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Old 10-17-12, 09:13 AM   #23
Daox
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I've been researching the idea that purging the parallel system would be troublesome because you need enough flow velocity in each parallel branch to move air pockets. This appears to not really be that large of an issue. The typical flow velocity needed to move air is a recommended 2 ft per second. This is the velocity required for inclined piping with water flowing downard trying to push the air down. For horizontal lines that are in a floor, the required flow velocity is actually much less.

According to this paper, the flow velocity for 3/4" HDPE pipe (it is a paper on ground source heat pumps so that is as small of pipe as they tested), the required flow velocity for horizontal piping is only .9 feet per second. I've copied the chart from the paper below, and you can see as you reduce the pipe diameter, the required flow velocity is reduced. For 1/2" piping I'd imagine I only need around .6-.7 feet per second.



Another interesting aspect here is that 50-60 degrees is the actual worst angle for air purging according to the paper.

Anyway, to achieve .7 feet per second in 1/2" pex tubing, you need a flow rate of .4 gpm. Under normal conditions we're flowing .167 gpm. So, we need to boost the flow rate by about 2.5X to purge the air. This doesn't seem like it should be all that hard to do IMO. If you have multiple zones on a home run type system, you should be able to boost any one loop by 2.5X the flow rate.
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