Can canister filters be placed at the same level as tank? - The Planted Tank Forum
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post #1 of 26 (permalink) Old 06-01-2004, 07:58 PM Thread Starter
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Can canister filters be placed at the same level as tank?

Hi, I tried searching for the answer, but couldn't find it. I vaguely remember someone saying that you can't place the eheim canister filter right next to your tank cuz it will harm the motor and that canisters always go underneath the tank for that reason. I just want to run that by you and see what you think. Thanks.

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post #2 of 26 (permalink) Old 06-01-2004, 08:06 PM
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Canister filters rely on Gravity to help force the water through the canister.. the impeller is simply there to help it along.. By having the filter at the same level as the tank you no longer have the benefit of a syphon fed water source so the canister has to work much harder to get water through the unit.


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post #3 of 26 (permalink) Old 06-01-2004, 08:21 PM
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Man, I hate to disagree all the time

I really doubt that gravity plays a role in that. The weight of the water going down the pipe is completely offset by the weight of the water going up. Gravity would only help if the water would flow out onto the carpet. But canister filters are closed systems, and the water surface is on the same level for inlet and outlet hose.

I am not saying that it is okay to put the canister on the same level, but not for gravity reasons, but for problems priming it and getting any air out of it.

In any case, here is the discussion that we had a little while ago...
https://www.plantedtank.net/forums/showthread.php?t=5445
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post #4 of 26 (permalink) Old 06-01-2004, 08:47 PM
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Quote:
Originally Posted by Wasserpest
Man, I hate to disagree all the time

I really doubt that gravity plays a role in that. The weight of the water going down the pipe is completely offset by the weight of the water going up. Gravity would only help if the water would flow out onto the carpet. But canister filters are closed systems, and the water surface is on the same level for inlet and outlet hose.

I am not saying that it is okay to put the canister on the same level, but not for gravity reasons, but for problems priming it and getting any air out of it.

In any case, here is the discussion that we had a little while ago...
https://www.plantedtank.net/forums/showthread.php?t=5445
Don't feel bad.. you actually added something to my argument ;-)

If you place the canister filter on the same level as the tank (asuming that the canister is same level as the water level..) The Canister then has to "Pull" the water up from the tank instead of allowing the Syphon to draw the water out.

Although Gravity does cancel out the net effects of hte syphon on teh return line.. this means that the impeller only has to apply enough force to the still water to keep it moving.. if the canister is above the syphon point then the impeller is doing a lot of extra work that would normally not be required.


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post #5 of 26 (permalink) Old 06-02-2004, 04:09 AM
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indeed... There is also a siphon on the output hose as well, making it equal. Raising the cannister will lessen the down-force on the in-hose, but also on the out hose, so it's the same performance. Also less water pressure, so less likelyhood of leaks.

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post #6 of 26 (permalink) Old 06-02-2004, 04:15 AM
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Everyone makes great points, but I must question why then do the major canister filter manufacturers recommend placing the filter at least 24inches below the surface of the water? I am not an engineer and dont play one on tv either, but I would suspect there is a reason for this....

Jason
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post #7 of 26 (permalink) Old 06-02-2004, 04:58 AM
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If your pump gets a tiny leak:

With the canister below the tank level the pump will still run and depending on the leak a siphon will start preventing the pump from airlocking.

With the canister above water level, the siphon can not start and the pump will eventually air-lock. This kills your pump from overtemp.
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post #8 of 26 (permalink) Old 06-02-2004, 06:27 AM
 
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I just recently moved my tank and the canister is now on the same surface as the tank itself. It was quite a PITA getting the air out and the "siphon" going, but after that, it worked quite well. I actually have more water flow now, most likely due to a huge reduction in head pressure.

It's true the pump gets a good gravity siphon reducing the force needed to get the water into the canister, but the pump still has to pump the water back UP the output. If it's too far below the tank, you won't get as good a flow (easily proveable).
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post #9 of 26 (permalink) Old 06-02-2004, 07:21 AM
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There is most defaintely a performance decrease based on the distance of the water path because despite the syphone creating a point of balance between teh inlet and outlet you are still fighting gravity to push the water up... This is not even close to the presure generated from moving water from a standing body at the same levels as the pump however (its not like calculating flow rate for a sump at all). Generally speaking you don't have to worry "too much" about distance from the surface of the water... I have a barely noticable performance difference (less then 50gph.. I measured) from 2-4 feet of distance from surface of the water using a Fluval 404.


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post #10 of 26 (permalink) Old 06-02-2004, 07:58 AM
 
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All I can add is that when I had my Fluval 304 on a tank where the water level was only a few inchs above the filter inlet hose, I had heck of a time getting the thing started. and even then I couldnt get all the air out of it.

Soon as I moved the tank and the filter is now about 3ft below the water line it runs like a dream, the air expels itself all I do is plug it in.

Ken
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post #11 of 26 (permalink) Old 06-02-2004, 03:29 PM
 
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i run an eheim 2224 at tank level. that is, the base of the ehiem is sat on the same table as the base of the tank.

the filter works perfectly, with no apparent loss of flow, no leaks, nothing.

HTH
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post #12 of 26 (permalink) Old 06-02-2004, 04:50 PM
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For me, as long as the impeller is lower than the water level things run fine, except that air don't get push out by siyphon as much as placing the canister under the tank. Apart from that, I found no difference.
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post #13 of 26 (permalink) Old 06-02-2004, 04:54 PM
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I can only repeat... no gravity involved after the filter is primed!

The longer the hoses, the more friction, the more the impeller/pump has to work to push water through. But it doesn't make the slightest difference if the canister is below, at, or above tank level.

Why do manufacturers suggest to place them below then? Because you can't get a siphon if the canister is placed above! Plus, usually ppl would want to place it into the stand anyway, so to avoid all of the "Can I place it in the stand, below the tank?" questions that's what they recommend.

Regarding the airleak, it really depends where the leak is, and again, totally independent from the placement of the canister. If the leak is before the impeller, the filter will airlock. If the leak is after the impeller, the carpet gets wet.

As long as the pump doesn't leak, and you get all of the air out, you can place it wherever. The longer the hoses, the more resistance, the less water comes out on the end.

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post #14 of 26 (permalink) Old 06-02-2004, 08:38 PM
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Quote:
Originally Posted by Wasserpest
I can only repeat... no gravity involved after the filter is primed!

Incorrect.

If you manage to get a get a filter primed "above" the natural syphon point of the tank (regardless of height.. it doesnt matter, as you said) then the pump is pulling the water instead of moving the flow... This puts more strain on the motor. Canister filters are designed to operate below the syphon point.. Thats why Syphon Presure down = Verticle lift up.... If there is no syphon presure down.. then the canister is doing all the work.

Now.. In practice... It is very difficult to get a canister filter "above" the natural syphon point in a tank unless the impeller is above the water line.. as long as the water line is above the impeller then the canister will work just fine..
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post #15 of 26 (permalink) Old 06-02-2004, 08:59 PM
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As long as the canister is below the water level in the aquarium you should be fine. If you try to raise the canister above the water level in the aquarium you could run into a host of problems already mentioned. Runing the filter just below the level of the water in the tank, rather than 3 feet lower, will reduce the head loss on the pump and allow for close to the maximum flow rate. Someone could always experiment and start a filter at 3 feet down then pick it up to even, then try it above. I don't know how fast you can achive air lock but it might be interesting to find out, with someone else's filter.

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