|This picture is of a Watts 9D backflow
preventer it is installed on the feed line to a
hot water boiler, it's required by code on all
hot water boilers. it's purpose is to prevent
the dirty, toxic water from the boiler from
getting into the domestic water system (the
water you use to cook bath and drink) One
side of the copper pipe connects to the boiler
fill valve that sends water to the boiler, the
other pipe is connected to your drinking
water. The opening in the middle facing down is
the vent or air gap that will stop a vacuum that
would allow the boiler water from coming into
your drinking water. this fitting needs to be
open to allow air into it if a vacuum happens.
Some knuckle head saw that the vent was dripping, so he or she installed a boiler drain in the vent opening, (instead of fixing the Watts 9d Backflow preventer) and it looks like the boiler drain also leaked so a cap was installed on the threads of the boiler drain. This is equal to removing the backflow preventer all together. A real licensed plumbing contractor would have never made such a foolish mistake. Some boilers contain antifreeze, antifreeze in your drinking water could kill you.