Water and Polypropylene has about the same degree of thermal expansion
so I don't believe that is the reason. Although it hasn't really been
discussed by the TC, the stated premise on SMALL antifreeze systems is
that there is no trapped air. Conversely, we expect a lot of trapped
air in a standard wet pipe system. Having said that, the corrosion
driven efforts to reduce trapped air has NOT been correlated with the
expansion concerns. Somewhere along the way, the two opposing efforts
will bump heads.
Roland
On Jul 31, 2012, at 7:55 AM, Brad Casterline wrote:
the reason we do not need expansion tanks on wet systems is due to
water's
low coefficient of thermal expansion, but since it is not zero, and
assumed
potable, we use a 1/2" relief valve. why on grids but not trees?,
(until
13-2013)-- because with an aux drain on the secondary main, most of
the air
(cushion) can be purged at system charge or recharge. now with the
emphasis
on getting the air out, best to use a relief valve on trees too.
this is my
understanding-- i might be mistaken.
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