Quick Answer:
If you see air spit from a faucet after a backflow device was installed, it’s usually trapped air released as the system refills, not the house drawing air or losing pressure. Open a nearby fixture fully to purge the line. If the spitting only appears at one fixture, you can often clear it yourself; if it repeats across fixtures or across floors, a plumber should trace the source safely.
Why This Happens
- Installing or working on a backflow preventer can introduce air pockets into the supply line. Those pockets move and spit at fixtures as lines refill.
- Shutoffs, PRVs, and new fittings change flow patterns and can trap air until a full purge occurs. For similar situations after changes, see Air spitting after PRV adjustment.
- If the house was without water for a period (outage) or the water heater was drained and refilled, air can build up in sections of the system. A common related event is when water sits unused for a while — see Air from faucet after long vacation.
- Upstairs fixtures are often the last to purge because they sit at the top of the system; that can make the problem seem location-specific even though air is in the main line.
Step-by-Step What to Do
1. Open one faucet fully to purge air
Choose the fixture nearest the backflow device or the lowest fixture in the house. Open it fully (both hot and cold if it’s a single-handle mixer) and let it run until the sputtering stops and flow is steady. This forces the trapped air out of the line.
2. Test hot-only vs cold-only
At a nearby fixture, turn off the hot supply and run cold only, then run hot only. If spitting happens on hot but not cold, the air may be tied to the water heater or its return lines. If it’s only on cold, it’s likely in the supply side before the heater.
3. Note timing — outages and heater refills
Write down when the spitting occurred relative to any water outage, backflow install, or heater refill. Air after a refill or outage is normal while lines settle; repeated spitting long after those events suggests a venting or supply issue.
4. Check upstairs vs downstairs behavior
- Try the same tests on an upstairs fixture and a downstairs fixture. If upstairs continues to spit after downstairs is clear, the air is trapped higher in the system and may require a longer purge or a plumber’s targeted tracing.
- If multiple floors are affected at the same time, treat it as a system-level issue that a pro should inspect.
What Not to Do
- Don’t keep cycling the main shutoff valve in an attempt to clear air — this can introduce more air and may disturb the backflow device or PRV settings.
- Don’t dismantle cartridges, ceramic cartridges, or shower valves yourself to chase air unless you have experience; that can cause leaks or damage and may void warranties.
- If air returns after purging or it affects multiple floors, don’t try repeated, aggressive DIY fixes — let a plumber inspect venting and supply components so the source is properly traced and corrected.
When to Call a Professional
- If spitting continues after a thorough purge and testing hot vs cold.
- If air appears across multiple fixtures or on more than one floor.
- If you suspect the backflow device, pressure reducing valve, or water heater is involved and simple purging doesn’t stop the problem.
- If there is a noticeable drop in pressure, banging pipes, or loss of water service — a plumber can safely trace and isolate the affected section.
Safety Notes
- When testing hot water, be careful of scalding. Run hot slowly at first and have a towel or container ready.
- Use a bucket or basin to catch sputtering water and prevent slips or water damage.
- If work is needed on the backflow preventer or other backflow-protection components, a licensed plumber should perform it to keep the device functioning and compliant with local codes.
Common Homeowner Questions
- Will the air stop by itself? — Often yes after a full purge, but recurring air needs a plumber.
- Can the backflow device be causing pressure loss? — Not usually immediately; persistent pressure loss should be inspected by a pro.
- Is the heater likely the problem? — If spitting occurs only on hot lines, the water heater or its connections are a likely source and should be checked.
For more related articles, see the Air Spitting From Faucets hub.