Backflow (plumbing)
Encyclopedia
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 code
s mandate a series of measures to prevent backflow.
Backflow occurs for one of two reasons, either back pressure or back siphonage.
Back pressure is the result of a higher pressure in the system than in its supply, i.e. the system pressure has been increased by some means. This usually occurs in unvented heating systems, where thermal expansion increases the pressure.
Back siphonage is the result of supply pressure being lowered below that of the system. This occurs when a supply is interrupted or drained down.
s 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 maunfactured as a convenient single unit, or even integrated into tap (faucet) fittings). Category 5 requires an air gap, not merely a valve. A recent introduction to the UK has been the Reduced Pressure Zone
(RPZ) valve, a form of double check valve where the intervening zone is drained and normally kept empty. 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 sprinkler
s where the system has an antifreeze additive.
(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. The size of the acceptable gap also depends on the capacity of the incoming supply, such that a stuck-open flow cannot overfill the cistern and close the gap.
Air gaps may also protect against back pressure, and are generally favoured for this. 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:
Plumbing
Plumbing is the system of pipes and drains installed in a building for the distribution of potable drinking water and the removal of waterborne wastes, and the skilled trade of working with pipes, tubing and plumbing fixtures in such systems. A plumber is someone who installs or repairs piping...
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 code
Building code
A building code, or building control, is a set of rules that specify the minimum acceptable level of safety for constructed objects such as buildings and nonbuilding structures. The main purpose of building codes are to protect public health, safety and general welfare as they relate to the...
s mandate a series of measures to prevent backflow.
Backflow occurs for one of two reasons, either back pressure or back siphonage.
Back pressure is the result of a higher pressure in the system than in its supply, i.e. the system pressure has been increased by some means. This usually occurs in unvented heating systems, where thermal expansion increases the pressure.
Back siphonage is the result of supply pressure being lowered below that of the system. This occurs when a supply is interrupted or drained down.
Risk of contamination
The precise measures required to prevent backflow depend on the risk of contamination, i.e. the condition of the water in the connected system. This is categorized into different risk levels:- Category 1: No risk. Potable water
- Category 2: Aesthetic quality affected, e.g. water may have been heated
- Category 3: Slight hazard from substances of low toxicity, e.g. cold water storage tanks
- Category 4: Significant hazard, e.g. pesticides.
- Category 5: Serious health risk, e.g. human waste.
Measures to avoid backflow
Backflow prevention must be automatic, and so manual valves are not usually acceptable.Check valves
Automatic check valveCheck valve
A check valve, clack valve, non-return valve or one-way valve is a mechanical device, a valve, which normally allows fluid to flow through it in only one direction....
s 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
Double check valve
A double check valve or double check assembly is a backflow prevention device designed to protect water supplies from contamination. It is also a valve used in air brake systems on heavy trucks.Usage in Water Supply:...
(these are maunfactured as a convenient single unit, or even integrated into tap (faucet) fittings). Category 5 requires an air gap, not merely a valve. A recent introduction to the UK has been the Reduced Pressure Zone
Reduced pressure zone device
A Reduced Pressure Zone Device is a type of backflow prevention device used to protect water supplies from contamination...
(RPZ) valve, a form of double check valve where the intervening zone is drained and normally kept empty. 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 sprinkler
Fire sprinkler
A fire sprinkler system is an active fire protection measure, consisting of a water supply system, providing adequate pressure and flowrate to a water distribution piping system, onto which fire sprinklers are connected...
s where the system has an antifreeze additive.
Air gaps
Back siphonage may be prevented by use of an air gap. This may be a small gap, such as provided by a tundishTundish
The word tundish originates from a shallow wooden dish with an outlet channel, fitting into the bunghole of a tun or cask and forming a kind of funnel for filling it. These were originally used in brewing.- Plumbing :...
(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. The size of the acceptable gap also depends on the capacity of the incoming supply, such that a stuck-open flow cannot overfill the cistern and close the gap.
Air gaps may also protect against back pressure, and are generally favoured for this. 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 cisternCisternA cistern is a waterproof receptacle for holding liquids, usually water. Cisterns are often built to catch and store rainwater. Cisterns are distinguished from wells by their waterproof linings...
s, 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. Such silencing may be acceptable if it is a soft collapsible tube, as this cannot syphon. - Hand-held showerShowerA shower is an area in which one bathes underneath a spray of water.- History :...
s must have their hoses fastened such that the shower head cannot rest below the water level in a bath or basin.