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Old 11-26-2014, 01:37 PM   #1
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canister diy question

I have plenty of stuff laying around to try a diy canister filter. One question that i have seen several places is head pressure to pump up to tank. Water seeks its own level, so if it is a closed system, would the pump (power head) just be moving water as it is in an under gravel system? My Grandfather had a gravity filter on a 55 back in the eighties with two inputs and one output. Once he got the siphon going it would run on its own with no pumps. So a 1" overflow with 1/2" return should work. I am using dollar store plastic scrubbies for media. I plan to use a 170gph powerhead. First try will be on my ten gallon then this spring will be a pond or bigger tank.
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Old 11-26-2014, 03:52 PM   #2
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How did it work without a pump?
Once the water level has reached equilibrium, any flow will stop.
The easiest way to make gravity work for you is to put the return pump/powerhead as close to the surface/end of the hose run as possible pulling water through the filter.
That way, as you pointed out, the water will seek it's own level and then the pump only has to move it a few inches and head pressure is no issue.
If you try to push the water out of the tank and through the filter, then you have to contend with head pressure and all that.


hope that made sense
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Old 11-26-2014, 04:23 PM   #3
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I'm not completely sure of your question, but pump sizing is related to static head pressure (water height, to over-simplify, in this case zero) as well as dynamic pressure from friction in the piping. There are formulae and tables for calculating equivalent head pressure based on pipe size, fittings, and (one of the most important) water flow velocity. Dynamic head pressure (or more properly "frictional head losses") vary as the square of the velocity, so larger pipe makes a huge difference.

Placement of the pump in a closed system makes no difference. But an overflow may not be a closed system in a pressure sense. Water falling freely into a sump for example "wastes" the energy as it falls. Pumping back up from a sump into the main tank then has the static head pressure of the difference in water levels in the main tank and sump, PLUS the dynamic head loss from friction due to length and size of pipes.

So your comment on overflow seems at odds with the comment on it being a closed system. Unless the overflow is kept completely full at all times, somehow.
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