boiler problem

Dave Hartwig

I am working at a sewage treatment plant on a hot water boiler that they use to treat the sewage with. The problem i an having is everytime the circulating pump shuts off the low water cutoff gets air bound, creatin a low water alarm. I installed a new bleeder on top of the cutoff that bleeds the air off,bit by then ther burner controller is in flame failur mode. We changed the flow valvwe on the supply line,because the check valve was sticking open when the pump would shut off creating a vacuum that would suck the air out of the expansion tank,causing it to fill up. This solved the expansion tank problem and i thought this was the reason the low water cutoff was getting air bound. I have manual bleeders on the supply line on the highest point of the boiler, which i have to bleed constantly. I am still getting these low water cutoff alarms when i shut the pump off. any sugestions thank you Dave Hartwig dhart09095@aol.com