Quote Originally Posted by jaded
I was thinking about a dual float switch (no timer) The main float switch would open the solenoid when the water dropped and the backup float switch would always be "open" unless the water reached high enough to close the solenoid. In other words the backup would be an open circuit all the time unless the main switch failed... if that happened the rise in water level would close the solenoid. If they both fail I would have a mess...

Is there a problem with using this solenoid valve It has a brass fitting. I'm not sure what that would effect if anything
You're going to have the issue of your filter turning on-off-on-off constantly with the set up you described. I've heard of using a 2 float switch arrangement in which both have to be either on or off for the solenoid to fire. The switches are set at slightly different depths. This way, the level drops to open the first (higher) switch, nothing happens, it keeps dropping and opens the second switch, water comes on, starts rising, closes the second switch, stays on because both switches need to close, rises to close the first switch, then the water stays off until it drops below the second (lower) switch. I think some older thermostats used this type of principle to keep the AC/heat from constantly going on-off as the temp fluctuated slightly.

Don't know about the solenoid, just obviously anything with brass in it should go before the filter. I got mine from Mcmaster-Carr, but I have no idea what the number is. It's all on that monstro thread on reefcentral from a couple of years back "DIY auto top off with solenoid". As far as pre-filter pressure goes, you're looking at residencial line pressure, which I believe never exceeds 75 psi; probably quite a bit lower, and I'm sure any plumber could tell you.

Good luck!