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.