Close the upstream isolation valve where water enters the backflow preventer. Turn it slowly to the marked closed position: a quarter-turn handle is usually closed when perpendicular to the pipe, while a handwheel usually closes clockwise. Follow the valve markings if they differ. Do not operate test cocks, dismantle the device or force a seized valve. Isolate only if the valve is clearly identified, you are authorised to operate it and the shutdown will not disable a fire service or other safety-critical supply. Understanding how drinking-water backflow is prevented explains why the assembly must not be bypassed.
Find the upstream isolation valve
The backflow preventer isolation valve is normally immediately before the assembly, on the side indicated by the flow arrow. If there are valves at both ends, the upstream valve is closest to the incoming supply.
Look for a substantial lever or handwheel. Do not confuse it with the smaller test cocks used for professional testing. If the arrow is unreadable or you cannot identify the correct supply, do not guess.
Close it safely
- Stop using fixtures or equipment supplied by the line.
- Confirm the inlet side using the flow arrow or clear labelling.
- Turn a quarter-turn handle 90 degrees, slowly. It is generally open when parallel to the pipe and closed when perpendicular.
- Turn a handwheel gently towards its marked closed position, usually clockwise. Do not overtighten it.
- Check a normal downstream outlet supplied by that line. If water continues flowing, do not adjust or repair the assembly.
The device and pipework may remain pressurised after isolation. Keep clear of spraying water, and do not loosen fittings, remove covers or open test cocks. Backflow devices protect against contamination, so testing, depressurising and repairs are not homeowner tasks.
If the valve is damaged, corroded, leaking, seized or difficult to identify, stop. Check the signs a backflow preventer may need repair or replacement, then contact the building manager and our licensed backflow prevention plumbers, as appropriate.