Water Backflow Preventer

A backflow preventer is a safety device that prevents the water from faucets, sprayer, sprinklers, and any other water fixtures in homes coming back into the city water system.

In this article, we are going to talk about Backflow Preventers, backwater valves, and Pressure Vacuum Breaker

We’re going to talk about how they work, or why you need them in your home or business.

What Is A Backflow Preventer?

A backflow preventer is a safety device that prevents the water from faucets, sprayer, sprinklers, and any other water fixtures in homes coming back into the city water system.

A backflow preventer is also known as a backwater valve, or a vacuum breaker.

A backflow preventer can be a small one like this:

backwater valves

… a bigger one like this:

Backflow Preventer

… or a large one like this:

Pressure Vacuum Breaker

How A Water Backflow Preventer Works?

A water backflow preventer (backwater valve) works as a physical barrier between the home water system and the public city water system or sewer.

Water from the city water system flows through the valve, so the waste water from bath, shower, sink, toilet, and other drain would normally flow through a valve into the public sewer.

The public sewer can back up due to blockages from tree root, grease, or from overburdened systems caused by extreme rainfall.

This can happen to the drinking water system as well.

This device acts as a physical barrier protecting your home from sewer back up, or protecting the city drinking water system from any potentially contaminants from water backup from hoses and taps.

It is important to know that backflow preventers only work through a combination of correct installation and regular maintenance.

So you will need a plumbing professional to do the work for you.

Types Of Backflow Preventer Valves

Small – Home Use

With this type of backflow preventer, you can screw it on a water tap, and hook your garden hose up to it.

When you hook a hose with a sprayer to water your garden, wash your car or use it for anything, sometimes it applies a back pressure when you shut the sprayer off, the remaining water in the hose will leak out at the valve, not coming back to the water system.

It’s quite annoying but it is doing its job to prevent any potential contaminants from the hose getting into the city drinking water. 

Large Backflow Preventer

The large backflow preventer you would find probably on a small to medium-sized business or commercial building.

This type of backflow preventer is more complicated.

Following the water flow, there is a ball valve at the water inlet side, then a vent, and then 2 check valves at the water outlet side.

If there is any back pressure coming in from the water discharge side, the water will be vent out at the bottom to the water drain, or outside where you install the backflow preventer.

That way, any contaminants don’t get back to the city drinking water system.

L-Shape Backflow Preventer

The L-Shape backflow preventer that you see on a lot of well and irrigation lines.

The water comes up from the bottom through the backflow preventer, and discharges out to the irrigation on a side.

The reason we use this is because a lot of people fertilize their garden or lawn to make their lawn look nice and green.

They will put down fertilizer, and these are chemicals.

If they have the pop up water sprinklers in their water irrigation system.

Let’s say, by some chance we lose water pressure in the city or in their neighborhood, and the contaminants or the chemicals they put in their lawn could potentially go back down into the water sprinklers’ head, though the water system, and into the main city drinking water system.

That can be a very dangerous thing.

They can have chemicals in the water system when they turn the water back on or charge the system back up.

People can showering in chemicals, drinking chemicals in their water.

Water backflow preventer is a safety feature.

If the city water system loses the pressure, if there is back pressure on the backflow preventer, the water will be vent out at the top.

Choosing a Backflow Preventer

Anti-Siphon Valve

Anti-Siphon Valve

An anti-siphon valve is generally less expensive and less work to install.

It’s available in 2 versions, manual and automatic control valve with built-in atmospheric vacuum breaker.

It must be installed 12 inches or higher than the highest sprinkler head or dripping emitter outlet.

If you install any valves of any type on the pipes downstream of the anti-siphon valve, the anti-siphon valve will not work.

The downstream valve creates back pressure on the anti-siphon valve which causes the vent in it to jam in the closed position.

If this vent can open, the anti-siphon valve will not prevent backflow.

Anti-siphon valves may not be installed in any location where it might ever be submerged under water.

A pressure vacuum breaker or PVB is similar to an atmospheric vacuum breaker, except you only need to install one.

It is attached to the main line leading to the control valves. 

It’s like the atmospheric vacuum breaker, it must also be installed above the ground, and it must be at least 6 inches higher than the highest sprinkler head, or dripping emitter controlled by other valves.

In a sloped yard, it would typically need to be installed at the highest point of the yard, with a mainline pipe running up to it from the water source, and then another main line running back to the control valves.

Some local municipalities require that a pressure vacuum breaker must be installed within 18 inches of the connection to the water source, or prohibit their use of the drip irrigation systems.

Others don’t allow the use of pressure vacuum breakers at all.

Make sure to check the codes regulation and laws in your area before installing one.

Pressure vacuum breakers are known to spew spit or spill water from under the cap when it backflow occurs because of this, it should be installed in a location where the water spillage will not cause problems.

Commercial Backflow Preventer

Commercial Pressure Vacuum Breaker Backflow Preventer

The standard for commercial irrigation installations (and a bit more expensive) option is to use a reduced pressure backflow preventer, or RP assembly. 

It’s like a PVB, it should be installed in a location where it will never be submerged.

It has to be installed at least 12 inches above the ground, but it does not have to be higher than the sprinkler head in the system.

All backflow preventers must be installed correctly.

Backflow should be checked at least yearly for proper operation, and it should be protected from freezing.

Backflow Preventer Testing

We have to test the water backflow preventer on an annual basis, and report it to the local city by city code.

There is a little test port on the backflow preventer, just to make sure it’s operating correctly.

You can get a professional plumber to test it.

It usually runs anywhere from 80$ to 120$ to have it tested on an annual basis.

They will leave you a piece of paper work, and they usually send it off to the city for you and get everything set up. So you don’t have to worry about it.

Backflow Preventer Repair

One of the common problems from a backflow preventer is leaking.

That is because there is something wrong internally with the device, and mostly the problems come from the sealing and washer parts.

So what you have to do is disassemble the device, there are a few pieces that you can easily check and fix or replace it.

Steps How To Fix Leaking On A Backflow Preventer

  • Step 1 – Remove the cap
  • Step 2 – Remove the bonnet with your hands or a stick like a screwdriver.
  • Step 3 – Inspect the sealing parts include washers, and o-rings
  • Step 4 – Replace the damaged parts with a new one
  • Step 5 – Reassemble all the parts by putting all the components into the right place, right order, nice and flat, then screw back the bonnet and the cap.
  • Step 6 – Check again to make sure no more leaking

In order to buy the right replacement parts of the device, you can check on the device’s side or on the cap for the brand and model number.


It’s really important to have a backflow preventer in your water system, or if you are thinking about putting an irrigation system in, make sure you put one in.

You need it only for the city water, if you are using water from a water well, you really don’t need it.

If you have any questions about the backflow preventer, please leave your comment below.

We will talk more.

Thank you.

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