Automatic Water Changes and Pump Recommendation

norm3000

norm3000

AC Members
Sep 11, 2004
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Clawson, MI
I'm working on a system to do automatic 40% water changes 3 times a week in preparation for a Discus tank. (plus a manual waterchange and vacuuming every weekend). It's pretty simple really, I've got a Reverse Osmosis unit feeding into a 40 gallon rubbermaid tub with a float switch to turn it off when it fills up. Next are two pumps that run on timers. One pumps out 20% of the water, that's all it can do because the PVC pipe going into the tank only goes down 40% of the way. The second pump pumps water from the Rubbermaid tub back into the tank. This one is triggered by a timer but has an additional float switch to make sure the tank doesn't overflow. This one also has a check valve to make sure that once the pump turns off gravity does start siphoning the water back out of the tank and into the rubbermaid.

Ok, now to my problem and question. The pump that drains the tank is connected to a "U" section of PVC that hangs over the edge of the tank. The pump runs until it's dry making sure it's drained 40% and making sure that at the end of the cycle the drain hose is empty and won't siphon the tank further. Problem is that the pump I bought isn't self priming so once it runs dry it won't work until I prime it manually again. Any suggestions on a pump here that would be able to pull the water through the PVC "U" and prime the pump even when dry? The pump would obviously need to be able to run dry for at least a few minutes without overheating and damaging itself.

Suggestions would be very appreciated.
 
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Houston, we have a problem ! I'm not aware of ANY affordable plastic construction pump which is capable of 'automatically' repriming with the amount of air volume trapped in your U-tube approach. The only affordable, dependable and practical option that I can think of is to hide a small submersible pump positioned 40% of the way down into your tank which will then 'push' water up the U-tube to your drain. To avoid an undesireable potential siphoning effect, you'll also need to make yourself a 'vacuum breaker' (like a Y fitting and a standpipe with the height of the Y matching the normal surface level of your tank) to guarantee that the flow to the drain will stop when the pump is shut off/runs dry.
 
Thanks for the feedback Melonie. I was hoping to avoid a pump in the tank for esthetic reasons, but looks like I'll have to do it. The pump that I have has a 1/2 MTF that I can attach directly to the U tube and hang at the right height. I can make a box out of smoke or white acrylic to house the whole thing so you don't see a pump in the back of the tank.

As far as the vacuum breaker, I'm not sure that a Y in the PVC would work when it's under pressure from the pump. The waters going to go "the path of least resistance" and if there's an open Y I think it'll go out both ends of the Y. If it were just siphoning we were talking about I think that would work but not under pressure.

Regardless, I'm thinking that if as long as the pump runs until it is dry the out tube will fill with air and I won't have any siphoning problems. Worst case the tank drains 40%, no catastrophe. If all else fails I think I can buy valves that are normally closed and won't open unless under pressure from the pump.
 
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You may be correct about the Y vacuum breaker if your pump flow rate is higher than the 'natural' drainage flow rate of whatever size drain pipe you are using. I agree that a spring loaded check valve like MarineDepot offers (which requires 1/2 lb + of pump pressure to force open) is a simpler and safer alternative.
 
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I'm not an expert on mechanics (or for that matter chemistry of tanks) but the other concern I would have is adding 100% pure RO water, how are you going to add the elemnts to it to have a balanced tank? ie keep the alkalinity and PH in range.
 
acefred - yeah, the next step is to add tap water to the resevoir to make the mixure 50/50 RO and Tap water. I'll have to use a solenoid valve (automatic sprinkler valve) for that and again a float switch to insure it will only fill 50% of the resevoir. The water will be sitting for 24 hours so the chlorine will evaporate overnight.

mvigor - I'm using digital timers so I don't expect that they will make any gross errors. Still, the worst that could happen if one malfuntioned would be my tank would get drained 40%. The float switched insure that neither the resevoir nor the tank can ever overfill. In the long run I'd like to program my own Integrated Circuit to control everything. That way there is one unit that handles all the timing.

svic - thanks for links. I'll read them over for some ideas.

What seemed simple at first has gotten fairly complicated but I'm still confident it'll work. Someone else said it best. "Laziness is the mother of invention."

-Norm
 
norm3000 said:
I'll have to use a solenoid valve (automatic sprinkler valve) for that and again a float switch to insure it will only fill 50% of the resevoir. The water will be sitting for 24 hours so the chlorine will evaporate overnight.
Click to expand...

I was getting ready to say that. I thought of doing an automatic water changing system one day when I can afford a house and my big tank

The system I have in my head works primarily on siphons and checkvalves with a programmable multlidevice timer switch ( not aware of and on the market, probably have to flash a rom and build my own .. but first i got to learn how to flash a rom :) )

I'll be briefish since most of this is theoretical. You will need 2 extra tanks/tubs half the capacity of your tank, one positioned above your waterline and one below. run a syphon tube from each to your main tank with a soloniod switch on both tubes. then the following happens

1: top tank gets filled with water using plain water pressure. safety devices as needed (translation not figured that out yet :) )

2: air pumps comes on running bubble wands in top tank to facilitate dechlorination ( or a premeasured amount of declorinator is dumped in before its filled which in case no delay is needed)

3: 24hrs later the syphon to the drain tank is openedd allowing a predeturmined amount of water to drain out of main tank (insert safety devices). this tank is modified (???) so that the overflow goes into a drain. reason. im parinoid it will suck something up important (fish plant, etc) so i want a "trashcan" effect to borrow from computer terms.

4: drain syphon solinoid is closed, top one is opened, top tank drains into main tank bringing it full circle.

system shuts down for whatever duration then it repeats or maybe it could be a oneshot circut. sort of like flushing toilet.. push a button and it works.

heaters probably be a good idea in the top tank.

I hope you find something useful in this information.
 
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