
Yep, they spec that so that condensation is unlikely
to occur in the main (pri) heat exchanger, which could
cause early failure.

It can become a warranty issue (if you care).

Jim


it becoming a warranty issue - how can they _prove_ I ever allowed
the temp to drop below 55F?


Of course, unless you bought the extended warranty, the heat exchanger
is likely to corrode through after your warranty expires.

M Q wrote:
wrote:

Speedy Jim wrote:

...
Yep, they spec that so that condensation is unlikely
to occur in the main (pri) heat exchanger, which could
cause early failure.

It can become a warranty issue (if you care).

Jim


it becoming a warranty issue - how can they _prove_ I ever allowed
the temp to drop below 55F?


Of course, unless you bought the extended warranty, the heat exchanger
is likely to corrode through after your warranty expires.



I would also think that any concer over the return air temp being below
55 is likely predicated on it being under that temp for very long
periods do to some unusual furnace application not seen in residential
environments. It seems difficult to believe occassional operation
like that for say a vacation home, where it will only run below 55 for
short periods, followed by heating fully to normal temps, is going to
cause problems.

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

wrote:

M Q wrote:

....
Of course, unless you bought the extended warranty, the heat exchanger
is likely to corrode through after your warranty expires.




I would also think that any concer over the return air temp being below
55 is likely predicated on it being under that temp for very long
periods do to some unusual furnace application not seen in residential
environments. It seems difficult to believe occassional operation
like that for say a vacation home, where it will only run below 55 for
short periods, followed by heating fully to normal temps, is going to
cause problems.


In a vacation home, you are likely to be operating it below 55 for
extended periods: as low as possible to keep the costs down,
but above 32 to keep the pipes from freezing.

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.

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.

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.

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.

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.

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

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.

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

writes:

"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
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

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

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?

Nick


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?

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

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

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

On Jan 24, 4:00 am, wrote:
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



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.

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.

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

On Jan 24, 3:57 pm, Bubba wrote:
On 24 Jan 2007 06:55:45 -0800, wrote:

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.Gee, where did I hear that answer about a week ago when you posted
your question?
Oh, thats right. It was from me!

"Trust, but verify"

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

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

On Jan 25, 3:54 pm, wrote:
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- Hide quoted text -- Show quoted text -


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
won't
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:

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 inversely
proportional to
the thickness of the heat exchanger.


And second, from an industrial company that actually makes air to air
heat exchangers:


http://www.anguil.com/downloads/Heat...ate-Anguil.pdf
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?

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

On Jan 26, 5:25 am, wrote:
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?




Try shoving them up your ass and get back to us on how many fit.

wrote:

On Jan 25, 3:54 pm, wrote:

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- Hide quoted text -- Show quoted text -



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
won't
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:

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 inversely
proportional to
the thickness of the heat exchanger.


And second, from an industrial company that actually makes air to air
heat exchangers:


http://www.anguil.com/downloads/Heat...ate-Anguil.pdf
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
across it.

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?

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