ThomasComstock's blog
14 Quick, Easy & Inexpensive Ways to Improve You Home Inspection Report: Tip #1
They are called Vacuum Breakers, Backflow Preventers, or Anti-Siphon Devices and they are now Code for all structures with outdoor hose bibs or faucets. These small devices that easily thread onto the "hose bib" or
outdoor faucet, act as a one way valve, preventing the water that has already flowed past it from being able to be "siphoned" back into the potable water system. Many of today’s fertilizers, weed killers and insecticides come bottled within a "attach a hose" applicator, and any sudden loss of pressure could result in the chemicals being sucked back into the water supply, creating a dangerous, poisonous situation for anyone that might drink from that water 
system. Vacuum Breakers are designed to break that suction and prevent anything from flowing back into our drinking water.
Vacuum Breakers are required on every outside faucet, or "Hose Bib". They can be found easily at any hardware store or builders warehouse, like Lowe's or Home Depot, ranging in price from $6.00 to $8.00 as of 10/01/06. To install a vacuum breaker, simply thread it onto the faucet as you would a regular hose and tighten the set screw so it will not come loose when you tighten and un-tighten your hose. The male end of the Vacuum Breaker will accept your hose, just as the regular faucet did.
Once the set screw is tightened, you are done. You have protected the potable water supply in your area, and you have knocked one more "In Need Of Repair" from the Home Inspectors report.
