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3/4" heater hose is adequate for a thermosiphon setup.
+1 for Mike's method. His part number for the NAPA gasket was very helpful. A friend with a plasma cutter made a 3/8 steel plate as a spacer.
Mike,Again, good work !We have found a 1/8" weep hole will really help in the t-stat.Makes the system easy to get rid of hidden air pockets.Gary
+1 on the hole(s). I've gotten better performance (less hot/cold) going up in size from 1/8", now use two 3/16" holes on my thermosiphon setup.