The only way what you're saying would make sense, is if you *didn't* get the shut off kit I told you about 3 pages ago. Is that the case? See, I thought you DID get the shutoff kit, but were still concerned because it was turning on and off frequently, because even though waste water isn't going down the drain when shut off, it was still turning on and off very often.
This is because the float valve only knows when to shut off. The water level drops 1mm, or maybe even less, and that's enough to open the float valve. So the water drops 1mm, the pressure drops in the RO/DI output line, the solenoid kicks off, the RO/DI is on. 10 seconds later, the float valve is back up again, the line repressurizes, the RO/DI shuts off, and we wait until the water level drops again that 1mm.
This is *awesome* for your reef tank, because the salinity doesn't waver by the slightest. But, meanwhile, the RO/DI filter is working it's butt off.
This is why you want a "buffer" between the float valve and the source. A reservoir is this buffer. It releases water as it's needed, so the tank gets it's water on demand, but, the RO/DI doesn't have to do more work than is needed. Ie., the output rate is reasonably steady-state, but the input is on-demand.
What you're talking about what your friend has, he has the shutoff kit, so to speak (it may not be a "kit" per se, it was probably part of the RO/DI package). The pressure tank is the reservoir. It works on a slightly different principle, it has an air bladder inside which compresses as the tank fills, thus storing pressure as it releases the water. What I'm not sure of is, how much water can be bled off before the pressure starts to drop. If it can handle a couple gallons at a time before releasing the pressure, then that might be buffer enough. I once toyed around with a small pressure tank but it could not handle more than about a gallon, possibly even less, before it experienced a pressure drop. So it does depend on the tank a little bit I guess.
So you see, if you had the shut off kit, with a reservoir - what happens is you put a float valve in the reservoir. This float valve shuts off your RO/DI so you don't "have to remember to shut it off". The thing is, you will remember eventually, but in the meantime, your reservoir isn't overflowing everywhere (and you're not dumping water down the drain).
Basically it's a low tech, low cost, fail safe mechanism. No reliance on electricity, and yet, full blown automation. But -- it only works if you have the shut off kit. If you don't have that - well then - yes I agree - you took something that was supposed to be simple and made it completely ridiculous (are you sure you're not an engineer?
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