On Sun, 08 Jul 2012 12:47:30 -0400, Reed wrote:

There is such a thing that is used in underground lawn sprinkler
systems. I have them installed on mine in both the constant pressure
runs and the timer controlled runs. On winter shutdown, you will need
to open highest faucets etc to allow air into lines, and releave
pressure in lines.

http://www.rainbird.com/homeowner/pr...DrainValve.htm

I read of these in irrigation systems. Called a "positive drain
valve". I could not find it under that name (couple of tries). Your
link appears to be the same / similar, named differently. In the
irrigation, the valve is on a long end run and drains into a sand dry
well. When the sprinklers turn off the pipes drain as the valve opens.

My pool solar has a drain valve at the top. When water pressure is
off the solar unit drains completely. No need for winterization.

Thanks for that link.