Attention Customers

New rates effective 7.1.24   Estimate your next water bill

Estimated Bill?

Send us your meter reading

Water Service Turned Off?

Find out how to get your water turned back on.



The following is presented to provide a general explanation on the subject of backflow prevention. It is not intended to be used in place of Code requirements and professionals should always utilize the code when making design, installation, and/or maintenance decisions. Definitions of italicized words follow at the end.


When drinking water piping connects to various plumbing fixtures or water utilizing equipment a cross-connection is created. If improperly protected, contamination can result when a backflow event occurs; allowing contaminates to reverse flow from the fixture/equipment back into the drinking water piping.

What Can Cause a Backflow

Conditions that are likely to create a backflow event are typically common, but unfavorable, hydraulic events that occur within the public or private segments of the water distribution system such as: an owner or worker draining down a house or building to make a plumbing repair; a broken water main or distribution pipe; severe/widespread power outages; fire fighters using large quantities of water to extinguish a fire, or; a water utilizing piece of equipment or process creating pressures greater than those present in water distribution system.

When these unfavorable hydraulic events are occurring, contaminates can be back-siphoned into the drinking water due to negative pressures; or contaminates can back-pressured into the drinking water due to excessive pressure imposed by the water utilizing piece of equipment or process.

Built-in Protection

Modern plumbing fixtures generally have built-in backflow protection. For instance, a faucet spout terminates above the flood rim level of the sink or tub. So if the sink or tub is full of dirty water or worse, backed-up sewage, there is no possible way for backflow because of the air gap created by the elevated spout. Toilet fill valves, clothes washers, dishwashers, and refrigerator/ice makers also employ some type of built-in air gap as their method of protection.

So around the home and office, for the most part, standard plumbing fixtures do not present a hazardous condition. However, additional protections are needed for household items such as: hand held shower heads; hose bibbs; lawn irrigation; and boilers.

Additional Protection Needed

In addition to the few items around the home that require additional protection, there are numerous applications within commercial and industrial processes that require additional protection. In general, these applications cannot utilize the air gap method of protection because the equipment or process requires a direct connection in order to utilize the dynamic pressure and flow that already exists within the water distribution system.

Therefore, directly connected water piping requires a different method of protection; the backflow preventer. A backflow preventer is a “one-way” appurtenance (an assembly of check valves or a vacuum breaker), that only allows water to flow in the desired direction and physically impedes reverse flow.

Types of Backflow Preventers

There are two basic types of backflow preventers: testable and non-testable.

Testable Backflow Preventers

Also referred to as Backflow Prevention Assemblies; Backflow Assemblies; Testable Assemblies; or simply, Assemblies. Backflow Prevention Assemblies are generally required on the more hazardous cross connection applications, see below. By federal, state and local requirements as well as the manufacturer’s product listing, annual testing is required to ensure the assembly is good working order. This is due in part because the working components of a backflow assembly have a fairly short life expectancy and/or because sediment and debris can easily block their proper function.

When required testing fails to produce satisfactory results, assemblies must be cleaned and/or rebuilt as needed and retested. Un-repairable or obsolete assemblies must be replaced. See more below, for reporting, permitting, and licensing requirements.

Non-Testable Backflow Preventers

Also referred to as Backflow Prevention Devices; Backflow Devices; Non-Testable Devices; or simply Devices. Backflow Prevention Devices are generally required on the less hazardous cross-connection applications, see below. Some devices are required to be rebuilt or replaced every five years; while others are good for the life of the fixture they serve or until they visibly fail (leak externally). See below.

Application of Backflow Preventers

The following is a quick view of applications that require either a testable or non-testable backflow preventer. For a comprehensive look at selection criteria please visit in the WSSC Water Plumbing and Fuel Gas Code. In fact, take a look at Chapter 5 in its entirety; it is all about Cross-Connection Control.

Testable Applications:

  • Irrigation, in ground (All Homes & Businesses)
  • Commercial Boilers
  • Cooling Towers
  • Medical Equipment
  • Laboratory Uses
  • Commercial Water Treatment
  • Vehicle Washing Facilities
  • Commercial Fire Sprinklers
  • Processing Plants

Non-Testable Applications:

  • Residential Hose Bibbs
  • Hand Held Shower Heads
  • Emergency Eye Wash
  • Residential Fire Sprinkler
  • Residential Boilers
  • Commercial Ice Makers
  • Beverage Dispensers
  • Residential Humidifiers
  • Food Service Equipment

Non-Testable Devices: Every 5 years, the following devices must be either rebuilt with new check modules or replaced.:

  • ASSE 1012 – Dual Check Valve; with Atmospheric Vent
  • ASSE 1022 – Dual Check Valve; with Atmospheric Vent for Carbonated Beverage Dispenser
  • ASSE 1024 – Dual Check Valve
  • ASSE 1081 - Backflow preventer with Atmospheric Vent and Pressure Reducing Valve

These devices require tagging to identify the installation and/or expiration date. Homeowners can replace these devices without permit or inspection.

Cross-Connection Technicians

Also referred to as Backflow Technician; Backflow Tester; or simply, Tester. By Maryland Law, and subsequently by WSSC Water Plumbing Code, Backflow Techs must be registered as, at minimum, a Journeyman Plumber and also have completed a WSSC Water Approved 32-hour certification course for Cross-Connection Technicians (and recertify every 3 years).

Plumbing Services Firms

Under Maryland Law, only firms registered under a licensed Master Plumber can perform Plumbing Services which includes the installation, repair and testing of backflow prevention assemblies.

Therefore, cross-connection technicians have to be performing their duties under the direction of a registered plumbing services firm.

Courtesy List of Service Providers

The following link is a list of approved service providers who have declared they provide this specialty line of services. WSSC Water has validated their licensing credentials, but does not endorse these firms; you are free to choose any licensed firm.

Test Reports

Numbered forms can only be purchased by registered plumbing services firms. Upon completion of a test on a testable assembly, registered cross-connection technicians are required to electronically submit their reports, to WSSC Water, within 30 business days.

Long Form Permits

The “first time” installation of a new testable backflow assembly or relocation of an existing testable assembly requires a long form permit and inspection.

Short Form Permits

The replacement of an existing backflow assembly, in kind and location, requires a short form permit and inspection.

Seasonal Water Use Systems

Testing of backflow assemblies shall be performed in the spring when system start-up/reactivation occurs. This typically applies to irrigation systems, pools, pool houses, summer/beach houses, campgrounds, park restrooms, amusement rides, etc. WSSC Water is systematically resetting all annual test due dates to May 1. Note: testing is only required once per year, but from this point forward, it should be done in the spring, before May 1.

Rationale: Backflow assemblies that have been de-pressurized, removed or winterized in-place are at the greatest risk of failure because they were subjected to “no-flow”, “no-pressure” conditions. Like most mechanical devices, lack of movement is bad on the working parts, not to mention the affect dried residue and/or nesting bugs can have on them as well.

Hint for owners: for pools and irrigation systems, make the testing part of your spring start-up contract. The pool and irrigation firms may be able to procure the most favorable pricing due to the ability to deal in volume with plumbing services firms.

New Construction

Industry experts recognize that WSSC Water Code Officials (Plan Reviewers and Inspectors), Design Engineers, Plumbing Installers and knowledgeable owners have done an exceptional job to ensure that the right form of backflow protection was specified, approved, and installed, over recent decades, for tens of thousands of cross-connections. However, perpetual maintenance is equally as important.

Federal, Safe Drinking Water Act: Follow-up

Federal, state and underwriting authorities mandate that water purveyors, like WSSC Water, follow-up on cross-connections and backflow prevention, in addition to initial construction efforts. Since 2010, WSSC Water has increased its efforts in this area.

WSSC Water began with its own treatment plants and pumping stations to ensure that we are in full compliance; we are. Then inspections commenced on a priority based matrix, where the highest priority was given to commercial properties, containing the highest degree of hazards, coupled with an assessment of their hydraulic vulnerability.

The Cross-Connection Control Program

is responsible for inspecting existing buildings for un-protected or improperly protected cross-connections. WSSC Water currently has eight full time inspectors canvassing the service area on the established priority basis. The program also utilizes a strong administrative support staff to maintain our database, send reminder notices, and administer our self-survey program, more below.


WSSC Water maintains a database of annually tested backflow preventers. Information from electronically submitted test reports is directly uploaded to this database. In addition, the database can determine when backflow assemblies are out-of-date for testing and generate reminder notices.


WSSC Water needs your help to catch-up on years of non-reporting and a general lack of follow-up compliance (testing). To enhance the database and ensure follow-up compliance, WSSC is asking its commercial and industrial water customers to complete a self inspection or “self survey” of all connections to the water system within their property. Homeowners who have a testable backflow assembly are asked to self report as well; a simple call or email will do. See contact information below.

The survey may be completed by a knowledgeable representative of the owner or tenant or by a licensed plumber who has a current cross-connection certification. If you are a property or building owner, please have the survey completed for the backflow assemblies under your ownership and control. If you are a tenant or unit owner, please complete the survey for the backflow assemblies under your ownership and control. Please share this notice openly between ownership and tenants so the overall results will be all inclusive and accurate with respect to controlling entities.

When discovered, un-protected water uses, or protected outlets that are overdue for a test, must be corrected. Maryland State Plumbing Code, and subsequently WSSC Water’s Plumbing Code, requires the installation and testing of backflow prevention assemblies to be performed by licensed plumbers who also possess certification as a backflow technician.


Compliance may also help in reducing your liability. Note: if you are responsible for a cross-connection (whether un-protected, improperly protected, or out-of-date for service) and a backflow event occurs, you may be liable for the resultant sickness, death, or property damage. 

Yes, it can be that serious.

Contact Us by email at: or by phone: 301-206-4004


is the physical connection between the potable water system and an “end-use” where a potential hazard exists.

is the undesired reverse flow of contaminants into the potable water from an “end-use” hazard and is typically driven by common, but unfavorable, hydraulic events in either the public or private water distribution system.

Air Gap
the un-obstructed vertical distance between a water outlet and the flood rim level of a potential source of contamination; the air gap shall not be less than twice the diameter of the water outlet and never less than one (1) inch. 

Direct Connection
water distribution piping connected, without an air gap, to an “end-use” where a potential source of contamination exist.

Backflow Preventer
is a “one-way” appurtenance (an assembly of check valves or a vacuum breaker), that only allows water to flow in the desired direction and physically impedes reverse flow.

Plumbing Services
means to install, maintain, extend, alter, or remove piping, a plumbing fixture, a plumbing appliance, a plumbing appurtenance, or other plumbing apparatus within a building or on-property where such plumbing is connected to a public or private sewerage system or connected to a private or public water system.


List of Cross Connection Approved Firms

Last Modified: December 20, 2023, 11:28 am EST