What is backflow in a plumbing system?

Backflow is the unwanted reversal of water flow in a plumbing system. Water normally travels one direction, from the city main into your building. That flow reverses when pressure drops in the city main, called back-siphonage, or when pressure rises on your side of the meter, called back-pressure. If it reverses through an unprotected cross connection, contaminated water can be pulled or pushed back into the clean drinking water supply.

What is a cross connection?

A cross connection is any point where the drinking water supply can come into contact with a non-potable source. Common examples on commercial properties are irrigation systems, boilers and chillers, cooling towers, swimming pools, fire suppression lines, chemical dispensers, and hose bibbs. Each one needs a backflow preventer matched to its hazard level.

How does a backflow preventer work?

A backflow preventer is a mechanical assembly installed at the cross connection that physically blocks reverse flow. Reduced pressure zone (RPZ) assemblies use two independent check valves plus a relief valve that dumps water to atmosphere if either check fails. Double check (DC and DCDA) assemblies use two spring-loaded checks in series. Pressure vacuum breakers (PVB) and spill-resistant vacuum breakers (SVB) use an air inlet that opens to break a siphon.

Why is annual backflow testing required?

Backflow assemblies depend on small internal parts — springs, rubber seats, diaphragms, and poppets — that wear out, foul with debris, or take on mineral scale. A device can look fine and still fail to hold pressure. Florida water utilities therefore require a certified tester to verify each assembly on their service connections every 12 months, which catches worn checks and weeping relief valves before contamination occurs.

What happens if a backflow test is not filed on time?

Testing without filing does not count toward compliance. If the passing report does not reach the county or city utility that holds your record, you can receive a violation notice, escalating fines, and in persistent cases a water service shutoff. The report must be filed with the utility, not just kept on site.

Which backflow device does my property need?

The device is determined by the hazard level of the connection and the local utility's requirements, not by preference. High-hazard connections such as chemically treated irrigation, boilers, and process water generally require an RPZ. Low-hazard connections and most fire lines commonly use DC or DCDA assemblies. Irrigation without chemical injection may be allowed on a PVB or SVB. A certified tester or the utility's cross-connection control program confirms the correct assembly.

Can a backflow preventer be installed underground or inside a box?

Testable assemblies must remain accessible and, for RPZ devices, must be installed above grade so the relief valve can discharge freely. Devices that are buried, boxed in, or installed below the required height cannot be tested or serviced properly and are commonly cited by inspectors. Access problems like these can be corrected by relocating or re-plumbing the assembly.

How long does an annual backflow test take?

A routine test on a single assembly typically takes 20 to 30 minutes and requires a brief interruption of water through that connection. Larger properties with multiple assemblies are usually completed in one visit, and the digital report is emailed within minutes of the test and filed with the utility the same day.

Why it matters

A 60-second guide to backflow — and why it can ruin your water.

Backflow is the unwanted reversal of water flow. When it happens at the wrong cross connection, contaminated water can be pulled back into the clean supply that feeds your tap.

Published January 8, 2026Last updated August 3, 2026Reviewed by the AlphaBackflows certified testing team

What is backflow?

Normally, water flows in one direction: from the city main into your building. Backflow is what happens when that flow reverses — either because pressure drops in the city main (back-siphonage) or because pressure rises on your side (back-pressure). When it reverses through an unprotected cross connection, anything downstream can be sucked or pushed back into the drinking water.

Why it's dangerous

Cross connections are everywhere: irrigation systems, boilers, swimming pools, fire suppression lines, soap dispensers, even a garden hose left in a bucket. Without a backflow preventer, fertilizer, antifreeze, pesticides, or bacteria can end up in the same line that feeds your kitchen sink. Documented backflow incidents have caused hospitalizations and forced city-wide boil-water notices.

What a backflow preventer does

A backflow preventer is a mechanical assembly installed at the cross connection that physically blocks reverse flow. Different devices (RPZ, DCDA, DC, PVB) handle different hazard levels — but all of them rely on small internal parts that wear out over time. That's why annual testing is required.

Why annual testing is required

Florida water utilities require backflow assemblies on their service connections to be tested annually by a certified tester. If yours isn't tested and filed on time, you can receive a violation notice, fines, or even have your water shut off. Annual testing catches worn checks and weeping relief valves before they fail.

Quick checklist for homeowners and property managers

• Do you have irrigation, a pool, a boiler, or a fire line? You almost certainly have a backflow preventer.
• Has it been tested in the past 12 months? If not, you're likely out of compliance.
• Did the last test report get filed with your county utility? Testing without filing doesn't count.
• Is your device visible, accessible, and not buried or boxed in? We can help correct access issues.

Schedule your annual test

Backflow prevention FAQ

Plain-English answers to the questions we get most often about how backflow happens and why testing is required.

What is backflow in a plumbing system?: Backflow is the unwanted reversal of water flow in a plumbing system. Water normally travels one direction, from the city main into your building. That flow reverses when pressure drops in the city main, called back-siphonage, or when pressure rises on your side of the meter, called back-pressure. If it reverses through an unprotected cross connection, contaminated water can be pulled or pushed back into the clean drinking water supply.
What is a cross connection?: A cross connection is any point where the drinking water supply can come into contact with a non-potable source. Common examples on commercial properties are irrigation systems, boilers and chillers, cooling towers, swimming pools, fire suppression lines, chemical dispensers, and hose bibbs. Each one needs a backflow preventer matched to its hazard level.
How does a backflow preventer work?: A backflow preventer is a mechanical assembly installed at the cross connection that physically blocks reverse flow. Reduced pressure zone (RPZ) assemblies use two independent check valves plus a relief valve that dumps water to atmosphere if either check fails. Double check (DC and DCDA) assemblies use two spring-loaded checks in series. Pressure vacuum breakers (PVB) and spill-resistant vacuum breakers (SVB) use an air inlet that opens to break a siphon.
Why is annual backflow testing required?: Backflow assemblies depend on small internal parts — springs, rubber seats, diaphragms, and poppets — that wear out, foul with debris, or take on mineral scale. A device can look fine and still fail to hold pressure. Florida water utilities therefore require a certified tester to verify each assembly on their service connections every 12 months, which catches worn checks and weeping relief valves before contamination occurs.
What happens if a backflow test is not filed on time?: Testing without filing does not count toward compliance. If the passing report does not reach the county or city utility that holds your record, you can receive a violation notice, escalating fines, and in persistent cases a water service shutoff. The report must be filed with the utility, not just kept on site.
Which backflow device does my property need?: The device is determined by the hazard level of the connection and the local utility's requirements, not by preference. High-hazard connections such as chemically treated irrigation, boilers, and process water generally require an RPZ. Low-hazard connections and most fire lines commonly use DC or DCDA assemblies. Irrigation without chemical injection may be allowed on a PVB or SVB. A certified tester or the utility's cross-connection control program confirms the correct assembly.
Can a backflow preventer be installed underground or inside a box?: Testable assemblies must remain accessible and, for RPZ devices, must be installed above grade so the relief valve can discharge freely. Devices that are buried, boxed in, or installed below the required height cannot be tested or serviced properly and are commonly cited by inspectors. Access problems like these can be corrected by relocating or re-plumbing the assembly.
How long does an annual backflow test take?: A routine test on a single assembly typically takes 20 to 30 minutes and requires a brief interruption of water through that connection. Larger properties with multiple assemblies are usually completed in one visit, and the digital report is emailed within minutes of the test and filed with the utility the same day.

What customers say about our guidance

5.0 / 5
I did not understand why one connection needed an RPZ and another was fine on a double check. They walked our maintenance team through the hazard levels and the testing procedure in plain language, then documented it for our records. The education pages on their site say the same thing, which made budgeting easy.
Marcus FeldOwner, industrial flex park (Broward County)February 2026
Backflow education