Would Wide Solar Collectors Work Better With Horizontal "Risers"?

DIYers will often build collectors that are quite wide (20+ ft) for solar space heating.  The usual approach is to run a manifold along the top and bottom of the collector, and then connect the two manifolds with closely spaced risers that have fins attached to capture the sun's heat  and transfer it into the water flowing through the risers.

It has been suggested by Alan Rushforth that it would save material and might also result in a more uniform water distribution to the risers if they were run horizontally instead of vertically.  You end up with a much smaller number of much longer risers (hisers?).   This reduces manifold material, and reduces the number of manifold to riser joints -- these joints are time consuming and expensive, so reducing the number of them would be a good gain.

In order to push this idea along a little further, I did a test by turning my old prototype copper tube/aluminum fin collector on its side so that the risers run horizontally and the manifolds are vertical. 

In a nutshell, the test appears to show that at least for this small collector, the startup process, flow distribution, and drain back all work without problems -- I think it looks promising. 
What do you think?

All the details on the results here...


This idea has been discussed some in the Yahoo Simply Solar group and the Yahoo Solar Heat if you want to see a bit more on it -- these are both good groups for solar heating questions (and answers :).

Gary