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.