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Air Problem in loop for indirect heater.
I'm having a problem with air getting into a loop for an indirect water heater. The system has several zones for air handlers and an indirect water heater which is in the boiler room. All zones have there own circulator. There is a spiro vent on the main header. The pumps are on the feed. We have changed the pump and also changed the location of the pump to be down low near the heater. We removed the watts flow check as we thought it by be sticking, and install a 3 speed pump with IFC. ( we tried the Taco 007E but it went into a lock out ) This worked for about a week ( it was every day before that ) but now the air is back. The system had been working ok before this happened for several years. Not sure what has changed to cause this, but the other zones some of which the air handlers are on upper floors have no problem. Any suggestions?