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Sizing for 1-1/4" mainline?

First, sizing. None of the backflow devices seem to be sized at 1-1/4". What size of device would be suitable for my situation?

Also, my understanding was that double-check valves could be buried in a valve box underground. The ones that I looked at had a requirement that they be installed at least a foot above the surface. Is there a difference between underground/above ground installation? If I go above ground, will my PVC be able to suspend the unit robustly?

Size the backflow preventor to the actual flow, not the pipe. Odds are that a one-inch device has enough capacity for you. Since there are testcock openings on double check valve assemblies, their underground usage may not be recommended.

Thanks for the reply. From your response, I gather that:
<ul>
<li>A double-check valve or a LPZ unit that is specified as 1" will work with my 1-1/4 supply pipe</li>
<li>I have to buy the appropriate size unit based on flow.</li>
</ul>

Is there a forumla for verifying that the capacity of the unit is sufficient for my needs?

You have to get the specifications on the backflow preventer you are interested in. The key would be to look at the Capacity/Pressure drop tables.

As an example, you can find charts for Watts 007 Double Check Valves at http://www.watts.com/pdf/ES-007.pdf
If you look at page 4, you will see that a 1" device has about the same amount of pressure loss for flows from 5gpm to 15gpm, with a suggested maximum flow of about 21gpm.

Unless otherwise stated in installation instructions or required by local codes, duel checks can be installed underground so long as the test cocks are readily accessible for testing. However, in looking for the above listed documentation, I noticed they also had something called "duel checks with atmospheric vents". I don't know what that is, but I wonder if you were looking at something like that. To the best of my knowledge, Watts 007's can be installed underground.

As for the PVC, you don't want to go there. For one, PVC shouldn't be exposed to sunlight. Additionally, some local codes require pressurized PVC main lines to be buried. You would also be putting your self at greater risk of damage from weed-eaters or other things that might hit the supporting pipes.