Backflow in one sentence

Backflow is water moving the wrong way through a pipe, from a customer's plumbing back toward the public main. It only takes a brief pressure change for that to happen, and when it does, whatever is sitting in the downstream piping can travel with it.

That is why water suppliers care about devices on private property. A sprinkler line full of fertilizer residue, a boiler loop with treatment chemicals or a car wash bay all connect to the same distribution system that feeds kitchen taps across Fullerton.

The two ways water runs backward

Testers and inspectors sort every backflow risk into one of these two causes, and the cause decides which devices can be used.

Backsiphonage

  • Supply pressure drops, for example during a main break or heavy hydrant use.
  • The falling pressure pulls water backward like a straw.
  • A hose left in a bucket or a submerged fill line is the classic example.
  • Vacuum breakers are designed for this condition only.

Backpressure

  • Pressure downstream rises above the supply pressure.
  • Pumps, boilers and elevated tanks can push water backward.
  • It can happen even while the street main is normal.
  • Needs a check-valve assembly such as a double check or RP.

Parts you will hear about

Check valve
A spring-loaded valve that opens for forward flow and snaps shut if flow tries to reverse.
Relief valve
On an RP assembly, an opening between the two checks that dumps water to the outside if pressure in the middle zone gets too close to supply pressure.
Air inlet
On a vacuum breaker, a port that opens to let air in and break a siphon when supply pressure falls.
Test cocks
Small valves on a testable assembly where the tester connects a gauge.
Shutoff valves
The two valves at the ends of a testable assembly, used to isolate it during a test.
Cross-connection
Any point where drinking water piping could come into contact with something that is not drinking water.

What happens inside an assembly when pressure drops

  1. Normal flow

    Supply pressure holds the check valves open and water moves toward your fixtures and sprinklers.

  2. Pressure falls upstream

    Flow stops and the springs push the checks closed, sealing the path back to the street.

  3. A check leaks

    On an RP, the relief valve opens and spills water so the dirty side cannot reach the supply. On a double check, the second valve is the backup.

  4. Pressure returns

    The checks reopen, the relief valve closes and normal flow resumes. A device that keeps dripping afterward needs a test.

Fullerton context

Who asks for tests
Your water supplier; for most addresses that is the City of Fullerton's water utility
Typical home device
A backflow preventer on the lawn sprinkler line
Typical business device
An RP or double check on the service line or fire line
Local rules
Deadlines, accepted testers and filing: check with the city

Good questions to ask a tester

  1. Which type of device is this, and is it the type my notice lists?
  2. Is it protecting against backsiphonage, backpressure or both?
  3. Is it installed at the right height and orientation for its type?
  4. Are the rubber parts original, and is a repair kit still made for this model?

Common questions

Does every house need a backflow preventer?

Not every connection needs a testable assembly, but irrigation systems and certain appliances normally need protection. Your water supplier decides what is required at your address.

Is a backflow preventer the same as a pressure regulator?

No. A pressure regulator lowers incoming pressure. A backflow preventer stops reverse flow. They often sit near each other, which is why people mix them up.

Can I see whether my device is working?

Not reliably. A dripping relief port or spraying vacuum breaker shows a problem, but a quiet device can still have a weak check valve. Only a gauge test confirms it.

General information for Fullerton, not a quote or professional advice. In an emergency, call 911.