Posted by Harold Kestenholz - Hydronic Network on December 22, 2000 at 15:26:10:
In Reply to: Hot Water Heat PSI? posted by Scott Stork on December 22, 2000 at 13:18:13:
If the system is the same as in the late 1890'S, it has a gravity expansion tank in the attic or high in a closet at the top floor. The pressure in the system is then determined by the height of the water above the point you are measuring. The intent of the tank was to pressurize the highest radiation enough so air bubbles would vent from the highest vents. This pressure was about 4 psig produced by having the tank water level 9 feet above the highest vent.
As this was the early intent, this did not change in residential systems. The top radiation still requires some small pressure to vent from the top of the system. Many installations now include a 12 psig automatic fill valve (pressure reducer.) this would provide enough pressure to push water as high as (8 times 2.31) 18.5 feet over the fill valve (and still have 4 psig at the highest vent.)
So as long as a modern hot water system requires air to be vented at the top, the minimum static water pressure at the fill valve will be 4 psig plus the pressure required to keep water at the highest level (which depends upon the difference in height between the fill valve and the highest part of the system.
If 12 psig is more than necessary to do the job, it is OK to leave the pressure reducing valve at the 12 psig setting and have extra pressure at the system top.
If the system is pressurized satisfactorily at the pressure reducing fill valve setting of 12 psig when the system is cold and the pressure in the system never exceeds 25 psig (the seating pressure of a 30 psig relief valve) the pressurization and expansion tank are in accord and there is no further cause for concern.
There is further explanation in the free design lessons at:
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