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Posted by HeatPro on August 15, 2005 at 21:48:50:
In Reply to: pex sizing to kompak registers posted by Equinox Builders, LLC on August 15, 2005 at 21:31:19:
1/2-inch pex that has a vapor barrier is OK if the circuit lengths are designed right to carry enough heat for a specified btuh total output on that circuit. Don't use pex without a vapro barrier or the oxygen will pass through the plastic to eat a iron pump or boiler, a copper boiler and bronze pumps won't have a problem with oxygen.
The common circulators used on hydronic home circuits can usually handle a maximum of 25,000 btuh through 1/2-inch tubing up to 130 feet from and to the boiler manifolds. I made my web site to help plumbers design the circuits without making the error of overlong circuits for certain usual pipe sizes:
http://64.226.150.132/hydronic/cir/pum00007.htm
http://64.226.150.132/hydronic/cir/start.htm
So if you have a ten-foot length of baseboard that gives off 600 btuh per foot at 180F degrees, you could put three ten-foot lengths of 3/4 on the loop or five 6-foot lengths. As long as the total feet of baseboard on the 130-foot 1/2-inch tubing doesn't exced 25,000 btuh it is OK.
If you need more heat, you'll need more circuits, never exceeding 25,000 btuh per 1/2-inch 130-foot circuit. It doesn't matter that the baseboard is 3/4-inch or 1/2-inch on the 1/2-inch circuit, the tubing diameter restricts the flow so it can't go fast enough to make noise with a commonly used circulator that makes a maximum of 14-foot head (6 psig).