hijinks

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What are people's feeling about auto topping off right from the ro/di unit?

I know using a electrical switch there is a chance for malfunction. I am using a mechanical one provided by Air Water & Ice.
 

Simon Garratt

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Heres a system that ive been using for a few years now that has proved completely reliable using two industrial solenoid valves and a float vlave plumbed directly to the sump.

It looks more complicated than it is to be honest. The basic principle is to deliver several timed inputs of RO water per day, with a short flush to waste prior to each run so that the buildup of high TDS water that accumulates after the membrane whilst on standby is flushed away prior to the DI pod to extend its life.

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Hers how it runs on my 490 gallon system.

Im running S2 for 15 min every 6 hours (the actual time required to clear the 'crappy water' thats built up on standby is about 5 mins until the TDS drops to around 6.0 on the output of the membrane...this water runs strait to waste) imediately after or within a minuet or two of this valve closing, S1 opens and allows the now lower TDS water to run to the DI pod and on to the float valve for 15 min which is about 5 min longer than required to top up the sump to its required level, and to allow for variances in evap dependant on the weather etc. As soon as the float valve is up and closed, the remainder simply flows the alternate route to waste..

for the six hours between fills, the system is in standby mode with both solenoids closed and the auto shut off closed keeping the unit under pressure.

The two one way valves on the flush line and the top up bypass line which link into the main waste line are there to prevent high TDS waste water back syphoning into the good side.

So the net result, is that im only just using a little more water than required to top up the system which flows to waste as soon as the float valve closes off, and Im only ever putting low TDS water into the DI pod becouse the initial crap that you get when first firing the membrane up each time is diverted to waste via the second solenoid. I usually have to re do the DI resin once every 3 months, which isnt bad considering the system evaporates nearly 5 gallons a day. thats about 140 gallons in a month.

As for faults....

well there are the following concerns to take into account..

Stuck solenoid...: It will either carry on running, or it wont...In which case the float valve becomes the second line of defence, 'if S1 solenoid jams on', it will simply keep running the surplus to waste..Chances are though, that if a solenoid is to stick, it will do it in the closed position (the springs on these 240V solenoids are bloody strong)..

Stuck float valve: likewise it will either be open or closed..

In the event of it staying open... The 4 x 15 min cycles in any 24 hour period ensure that even if the system was to stick on and the foat valve jammed open, it would take a week to oveflow the system past its normal evaporation capacity by about 10 gallons which would see a drop in salinity from about 1.026 to 1.024 on this sort of volume.

so all in all, its relatively bullet proof imo compared to some other methods.


The real trick is to set the timings up so that the on period is only just in exess of what you need to account for variances in evaporation rate and to devide it over 24 hours into 4 sessions to dstribute it more evenly and minimise salinity fluctuations.

Chances are though that with both a float valve and a solenoid controling things, you would have to have a multiple failure to have any real issues. chances are that if a solenoid fails, it will usually fail in the off position which simply means no top up at all which is far better than a flooded tank or home imo.

I dont see any reason this couldnt be employed on smaller systems just by simply choosing a lower output membrane and altering the timer settings.

One of the biggest advantages to this method imo, is that there is no water sitting around in vats or tanks etc collecting dust or pulling contaminants from containers etc, so you are guarenteed (as long as you keep an eye on your DI resin and TDS readings) to be topping up with 100% pure water.

Just thought this might be helpful to some.

regards

Simon.
 
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