Need help with sump/fuge

[Delphinus]

It's not necessarily a good idea to hook up the RO/DI to a tank anyhow, even if you have the solenoid shutoff control. The problem is that the evaporation rate is 24/7, so you would be constantly cycling the RO/DI on and off for short bursts. And actually, a higher GPD rating of the membrane compounds this, as you'll have it on for only a few seconds at a time before the level is maintained again. This is murder on a RO membrane and will contribute to early failure of the membrane.

As far as I'm concerned, the auto shutoff feature of a RO/DI unit should be used to avoid spillage when filling your reservoir. I.e. if it takes 10 hours to fill your reservoir you can just turn it on and forget about it, not have to worry about remember 10.5 hours later that you now have an overflowing reservoir.

You could probably make it a little better by expanding your deadband by creative use of float switches but even then .. it's still the same problem of repeated on/off cycles. I.e., say 40gpd replacement water per week, your membrane will last you much longer if you do one production run of 40g instead of 100 production runs of .4g.

Thus my recommendation is float valves on your sump fed by a reservoir, RO/DI only to fill the reservoir using a float valve on the reservoir and the auto shutoff kit on the RO/DI as a luxury item to afford you peace of mind when manually filling that reservoir as needed.

PS. My $0.02 as far as float valves failing .. generally speaking this is a very rare occurence if set up properly and maintained regularly with diligence. Also the likelihood of failing is more in the nature of a clog, rather than failing stuck open. So if you find your float valve clogged, you just clean it and away you go.