From: Alan Moorman@visi.com
Newsgroups: misc.consumers.house
Subject: Re: 2nd Water Heater
Date: Thu, 14 Dec 2006 11:45:13 -0600

On Tue, 12 Dec 2006 15:33:13 +0000 (UTC), ranck@vt.edu
wrote:

>breaks  wrote:
>> Ok, so the expansion tank serves multiple purposes?  My builder told me
>> that tank was used to maintain pressure in the pipes when multiple
>> faucets were running.  Then I heard from someone else the tank is used
>> to keep pressure in the lines to prevent gray water from contaminating
>> the source....Now I'm understanding the tank is also used to for when
>> heated water expands?
>
>Only that last use is fully correct.  Hence the name "expansion tank."
>When water is heated it expands.  Water, being non-compressible has
>to have room to expand.  Air is compressible, so an air cushion in
>an expansion tank allows the water to compress the air as it expands
>thus preventing leaks and very high pressures.  The reason hot water
>heaters need expansion tanks now is that backflow valves are much
>more common and the expansion can push back against the town water
>supply.  The backflow valves are what is supposed to prevent gray
>water contamination the source, so it's sort of related to the 
>expansion tank, but not really.  As for maintaining pressure,
>that is true for well pump expansion tanks.  Those help provide 
>some pressure on your water system when you first open a faucet 
>and before the pump comes on.  The compressed air expands pushing
>the water in the tank into the plumbing.  When the pressure drops
>to a set point the well pump comes on and keeps the flow going.
>
>I hope all that makes sense.
>
>Bill Ranck
>Blacksburg, Va.

In my metropolitan area, there are neither check valves nor
expansion tanks.



Alan Moorman 

The only reason some people get lost in thought 
is because it's unfamiliar territory.

Paul Fix

=================================