# Why is eheim 2217 inflow/ outflow different?



## msawdey (Apr 6, 2009)

really? i had no idea.. both of my eheims are exactly the same... if i had to guess which one was larger it was the outflow. Maybe its for flow reasons due to the motor being to strong for the unit? Interested in responses


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## cawolf86 (Dec 31, 2010)

A lot of places sell the inflow and outflows separately.

I have an Eheim with the same size inflow and outflow but I do know that restricting INTAKE flow may be bad for the motor. If you have more water leaving the filter than entering you may have times when the motor is running dry. So having a larger intake makes sense.


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## PlantedRich (Jul 21, 2010)

Looking at the economics may show some light. The Ehiem motor is built to EU standards which require much more economy than US standards. One way to get the economy which Eheim does is to have the intake help by syphoning more water down to the canister than the output has to push up. This makes the motor run better and require less power. The energy used by an Eheim compared to a similar US brand is quite remarkable. On their smaller canisters like 2211 and 2213, the hoses are the same size. I would guess they found the power savings not enough to bother with the size difference.


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## msawdey (Apr 6, 2009)

cawolf86 said:


> A lot of places sell the inflow and outflows separately.
> 
> I have an Eheim with the same size inflow and outflow but I do know that restricting INTAKE flow may be bad for the motor. If you have more water leaving the filter than entering you may have times when the motor is running dry. So having a larger intake makes sense.


Your right.. i said it backwards


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## AndreyT (Apr 28, 2011)

cawolf86 said:


> I have an Eheim with the same size inflow and outflow but I do know that restricting INTAKE flow may be bad for the motor. If you have more water leaving the filter than entering you may have times when the motor is running dry. So having a larger intake makes sense.


"Running dry"? That's not really possible. Canister filters are fully-flooded watertight/airtight systems with no air inside. In order to introduce a new dry volume into such a watertight/airtight system you'd need a pump that can counteract atmospheric pressure. A lab-grade vacuum pump would be capable of doing something like that. A impeller motor in a canister pump is completely and utterly incapable of even coming close to creating such condition.

To the OP: As long as you are operating within the capabilities of the motor, using a smaller diameter output tube will result in higher speed of the output flow (while keeping the throughput unchanged). Whoever installed that tube might have been aiming for that. Or maybe they used a smaller tube for no particular reason at all.


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## jreich (Feb 11, 2009)

AndreyT said:


> "Running dry"? That's not really possible. Canister filters are fully-flooded watertight/airtight systems with no air inside. In order to introduce a new dry volume into such a watertight/airtight system you'd need a pump that can counteract atmospheric pressure. A lab-grade vacuum pump would be capable of doing something like that. A impeller motor in a canister pump is completely and utterly incapable of even coming close to creating such condition.
> 
> To the OP: As long as you are operating within the capabilities of the motor, using a smaller diameter output tube will result in higher speed of the output flow (while keeping the throughput unchanged). Whoever installed that tube might have been aiming for that. Or maybe they used a smaller tube for no particular reason at all.


I think he ment cavetation whhich is totally possible in a canister. Thats why they tell u not to restrict the imput, only the output. I have had cavetation issues in a fully primed leak free 2213 due to restricted intake.

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## cawolf86 (Dec 31, 2010)

Sorry - I understand running dry won't really happen in a closed system. It will increase the workload of the motor as it will have to work harder and possibly against a stronger vacuum to move the water.


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## PlantedRich (Jul 21, 2010)

Are you folks missing that this is normal for 2217 filters? It isn't something somebody changed but the filter as designed. For the smaller Eheim Classics, the tubing is the same size but once you get to the 2217, they provide different sizes. I think the original question was why do they do it that way.


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## sayurasem (Jun 17, 2011)

PlantedRich said:


> Are you folks missing that this is normal for 2217 filters? It isn't something somebody changed but the filter as designed. For the smaller Eheim Classics, the tubing is the same size but once you get to the 2217, they provide different sizes. I think the original question was why do they do it that way.


correct! I believe all eheim classic 2217 has input of 5/8" (17mm pipes), and output of 1/2" (13mm pipes). Now I know where to buy the pipes...

Now I'm interested why did the German do it like this? Why can't both input/ output is 5/8"?


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## Zefrik (Oct 23, 2011)

I have wondered that as well. Would it have anything to do with the fact that there is less friction of the water going down into the canister and when the water in the canister filter is pumped back the smaller tubing assists in keeping a strong steady flow without making the pump work to hard. Does that make sense?


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## 150EH (Dec 6, 2004)

I think it has more to do with flow, if the out let was larger it would be too slow and lazy or in other words a 2215 with more media. If you think about it a 2217 is almost the same as the 2215, it draws 5 more watts of power and that's nothing but the flow is almost double and it all gets done with the 5 watts, 3 more impeller blades, and the larger intake.


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## deeda (Jun 28, 2005)

It's preferable to have a larger intake than an output in order to guarantee there is sufficient water to keep the impeller lubricated and prevent cavitation. This is just a simple explanation of why there is a difference.

There are formulas that the engineers use when deciding the design of various filters that is dependent on the type of pump used, the GPH and various other factors.


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## AndreyT (Apr 28, 2011)

Zefrik said:


> I have wondered that as well. Would it have anything to do with the fact that there is less friction of the water going down into the canister and when the water in the canister filter is pumped back the smaller tubing assists in keeping a strong steady flow without making the pump work to hard. Does that make sense?


Well, in a canister filter water is never really "pumped" back (if by "pumping" you mean "spending efforts on lifting water up"). Canister filter is sealed fully-flooded system. In such system both the intake and the output tube act as siphons, which perfectly and precisely counterbalance each other. This perfect counterbalancing is what essentially makes water in these tubes _weightless_. The pump does not have to spend _any effort at all_ to actually lift the water back to the aquarium level. 

This is actually the fundamental difference between sealed (canister) filters and non-sealed (wet-dry) filters. In wet-dry filter the pump has to _lift_ the water back to the tank. In canister filter the "lifting" comes for free: the pump doesn't have to spend any effort on it. This is the reason why a canister filter does not "feel" the height difference between the canister and the tank: regardless of how big that difference is, the pump does not have to work harder, since in canister filter the water is essentially "weightless".

In a canister filter the pump is only responsible for pushing the water through the media and for overcoming the water drag in the tubes. That's it. For this reason, having smaller output tubing cannot really help the pump to lift the water back into the tank, since the effort was zero anyway.


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## sayurasem (Jun 17, 2011)

AndreyT said:


> In a canister filter the pump is only responsible for pushing the water through the media and for overcoming the water drag in the tubes. That's it. For this reason, having smaller output tubing cannot really help the pump to lift the water back into the tank, since the effort was zero anyway.


Hmmm so are you saying smaller output has nothing to do with increasing flow?


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## deeda (Jun 28, 2005)

Canister flow is determined by the pump which involves the size of the impeller blades, the impeller chamber, suction port size and the number of watts (power); the amount, type and stacking order of the media can also directly impact how many gallons of water per hour the pump actually puts out, plus the dirtier the media gets, the less GPH output. 

The higher the water needs to be returned to the aquarium is also a factor and is common to all pumps, whether they are just a pump or are an integral part of a canister filter. The more water that is in the return piping decreases the lifting ability of the pump due to the weight of the water.


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## AndreyT (Apr 28, 2011)

deeda said:


> Canister flow is determined by the pump which involves the size of the impeller blades, the impeller chamber, suction port size and the number of watts (power); the amount, type and stacking order of the media can also directly impact how many gallons of water per hour the pump actually puts out, plus the dirtier the media gets, the less GPH output.


Correct.



deeda said:


> The higher the water needs to be returned to the aquarium is also a factor and is common to all pumps, whether they are just a pump or are an integral part of a canister filter. The more water that is in the return piping decreases the lifting ability of the pump due to the weight of the water.


Absolutely incorrect. 

Again, in a canister filter the pump does not have to work against the weight of the water. Amount of water in the return piping has absolutely no effect on the lifting ability of the pump. The pump, again, does not do any "lifting" at all in a canister filter. This is a fundamental property of a canister filter, the clever point around which it is designed. And this is why very large canister filters are able to get by with relatively tiny impeller pumps. This is, BTW, not some rocket science, but rather very basic high-school physics. 

Increasing the length of any tubing (intake or output) might make the pump to work harder simply because longer tubing creates more _drag_. But this a completely different issue, of course, which has nothing to do with the weight of the water.

Canister filter do have limitations with regard to the height difference, but these limitations have nothing to do with the pump. The limitations stem from the fact that bigger height difference means that the weight of the water in both tubes will produce higher water pressure inside the canister. If the pressure gets too high, the seals might give way and start leaking. I.e. these limitations are about seals, not about pump and its having to "lift" the water.


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