Quick Answer:
If water pressure dropped right after a backflow preventer was installed, the most common causes are a restriction from debris or incorrect installation, a closed valve, or trapped air. Start by isolating the repaired component, measure pressure before and after the repair point, and flush affected lines to clear debris. If those steps don’t restore normal pressure, call a licensed plumber or the installer.
Why This Happens
Backflow preventers add a device into the water line that can introduce a restriction if it’s installed with debris in the line, oriented incorrectly, or set up with a partial shutoff valve upstream. During installation, bits of pipe scale, solder, or thread sealant can move downstream and reduce flow. Air can also be trapped in branches that were isolated during work.
If a nearby fixture or shutoff was changed at the same time, the cause may be similar. For related situations, you can review Pressure drops after fixture replacement and Pressure low after main shutoff replacement for comparison and troubleshooting ideas.
Step-by-Step What to Do
1. Isolate the repaired component
- Close any isolation valves around the new backflow preventer so you only work on the affected section.
- If the assembly is in a bypass, close the bypass valves and open the test cocks as appropriate per the manufacturer’s instructions to relieve pressure.
- Isolating prevents water loss elsewhere and lets you test the device and lines independently.
2. Measure pressure before and after the repair point
- Attach a pressure gauge to a hose bib or a test cock upstream of the backflow device to get the supply pressure (before).
- Attach a gauge downstream of the device or open a fixture and use the gauge to measure outlet pressure (after).
- Record both readings. A big drop across the device (more than a few psi under normal conditions) indicates a restriction in or around the unit.
3. Flush affected lines
- Open the furthest fixtures one at a time (hot and cold) and let water run for several minutes to flush debris and trapped air.
- Check the downstream gauge or a nearby fixture while flushing to see if pressure restores as debris clears.
- If you suspect heavy debris, flush until the water runs clear and then recheck pressures.
4. Check valves, orientation, and test cocks
- Make sure inlet and outlet valves are fully open and that the backflow unit is installed in the correct flow direction.
- Inspect test cocks and any temporary caps—loose caps or partially closed valves can cause big drops.
- If there’s a pressure reducing valve (PRV) nearby, verify its setting hasn’t been changed during the work.
5. Re-measure and monitor
- After flushing and correcting valves, re-measure pressure before and after the device to confirm improvement.
- Run a few fixtures for 10–15 minutes and check that pressure is stable under normal demand.
- Document the readings in case the installer needs them for warranty or follow-up service.
What Not to Do
- Do not assume pressure will self-correct without flushing and inspection.
- Do not remove or bypass the backflow preventer if it is required by code; tampering may be illegal and unsafe.
- Do not force valves or pry open sealed components—this can damage the device or create leaks.
When to Call a Professional
- Pressure does not return after isolating, flushing, and re-measuring.
- There is a visible leak, loud banging, or repeated cycling of a PRV or pump after the install.
- You find debris or damage inside the device, or you suspect the unit is installed backwards or is the wrong size for the line.
- If the installer is required to certify the backflow preventer, contact them to inspect and correct installation issues.
Safety Notes
- Always shut off water and relieve downstream pressure before working on the assembly. Open a faucet to confirm pressure is relieved.
- Wear eye protection and gloves when flushing lines to avoid contact with discolored water or debris.
- Follow local codes: backflow preventers are often subject to testing and tamper rules—do not modify without proper authorization.
Common Homeowner Questions
- Why did pressure drop immediately after installation? Debris, a partially closed valve, or incorrect orientation of the device are the most common causes.
- Can I fix this by myself? You can perform the isolation, flushing, and pressure checks; do not remove or permanently alter the backflow device without a professional if code requires it.
- How long should flushing take? Usually a few minutes per fixture; continue until water runs clear and pressure readings stabilize.
Related Articles
If you’re troubleshooting a similar symptom, these guides may help:
For the full directory, see Pressure Loss After Plumbing Repairs.