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backflow preventer

I was just curious if anyone could explain the purpose of the extra port on a backflow preventer.  I have seen them piped down to the floor, but I am just not sure what exactly they do.



  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    "Extra" port on backflow preventer.

    The "extra" port on the backflow preventer is not extra at all.

    A backflow preventer contains two check valves in series. If it were not a matter of hygenic safety, you would need only a single check valve.

    What happens in normal operation is that the two check valves are open until the pressure on both sides of the valve are equal. Then the springs close the check valves. This is the easy part of the job.

    The problem occurs when the supply pressure drops. Ideally, the two checkvalves close. But in reality, they may not because of grit, wear, etc, so some boiler side water can flow back into the supply.

    To guard against this, a backflow preventer is set up so if the supply pressure drops, in addition to closing that check valve, it opens the assembly to the vent port. Thus, if any leakage from the boiler escapes that check valve, it dribbles out the "extra" vent port onto the floor instead of contaminating the supply water.

    The reason for the pipe to near the floor is because the boiler water could be quite hot and injure someone just as if a pressure relief valve on a boiler, or the pressure&temperature relief valve on a water heater would do. They too need a pipe to the floor.
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