This is one of the most common questions I get from homeowners that I do Truth-in-Housing Evaluations for. If you're selling your home in Minneapolis (or a number of other cities in the Twin Cities area), you will likely hear about this. A vacuum breaker, commonly referred to as a backflow preventer, is a device that prevents the potable water in your home, and possibly even your neighborhood, from getting contaminated.
How could your water get contaminated? Picture this scenario: I want to mix up some vegetation killer, so I buy the concentrated stuff, pour it in to a bucket, then put the garden hose in the bucket to fill it. I turn the water on, but I get sidetracked with a plumbing project. I shut off the water to my house, and then open up the laundry faucet to drain the water out of the pipes. This will create a siphoning effect, which could actually suck the nasty chemicals in the garden sprayer back in to my home, contaminating the potable water. An even worse scenario would be the city doing work on the water pipes, and the chemical gets siphoned back in to the city's water supply, contaminating a whole neighborhood!
While these occurrences are not likely, it has happened many times throughout the country, and the cost to fix a contaminated water supply for a city is huge. The cost of a vacuum breaker is very small - about five dollars. While only a few of the metro cities enforce this, it's required throughout Minnesota by the Minnesota State Plumbing Code, section 4715.2100 (D).
The two most common places where these are installed are at sillcocks (what you connect your garden hose to) and at laundry sink faucets. An external vacuum breaker is required at sillcocks if they don't already have one built in. How do you know the difference? The photos below show one sillcock with, and one without an integral vacuum breaker. If a sillcock doesn't have the little mushroom cap, an external vacuum breaker is required. At laundry sink faucets, a vacuum breaker is needed if there are threads present that a garden hose could attach to.
Reuben Saltzman, Structure Tech Home Inspections