Backflow occurs when water flows in the reverse direction from its intended flow, posing health risks by contaminating potable water supplies. Building codes mandate backflow prevention devices to isolate water systems and prevent backflow caused by back pressure or back-siphonage. Common devices include check valves and air gaps to prevent backflow into potable water systems from appliances, fire sprinklers, and cross-connection points. Proper installation and maintenance of backflow prevention devices according to hazard category standards helps ensure the safety of water distribution systems and public health.

1. Backflow prevention
Backflow is a term in plumbing for an
unwanted flow of water in the reverse
direction. It can be a serious health risk
for the contamination of potable
water supplies with foul water. In the most
obvious case, a toilet flush cistern and its
water supply must be isolated from the
toilet bowl. For this reason, building
codes mandate a series of measures
and backflow prevention devices to
prevent backflow.
Backflow means the undesirable reversal
of flow of a liquid, gas, or suspended solid
into the potable water supply;
a backflow preventer is designed to
keep this from happening. Points at which a potable water system connects with a non-potable water
system are called cross connections. Such connections occur naturally in appliances such
as clothes washers and dishwashers, but they must be carefully designed and installed to prevent
backflow. Another common location for a backflow preventer is the connection of a fire
sprinkler system to a water main, to prevent pressurized water from flowing from the fire suppression
system into the public water supply.
A backflow prevention device is used to protect potable water supplies from contamination or
pollution due to backflow.
In water distribution systems, water is normally maintained at a significant pressure to enable water
to flow from the tap, shower, or other fixture. Water pressure may fail or be reduced when a water
main bursts, pipes freeze, or there is unexpectedly high demand on the water system (for example,
when several fire hydrants are opened). Reduced pressure in the pipe may allow
contaminated water from the soil, from storage, or from other sources to be drawn up into the
system
Backflow occurs for one of two reasons, either back pressure or back siphonage.
Back-siphonage occurs when higher pressure fluids, gases, or suspended solids move to an
area of lower pressure fluids. For example, when a drinking straw is used to consume a beverage,
suction reduces the pressure of fluid inside the straw, causing liquid to move from the cup to inside
the straw and then into the drinker's mouth. A significant drop of pressure in a water delivery system
creates a similar suction, pulling possibly undesirable material into the system. This is an example of
an indirect cross-connection.
Back-pressure occurs for example when air is blown through the straw and bubbles begin to
erupt at the submerged end. If instead of air, natural gas had been forced into a potable water tank,
the gas in turn could be carried to a kitchen faucet. This is an example of a direct cross-connection,
with undesirable material being pushed into the system.
Backflow prevention must be automatic, and manually-operated valves are not usually acceptable.

2. Check valves
Automatic check valves are required to prevent back pressure. Regulations for these check valves specify the
design capabilities of the valve used, according to the hazard. Category 2 contamination may be prevented by a
single check valve, but category 3 requires a double check valve (these are manufactured as a convenient
single unit, or even integrated into tap (faucet) fittings). Category 5 requires an air gap, not merely a valve. If
the downstream valve leaks and permits backflow, this will drain out through the vent rather than building up
pressure against the upstream valve. These valves are complex, requiring certified installation and annual
checks. They are used for category 4 systems, such as fire sprinklers where the system has an
antifreeze additive.
Air gaps
Back siphonage may be prevented by use of a vertical air gap. This may be a small gap, such as provided by
a tundish (a combined overflow spout and catch funnel) or a large gap, such as a basin tap being above the
maximum level of the water in the basin. Standards for these air gaps group them by the amount of separation
that they provide and their acceptability for the various risk categories
Air gaps may also protect against back pressure, and are generally favoured for this.[3]
However most air gaps
also limit the system pressure that may be transmitted across them. In most cases they replace mains pressure
with the pressure of that from a raised gravity cistern.
Common examples of an air gap in domestic plumbing are:
 Taps above washbasins
 Cold water cisterns, where the float valve outlet must be above the overflow water level. The
previous practice of taking a "silencing tube" from the float valve to under the water level is no
longer acceptable. Under some plumbing codes. such silencing may still be acceptable if it is a
soft collapsible tube which cannot syphon.
 Hand-held showers must have their hoses fastened such that the shower head cannot rest below
the water level in a bath or basin.
Web: https:// abcplumbingbackflowprevention.com
United States
Email:plumber@ABCplumbing.com.
Phone: 256-810-2615
Address:4757 Ferry Street
Huntsville, AL 35816