A sewer backwater valve can close against flow coming back toward the building. Which fixtures it protects depends on where it is installed. A blockage inside the building can still cause a backup upstream of the valve.
Find out what the valve serves
Ask the plumber to identify the valve location and which branches connect on either side. If backups occur during storms, report the timing and which fixtures are affected. If only one sink backs up during normal use, the problem may be in a branch drain instead.
Keep the valve accessible
Ask how the valve should be inspected and serviced for its specific model. If it closes during a surcharge, avoid sending more wastewater into the protected piping until flow can resume. A plumber can evaluate whether the valve and sewer layout match the property’s needs.
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