At this time of year (early fall) I fill my collectors with anti-freeze, and generally "fire-up" my heating system.  This year as before I found that the circulation pump that runs the ground floor loop was cavitating.  Because this pump is mounted horizontally an air bubble gets caught in the pump preventing it from working.  I tried to purge the pump by forcing water through it with pressure from a garden hose brought over from the house 100 feet away.  (I don't have a water supply in the workshop.)  No good, it still cavitated.  A few years ago I had changed the orientation of the 2nd floor circulator  and I also added an automatic air vent to eliminate trapped air in the system. 

I decided to re-plumb the manifold for 2 reasons:
1> The pump was horizontal which is not recommended as it will trap air bubbles, so orienting it vertically would allow air to escape and be pumped out the top.
2> The pumps will run cooler if they are on the return side of the loop.  My solar tank/boiler provide 140F and sometimes up to 180F on very good solar days, whereas the return temperature from my 3" thick radiant concrete slab is always 70 to 74F, and the return from my 2 10ft radiators on the second floor is around 120F.  The cooler operating temperatures will extend the life of the Taco circulation pumps I'm told.

Before: Pumps are just after the Aquastar propane back-up heater.  They push water into the heating loops.	After: Pumps pull cooler water up from the heating loops and return water to the storage tanks via the manifold.

As you can see my plumbing is a bit wonky, and I burned the plywood board with the blow torch while removing the old plumbing -- but it holds pressure!   Best of all I no longer have cavitation issues!