backflow preventer?

Someone mentioned it's a good idea to install a backflow preventer on the house sewage line.
I searched google for such product and found a few different ones of different shapes and prices, some are called backwater preventer, some called flood guard. I still have no idea which is the correct type.
Could someone point me to the correct type to use for a family house? I have access to the sewage drain pipe (4-inch black plastic pipe) in the crawlspace and would like to install myself if it's just a matter of cutting the pipe and inserting the device.
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peter wrote:

It can be confusing. A backflow preventer is used on a potable water service (city supply).
A back WATER valve is the one used on a sewer lateral to prevent the city sewer from backing up into a house.
If you have access to the Main drain, you have it made! The latest PVC and ABS Backwater valves are slick indeed. See: http://www.superiorspecialty.com/Main/Plumbing/Legend/Thermoplastic.htm http://www.dafehr.com/Canplas/cisbwvalve.htm http://www.websitement-tm.com/z_websitement/Backwater-Valves.htm
Not needed in your crawl space, but this design features an extension to the ground surface, where the Main drain is buried, which allows the flapper to be pulled up for cleaning/repair/inspection without doing any digging! http://www.cleancheck.com/about.htm
Jim
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