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.
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.
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
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.
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