Exactly. I'm all for keeping things simple, and I like the idea of the NO AC solenoid valve plumbed into the O2 tank, but I'm still struggling with the notion of that solenoid being under power at all times except when the main goes down. Cecil's engineer undoubtedly knows far more about it than I do, and if he says it's good to go then that eases my concern somewhat. But I still wish we could set this up to avoid the always-on AC current scenario.
An A-B 700-HL Interposing relay only draws 0.3 watts at 120 VAC and can be held on (energized) indefinitely. No problem handling a 12V or 24V DC solenoid valve running from a backup battery. The relay itself is about twice the size of my thumb nail and about 3/16" thick. I use quite a few of these that are continuously energized in factories and infrastructure, and never heard of a problem.
Not sure what the electrical inspectors follow in Indiana, but NEC states you need to use a class 2 power supply, and or, a SELV rated power supply for control circuits. I forgot that this may be going into a school. I doubt you can just wire this directly to a plug and stick it into an outlet without following NEC in either scenario.
NEC 2017 is next, and it has some additional YIKES for everyone. 2014 should have closed the DIY electrical hole up quite a bit, but from what I have heard about 2017...
