Quick Answer:
If your pressure dropped immediately after a backflow preventer was installed, start by checking the device orientation, confirming the valve spring rating matches your supply pressure, and flushing out any installation debris. These three items account for most immediate drops. If you hear banging or see pressure that comes and goes, those are separate symptoms worth checking next: see water hammer and intermittent pressure loss.
Why This Happens
A backflow preventer adds extra parts in the water line: checks, seats, springs and test cocks. Right after installation a few common things can reduce flow:
- Incorrect orientation or a misinstalled check valve blocks flow more than expected.
- A spring sized for a higher opening pressure will restrict flow until pressure builds.
- Debris (sediment, pipe dope, or tape) introduced during work can lodge on a seat or screen.
- Partially closed isolation valves or air trapped in the line can make the drop obvious immediately after work.
Step-by-Step What to Do
1. Shut off and relieve pressure
- Close the isolation valves around the backflow device and open a downstream faucet to relieve pressure. Work only on a depressurized section.
2. Verify orientation
- Find the flow arrow or label on the body. The device must be installed with flow in that direction. If it’s backwards the checks will greatly restrict flow.
- Also confirm the device is upright if the manufacturer requires a specific mounting position.
3. Check valve spring rating
- Look for model numbers or stamped markings and compare to installation paperwork or the tag left by the installer. Springs are rated for a minimum opening pressure; a heavier spring can cut flow at household pressures.
- If the spring rating looks too high for your supply, note this for the installer or plumber — replacing springs usually requires the exact part and experience.
4. Flush debris
- With isolation valves open slightly, use the device’s test cocks or a temporary bypass to flush water through and remove trapped debris. Run water from a downstream faucet while a helper opens and closes the test cocks briefly to dislodge particles.
- Catch or direct the flushing water safely; wear eye protection and work slowly to avoid pressure surges.
5. Repressurize and observe
- Open the isolation valves slowly. Check pressure at a nearby faucet and listen for unusual noises.
- If the problem is intermittent or returns after a short time, keep notes on when it happens and what fixtures are affected.
6. Document and compare
- Record what you tried, the device model, and any markings. This helps a pro diagnose issues faster if you need help.
What Not to Do
- Do not remove internal springs — that can destroy the device’s function, void warranties, and allow contamination to flow backward.
- Do not work on the device while the line is pressurized or without shutting off and relieving pressure first.
- Do not substitute random springs or parts; use manufacturer-specified components.
- Do not ignore the manufacturer’s installation orientation and instruction tag.
When to Call a Professional
- If you verify orientation and flush debris but the pressure remains low.
- If the valve spring rating appears wrong or the device needs internal repair — a certified plumber or backflow tester should handle replacements and calibrations.
- If you detect leaks, persistent cycling, or contamination concerns. Backflow devices are safety equipment; improper repair can risk water quality.
- If local codes require certified testing after installation — a pro will provide the required certification.
Safety Notes
- Always depressurize the line and open a downstream tap before opening any test cock or removing covers.
- Wear eye protection and have a bucket or hose to direct flushed water to a drain.
- Do not perform internal repairs if you’re not trained — backflow assemblies are precision devices and must be reassembled and tested correctly.
- Keep the device’s tag and manufacturer instructions for reference; they often list safe service procedures.
Common Homeowner Questions
- Why did pressure drop right after installation? — Often orientation, a stiff spring, or installation debris; those are the first things to check.
- Can I bypass the device to restore pressure? — Temporarily bypassing may restore flow but can allow contamination; do not bypass permanently and check local rules first.
- How long before a professional is needed? — If basic checks (orientation and flushing) don’t help or you’re unsure about spring ratings, call a licensed plumber or certified backflow tester.
For more related articles, see the Backflow Preventer Malfunctions hub.