Posted by hj on February 17, 2004 at 22:46:23:  

In response to Re: Hot water supply sizing 

Still not quite right. Since pi is a constant for areas, it can be discarded for comparison purposes, Therefore for 3/4 it is 3/8 x 3/8 = 9/64 and for 1/2" it is 2/8 x 2/8 = 4/64 so it is actually 2 1/64 larger, using the pipe's nominal, rather than actual interior measurement. : : Run the 1/2", here's why. The cold water held in a 20 ft length of pipe is considerably more in a 3/4" than a 1/2" diameter pipe. There are formulas that I don't have or remember that would compute volume exactly, but the one I do remember should illustrate closely enough what I mean. The area of a circle is equal to the square of the diameter divided by .7854. It figures out to about .08 sq inches to .72 sq inches! That 9 times as much cold water in the 3/4" line that has to be purged before you get hot water. When I replumbed, I though it would be great to run 3/4" pipes for the main supply lines. Well, it's fine on the cold water side, but when I went to use the hot water in the bathroom some 30 feet of pipe away, it took forever to get hot water and many gallons of wasted water. I changed back to 1/2", and now it takes about 12 to 15 seconds, and the 1/2" flow volume is just fine for a few fixtures. : Gary; Check your math. A 3/4" pipe has approx 3 times the volume of a 1/2" pipe.. (not 9 times.) Hube

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