To avoid contamination of the fresh water supply, it's critical that no dirty water is allowed to be drawn back (siphoned) into the water supply system. This can be especially problematic in outdoor faucet spigots (sill-cocks), where a garden hose may lie on the ground or in water puddles. Should a sudden drop in pressure somewhere in the water supply pipes occur, dirty water can be sucked back into the pipes through the garden hose, thus contaminating your home's fresh water.
Where and When Do You Need Them
Some anti-siphon devices are available for situations in which a hose may be left dangling from a faucet—such as outdoor garden spigots or a faucet in laundry washrooms. Most other applications have no such need since the faucet spout is a located a safe distance from any drain water. These anti-siphon devices are essentially one-way valves designed to stop the flow of potentially contaminated water back into the drinkable (potable) water supply.
Anti-siphoning capabilities are required by code on certain plumbing devices, such as exterior faucets (sill-cocks) or utility sink faucets. Anti-siphon devices protect against the possibility of a low water-pressure event in the supply line sucking contaminated water back into the potable water supply.
Exterior faucets (sill-cocks), anti-siphon protection eliminates the chance that a garden hose immersed in contaminated water or attached to a chemical sprayer can draw contamination back into the water supply system. Today's plumbing codes require that the outdoor faucet is of a special type that includes an anti-siphon mechanism as an integral part of the faucet itself. The anti-siphon device can be a vacuum breaker or an air gap; this fitting is usually located in front of the handle portion of the faucet. These special faucets also keep freezing cold temperatures away from the water supply pipe. The faucet head is attached to a 6- to 20-inch-long stem, and at the end of the stem there is a fitting allows for connection to a threaded, soldered, or PEX tubing water supply line. On these faucets, the valve itself is located at the back end of the long faucet stem, inside the house itself. This means that the valve is never subjected to freezing temperatures and cannot freeze and rupture, as old-style outdoor faucets are prone to do.
For indoor faucets, such as those on laundry utility sinks, it is technically illegal to leave a hose attached to the faucet so that it lies in the bottom of the sink. However, this is very often done anyway, and for these situations, you can buy a vacuum breaker attachment that screws onto the end of the faucet spout. When a hose is attached to the threaded end, the fitting provides anti-siphon protection by allowing air instead of contaminated water to suck back into the pipes when a negative pressure event occurs in the water supply system. The same spout attachment that is used on indoor utility sink faucets is sometimes used on outdoor faucets, as well, when the faucet does not have built-in anti-siphon protection. Technically this is not allowed by Code—a true anti-siphon faucet is required—but such faucet spout attachments do somewhat protect the outdoor faucet from contamination, and may be an interim solution until you can install an approved anti-siphon faucet.