primary/secondary system pump

Paul Nebrasky

Recently, we installed a new, "engineered" primary/secondary system. The system includes 5 Teledyne Endurance boilers with multiple pumped zones. The engineer designed a "back up" primary pump into the system. The primary pump(s) are Taco 121's. The zone pumps vary in size but include a Taco 138 on one zone. The system ran fine for a couple of months, then the lead primary pump began leaking, apparently at the water seal. We isolated that pump and turned on the "back up" primary pump. Within days that pump leaked at the seal as well. In my discussions with both the Taco rep and the design engineer, they seem to feel that the system is designed and piped properly. The Taco rep feels that the "back up" pump, which was not isolated, saw back pressure from the lead primary pump. This doesn't sound correct! It also doesn't explain the leak on the lead pump. Could it be possible that the larger secondary pump is having a negative effect on the primary loop. Any input would be appreciated! I have also included a couple of pictures clarify the design.