Consider the flow rate needed to be between 0.5 and 1L/min/m2 of collector (aperture area, not gross area) being on the higher side for series installs. With 22mm piping and not too long a run, the head loss should not be too much. The issue is guaranteed flow through all tubes or the header and enough positive pressure. Also, make sure you have at least 0.5 bar at the top of the panel (when cold) or winter time flow may be an issue.
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