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parallel plumbed baseboards
I have a customer with a parallel plumbed baseboard heating system. The feed and return pipes are 60ft with 7 loops plumbed in parallel. Initially the farthest zone didn't get circulation, so I replaced the circulator, no change. I then put balancing valves on all 7 loops and achieved flow in the farthest loop, but now there are two baseboards in the middle of the line that do not circulate (they were working before the balance valve were installed). The valves to these two baseboards are wide open, but no heat. All other 5 baseboards are working, both closer and farther on the feed and return lines. When I pump the system with a purging pump, there is heat. Do I just increase the pumping power? Install a series circulator on the return side? It seem like the pressure in the middle is equal on both feed and return, hence no circulation at those points. Any suggestions? Could the circulator be cavitated? HELP PLEASE