[gordieax]

Thanks for the replies. tedindystries, you are 100% correct in stating the original irrigation system installer screwed up big time not providing a blowout port; this was done over 20 yrs. ago. Also the existing hose bib at the bottom of the piping is useless for blowouts b/c it's not post backflow preventer.
A follow-up question: Why would I need both a ball valve and also a hose bib installed post backflow preventer? Wouldn't just the hose bib be enough for me to attach the compressor hose for the blow out?
Thanks

 

[gordieax]

Well I was thinking to protect the backflow unit from the compressed air, to isolate it, then open the bib after it and all the air has to go towards the zone valves. I thought that because you said " I've always blown out the system using the test cock on the right of the backflow preventer, however, after having had a new backflow preventer manifold installed last spring I've learned that blowing out using the test cock can cause damage to the valves. " Is there still a test cock? How have you "learned" that blowing out would damage a valve and specifically, what valve?

Yes there is still a test cock to the right of the back flow preventer but it has a different fitting than the old one. The new test cock is a flare fitting so in my search to find a female match to connect the compressor hose both online and plumbing sources have stated it’s not advisable to blowout thru test cock for it can damage the back flow preventer valve. Who knew?? So now I’m seeking the best way to add what’s needed to properly blow out the system.

 

[gordieax]

This is why I thought it would be a good idea to isolate this backflow-preventer with a ball valve and then a hose bib which would allow an air hose with adapter to attach without putting air pressure on the backflow preventer.

Sounds like good advice to follow. Thanks much.