The usual way to make headers for copper pipe solar collectors is to use half or 3/8 inch pipe for the risers, which then connect into larger top and bottom manifolds (headers). The headers are usually 3/4 inch or 1 inch diameter. the most common technique for joining the risers to the headers is to drill the header to fit the OD of the riser, and then braze the connection. This makes a good solid connection, and the larger manifold allows several collectors to be hooked together by just connecting manifolds of the 2nd collector to the manifolds of the first collector.
I did not want to get into brazing. My first thought was to use reducing T's with the straight part of the T being 3/4 inch connections for the manifold, and the branch being half inch for the riser. This (I think) is feasible, but the price of these reducing T's is high (about $3 each here compared to $0.85 for half in T's). I decided to just use half inch T's I think this will work fine -- as long as the collector is feed through opposite ends, the flow path from the inlet, then through a riser, then to the outlet is the same for every riser, so the flow should be pretty uniform for each riser.
There is also the issue that collectors with larger manifolds drain back more reliably. But, with the full size half inch risers, and the relatively short manifolds, I do not think that good drainback will be a problem at all. In the shop, it drains back with great gusto.
I don't think its a good idea to connect several of these collectors in parallel with feeds to the manifolds on opposite ends of the parallel string of collectors. I suspect that the flow distribution to the several collectors would not be uniform enough with the small manifolds (but I've not actually tried this).
You also must remember to slop the collector itself toward the lower corner where the supply line from the pump comes in. This will allow the collector to fully empty when the pump shuts down.
Some nice work done by Kris on making collectors and headers ...
Gary September 23, 2008