Backflow Preventers Frequently Asked Questions

1. What is back siphonage?
2. What factors can cause back siphonage?
3. What is back pressure?
4. What factors can cause a back pressure backflow condition?
5. What is a cross-connection?
6. What is the most common form of a cross-connection?
7. What is an atmospheric vacuum breaker?
8. Will an anti-siphon vacuum breaker protect against a back pressure backflow condition?
9. What is continuous pressure?
10. Can an atmospheric vacuum breaker be used under continuous pressure?
11. Where is an atmospheric vacuum breaker used?
12. Where is a hose tap vacuum breaker used?
13. Where is a pressure vacuum breaker used?
14. What are typical applications for double check valve assemblies?
15. Where is a double check valve assembly used?
16. What is the concern over the contamination of safe drinking water by connection to a fire protection system?

1. What is back siphonage?

Back siphonage is the reversal of normal flow in a system caused by a negative pressure (vacuum or partial vacuum) in the supply piping.

2. What factors can cause back siphonage?

Back siphonage can be created when there is a stoppage of the water supply due to nearby fire-fighting, repairs or breaks in city main, etc. The effect is similar to the sipping of a drink through a straw, which induces a flow in the opposite direction.

3. What is backpressure?

Backpressure backflow is the reversal of normal flow in a system due to an increase in the downstream pressure above that of the supply pressure.

4. What factors can cause a back pressure backflow condition?

Back pressure backflow is created whenever the downstream pressure exceeds the supply pressure which is possible in installations such as heating systems, elevated tanks, and pressure producing systems. An example would be a space-heating boiler operating under 103 to 138 kPa coincidental with a reduction of the city water supply below such pressure (or higher in most commercial boilers). As water tends to flow in the direction of least resistance, a back pressure backflow condition would be created and the contaminated boiler water would flow into the potable water supply.

5. What is a cross-connection?

A cross-connection is a direct arrangement of a piping line which allows the potable water supply to be connected to a line which contains a contaminant. An example is the common garden hose attached to a tap with the end of the hose lying in a pool of muddy water. Another example would be a garden hose attached to a tap with the end of the hose submerged in a tub full of detergent.

6. What is the most common form of a cross-connection?

Ironically, the ordinary garden hose is the most common offender as it can be easily connected to the potable water supply and used for a variety of potentially dangerous applications.

7. What is an atmospheric vacuum breaker?

The most commonly used atmospheric anti-siphon vacuum breakers incorporate an atmospheric vent in combination with a check valve. Its operation depends on a supply of potable water to seal off the atmospheric vent, admitting the water to downstream equipment. If a negative pressure develops in the supply line, the loss of pressure permits the check valve to drop sealing the orifice while at the same time the vent opens admitting air to the system to break the vacuum.

8. Will an anti-siphon vacuum breaker protect against a back pressure backflow condition?

Absolutely not! If there is an increase in the downstream pressure over that of the supply pressure, the check valve would tend to "modulate" thus permitting the backflow of contaminated water to pass through the orifice into the potable water supply line.

9. What is continuous pressure?

This term is applied to an installation in which the pressure is being supplied continuously to a backflow preventer for periods of over 12 hours at a time. Laboratory faucet equipment, for example, is entirely suitable for a non-pressure, atmospheric anti-siphon vacuum breaker because the supply is periodically being turned on and shut off. A vacuum breaker should never be subjected to continuous pressure unless it is of the continuous pressure type and clearly identified for this service.

10. Can an atmospheric vacuum breaker be used under continuous pressure?

No! Codes do not permit this as the device could become "frozen", and not function under an emergency condition.

11. Where is an atmospheric vacuum breaker used?

Atmospheric vacuum breakers may be used only on connections to a non-potable system where the vacuum breaker is never subjected to back pressure, and is installed on the discharge side of the last control valve. It must be installed above the usage point. It cannot be used under continuous pressure. (Also see #7)

12. Where is a hose tap vacuum breaker used?

Hose tap vacuum breakers are small inexpensive devices with hose connections which are simply attached to taps wherever there is a possibility of a hose being attached which could be introduced to a contaminant. However, like the atmospheric vacuum breaker they should not be used under continuous pressure.

13. Where is a pressure vacuum breaker used?

Pressure vacuum breakers may be used as protection for connections to all types of non-potable systems where the vacuum breakers are not subject to back pressure. these units may be used under continuous supply pressure. They must be installed above the usage point. (Spill resistant models for indoor use are also available).

14. What are typical applications for double check valve assemblies?

They may be used where the degree of hazard is low, meaning the non-potable source is polluted rather than contaminated.

15. Where is a double check valve assembly used?

They can be used anywhere you need to protect direct connections from foreign material that might enter the potable system in concentration which does not constitute a health hazard.

16. What is the concern over the contamination of safe drinking water by connection to a fire protection system?

Fire protection systems are most commonly constructed of uncoated black iron pipe which is considered by plumbing officials to be unacceptable as water system piping material.