Solenoid for Outdoor Tankless

I'm looking at installing an outdoor tankless water heater (Rinnai). The installation manual recommends installing auto drain down solenoids so that if the power fails the unit will drain and not freeze. Has anyone done this type of install? What type of solenoids did you use and where did you get them? Any advice you can provide would be greatly appreciated.

I'm located in the Atlanta, GA area, so we rarely get temperatures below freezing... but I'd like to be prepared just in case.

valve

You need a normally open valve that is kept closed as long as there is power to it. IF the power fails the valve reverts to its open position and causes drainage.

Here's a link to the installation documentation...

http://www.foreverhotwater.com/pdf/r85e-io.pdf

Page 34 shows a typical install with freeze protection. With this layout, there are (2) solenoids installed. Am I correct in that if power is lost there will also be no water available down line at fixtures? Also, will all the water down line in the hot water pipe also drain?

Is this overkill or pretty much standard for outdoor units? Also, is there anywhere online you can recommend for me to purchase these solenoids?

Sorry for all the questions - I just want to do things the correct way and I'm not a plumbing expert (as you've probably already determined). However, I'm a pretty advanced DIY'er and willing to give this a try. Thanks for the help.

I would not mess with solenoid valves. What is the chance of simultaneous:

Power outage AND freezing weather in Atlanta AND you are not there to deal with it.

The valves must be continuously powered.

Maybe two valves sitting there powered open for a year or two between power failures will work, and maybe they won't.

If the improbable happens call a neighbor to open a drain valve.