Facts About Backflow Preventers

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Information about backflow preventers used in residential applications such as irrigation systems, fire sprinkler systems, boiler feed water, and domestic water supply.

Backflow preventers (BFP) are important devices to maintain the safety of drinking water. Most cities and many municipalities have made them required components on residential plumbing systems. Backflow preventers have been required on commercial and institutional systems for decades. Backflow preventers can be located on the water main coming into your home either before or after the water meter, on irrigation systems, outside sprinkler systems, residential fire sprinkler systems, and on city water lines supplying make-up water for hot water boilers.

The purpose of a backflow preventer is to ensure that potentially contaminated water does not reenter the city water supply. Unfortunately, even though many towns require that backflow preventers be installed, there are not always guidelines available for periodic testing and maintenance of the backflow devices. You should contact your local water company, building department or health department to determine the testing or inspection requirements for backflow preventers and cross connection devices.

Double Check Valve Backflow Preventer with Quarter Turn Shutoff

Types of Backflow Preventers

Reduced Pressure Type Backflow Preventer (RP) – Typically used on whole house at the water meter or on boilers and hydronic heating systems.

Double Check Type Backflow Preventer (DC) - This is the most widely used backflow prevention device on sprinkler systems. It is installed below grade in a standard valve box. Irrigation system can be turned off at 1 of 2 shut off handles.

Pressure Vacuum Breaker (PVB) - This device is approved for irrigation systems, however it is rarely used because of above ground installation and is subject to freezing during winter months. This device is also approved for chemical injection systems on sprinklers. Irrigation system can be turned off at 1 of 2 shut off handles.

Anti-Siphon Valve (ASV) – Used on hose bibbs and irrigation systems. They must be installed at least 6 inches above the highest sprinkler head. May not be approved in all jurisdictions as it cannot be tested; this is a newer version of the Atmospheric Vacuum Breaker.

Anti Siphon Irrigation Valve

Hose Anti Siphon Sillcock

Atmospheric Vacuum Breaker - This device is commonly found on older sprinkler systems, but is not approved for new installations because it is non-testable. It must be replaced by a Double Check Valve when upgrading irrigation system. No shut off handles to isolate irrigation system.


Hose bibbs or faucets that are connected to a municipal water supply or other drinking water supply should be equipped with hose connection vacuum breakers to prevent water in the hose from moving back into the water supply. Backward movement of water is called backflow, and it can occur either by siphoning or back pressure.

Backflow can occur due to siphoning if the pressure in the water supply suddenly drops to a low level. This can happen if the municipal water pumping system falls, a municipal water line breaks or when fire trucks pump from fire hydrants. In each of these cases, the pressure in the water supply lines may drop below atmospheric pressure as the lines drain, creating a vacuum which can pull water and any pollutants or contaminants from a garden hose and into the water supply lines.

Back pressure can occur in several ways. For example, if a spray nozzle which can be shut off with a valve is used on the end of the garden hose, and that valve is closed but the faucet is left open, the pressure in the hose will equalize to the city water supply pressure, and the hose will expand in response to the supply pressure. However, a sudden large water usage in the house or at another location can cause the supply pressure to drop.

This will cause the hose to contract, forcing water from the hose back into the domestic water system. Pressure can also build up in a pressurized hose if air is trapped in the hose and then expands as it heats in the sun. This pressure buildup can force water from the hose backwards into the water supply pipelines. Hose connection vacuum breakers will prevent backflow from occurring from these sources by opening to relieve the pressure build-up as soon as the pressure in the hose becomes greater than the supply pressure.

Woodford Double Check Backflow Preventers for Hoses

Boilers and Hydronic Heat

A boiler backflow preventer is designed to prevent the cross contamination of drinking water from boiler discharge and other contaminating water. The backflow preventer is required by code in most communities as necessary component for hot water and steam boilers.

If your building inspector or health department requires the use of a backflow preventer on a hot water boiler, this can be installed by a mechanical contractor to an existing system. Backflow preventers will already be included on newer systems.

There are three types of mechanical backflow preventers: reduced-pressure, pressure vacuum breaker, and the double check valve assembly. There is also a secondary type of mechanical backflow preventer, which is the residential dual check valve.

Most boilers are required to have a Reduced Pressure Backflow Preventer. You will need to obtain the manufacturer's pressure loss information for the reduced pressure backflow preventer used. Typically the pressure losses for reduced pressure backflow preventers range from 8 and 12 PSI. Pressure losses for reduced pressure backflow preventers are very high compared to other backflow preventers due to the method they use to prevent backflow. The pressure drop is used to detect the backflow and redirect the water out of the bottom of the backflow preventer. BFP of this type may drip water from the vent similar to a pressure relief valve on a hot water heater. Keep this in mind as this is what prevents the contaminated water from coming in contact with the city water. The drain or vent outlet should be piped to a sump pit or floor drain with an air gap pipe.

Air Gap drain line


A backflow preventer for a boiler costs between $15 to $30 and installation costs can add another $50 to $100. A fill valve will add another $30 to $50 for the materials cost. The part has a manual valve that can be engaged to prevent a backflow but it works on a siphoning process through the air gap that prevents the back up and cross contamination of water running through your plumbing system.

Watts Fill Valve and Backflow Preventer

Checking Codes

Check with your water company first, then building inspector or contractors in your community to determine the installation of and requirements for a backflow preventer in your home. In many locales it is necessary for a licensed plumber or someone who has a certification for backflow preventer testing to install the device. In many cases you will have to obtain a plumbing permit for installing a backflow preventer.

You may also run into the situation where you do not have a backflow preventer and none is required. In this situation, you should install a BFP in accordance to industry standards and the manufacturer’s specifications to protect your water supply.

Some jurisdictions require annual testing while others may have a 3 year testing schedule. All tests must be performed by a Certified Backflow Tester and verify that their certification is up to date. You can obtain a list of certified testers in your area from your water company.


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