Residential

What and Where?
Residential properties typically have a single backflow preventer installed between the water coming from your home, and your lawn and garden irrigation system.  The vast majority of these resemble the preventer pictured at right.  Some newer homes also have a preventer installed as part of a fire suppression system, but this is still rare.
 
Why?
Irrigation system backflow preventers are meant to keep the crud that your PVC pipes and sprinkler heads come in contact with from getting back into your home water pipes.  This includes fertilizers, pesticides, animal waste, dirt, etc.  If your home were to lose water pressure (e.g. you turn it off, the city turns it off, a water main breaks, the fire department opens the hydrant on the corner, etc.) and someone in your home turns on a faucet or flushes a toilet, water can be siphoned from your irrigation system back into the house pipes.  The result would hopefully only be noticeably gross water, but the possibility of sickness does exist if this water is consumed, especially by the very young or very old.
 
How?
In order to test these preventers, the water must be turned on at least to the preventer, if not your entire irrigation system.  A shut-off valve is usually located in your basement, crawl-space, or under a sink inside your home.  This valve is normally shut-off in the fall when your system is "blown-out" and turned back on in the spring when your system is restored to service.  The preventer can often be tested without putting your entire system in service by closing the valves (the blue handles shown in the picture at right) then turning on the shut-off valve inside the home (do not attempt without help if you are unsure).
 
Who and When?
It is very easy for these preventers to be tested without anyone being available at the residence.  No entry to your home is required.  However, if the preventer is in a fenced part of your yard, I will need gate access.  I am also good with dogs and appreciate the help they usually want to give :-), but that's certainly on a case-by-case basis.
 
Winter Freezing?
By far, the most common repairs I perform on residential preventers is caused by winter freezing.  Please be sure you, or the person draining your system in the fall, properly follow the recommended procedures for draining outdoor preventers.  You can find more information at my Freeze Protection page.
 
Many people choose to insulate their preventers.  This is fine, but not necessary if the preventer is correctly cleared of water in the fall.  If it isn't, the insulation is not likely to help when the deep freeze sets in.