What It Is
A vacuum breaker is a small backflow prevention device that allows air to enter a plumbing line when pressure drops, breaking the siphon that would otherwise pull contaminated water backward into the potable supply. It is commonly installed on hose bibbs, irrigation zone valves, and toilet fill valves as a simple, low-cost line of defense against back-siphonage.
What It Does
The job of a vacuum breaker is to interrupt a siphon before it can form. When water pressure in the supply line drops below the pressure at the outlet - as can happen during a main break, a pressure test, or heavy demand elsewhere on the system - a suction effect develops. Without intervention, that suction can draw whatever is at the end of the hose or pipe back into the supply. The vacuum breaker vents that suction to atmosphere by admitting air into the line, which collapses the siphon.
This is different from a full backflow preventer, which uses mechanical check valves to resist both back-siphonage and backpressure. A vacuum breaker only addresses back-siphonage and only works when installed above the highest point of the outlet it protects. For that reason, plumbing codes specify which type of device is appropriate for each hazard level.
Where It Is Located
On a standard outdoor hose bibb, the vacuum breaker is the small threaded cap or bonnet at the top of the spout - you may see it drip slightly after the hose is disconnected, which is normal. On irrigation systems, atmospheric vacuum breakers (AVBs) are installed at each zone valve, positioned at least 6 inches above the highest sprinkler head in that zone. Toilet flush valves also incorporate a vacuum breaker inside the tank to prevent toilet water from being siphoned back into the supply lines.
How It Works
Inside a standard atmospheric vacuum breaker is a small disc or poppet held against a seat by water pressure during normal flow. If supply pressure drops and a vacuum forms, the disc lifts away from the seat, opening a vent port to the atmosphere. Air flows in, the siphon is broken, and no water can flow backward. When supply pressure returns to normal, the disc reseats and the device returns to its closed position, ready to allow normal water flow.
Because the device relies on a moving part held in place by water pressure, it must always be installed so the supply is below the device and the outlet is above the potential contamination source. Installing a vacuum breaker below the high point of the connected hose or pipe renders it ineffective.
Common Problems or Signs of Failure
- Continuous dripping from the vacuum breaker after the hose is disconnected and water is shut off - indicates the poppet or disc is stuck open or worn
- Vacuum breaker cap is missing entirely - no protection against back-siphonage
- Device appears cracked or corroded
- Water sprays out of the side of the device during normal use - the air vent is stuck open
Can a Homeowner Check It?
- Look at the end of each outdoor hose bibb to confirm a vacuum breaker cap is present - it should be a small threaded bonnet above the hose threads
- After shutting off the hose, it is normal for a few drops to escape; continuous flow indicates a worn device
- Check that vacuum breaker caps are not capped off with a solid plug - doing so defeats the protection
- For irrigation systems, confirm each zone valve has a vacuum breaker installed and positioned above the highest head
When to Call a Plumber
Replacing a leaking or missing vacuum breaker on a hose bibb is a simple repair, and the replacement caps are available at hardware stores. However, if your irrigation system lacks proper backflow protection or you are not sure whether the installed devices meet current code requirements, a plumber familiar with backflow prevention can evaluate and upgrade the system. This is especially relevant if your municipality requires annual certification of backflow assemblies.
Related Plumbing Terms
Related Gray's Services
Have a Plumbing Question? We Can Help.
Gray's Precision Plumbing serves Northeast Ohio. Call or text for a straight answer on what's wrong and how to fix it.
Call 440-723-8850 Send a Message