circulator on supply or return ?

On a new Weil-McLain WTGO3 boiler servicing three zones (Taco zone valves), should the single circulator be piped in the supply from the boiler to the zones or in return from the zones to the boiler? The current boiler, which is to be replaced, has the circulator in the return line. However, the contractor/supplier suggests pumping away from the boiler. the reasoning for the placement would also be appreciated.Thanks, Mark S.
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http://www.heatinghelp.com/shopcart/product.cfm?category=2-16
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thanks. off I go to the library.

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They probably won't have that particular book at the library.

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Pumping Away??!!
I thought that was a soft porn movie Arnold Schwartzenegger did back in his early days....
;-]
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On Thu, 13 Oct 2005 14:03:48 GMT, "Mark Schofield"

Circulators for residential systems used to be mostly placed in the return side. The reason for this was they would be pumping somewhat cooler water which was easier on them. Modern circulators are usually rated to handle much higher temperature water than they ever see and can be placed on the outlet side of the boiler without shortening their life.
From a hydraulics perspective, you want the inlet of the circulator to be as close as possible to the connection point of the expansion tank. This means at the outlet of the boiler. The short explanation is that placing it there *increases* the pressure in the piping system when the circulator is running, while placing it at the inlet of the boiler *decreases* pressure in the piping system when it is running. Among other things, this makes it easier to vent the system, and entrapped air causes less trouble.
If you want the gory details, look for Siegenthaler's book: "Modern Hydronic Heating", which you may very well find in the library.
HTH,
Paul
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