From: "AKS" 
Newsgroups: alt.hvac sci.engr.heat-vent-ac misc.consumers.house alt.home.repair
Subject: Re: High Efficiency gas furnace - return air temperature
Date: Sun, 21 Jan 2007 20:13:21 GMT

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 
news:1169400506.363234.271480@38g2000cwa.googlegroups.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.
>