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Multiple boiler start-up trouble shooting

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BEAR04
BEAR04 Member Posts: 1
in THE MAIN WALL
In a 21 storey apt building, built in the 70's.
Three athmospheric gas boilers in the basement, 3500 MBH each, controlled in steps by a boiler controller based on outdoor temperature reset. The pumps give us 720 gpm and the boilers have a minimum flow to fire of 180 gpm.They are piped in reverse return.between the pumps and the system distribution.
So, boilers 1 and 2 cycle properly but not boiler 3. Each boiler has 2 stages. The contractor says it's a flow issue. That when he opens isolating valves to boiler 3 (there are no control valves to the boilers) eveything shuts down because of the flow proving switches.

Any clues of what could be the issue and how to solve this?


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