# why is 'height' relevant for pumps?



## greenink (31 Aug 2011)

I'm confused about why external pumps and filters have maximum 'head heights' - the distance below the surface of the tank that you can put them. (My tank is mounted in a wall, pretty high up, and I need a big height gap between the surface of the tank and the filter/pumps).

Why aren't the only relevant factors the pressure differential between the intake and the output of a pump (e.g. if one is much higher), and the length and curvature of the pipe (for flow resistance) regardless of whether it goes up or down? Any filter or pump is a closed circuit (in my understanding) and so should have equal pressure throughout once it's filled with water - so it shouldn't make any difference at all whether the pump is below or at the same level as the surface of the tank.

Basically, for each ml of water you pump up, another ml is coming down, equalising the pressure on the pump... no?   

Maybe the issue is to do with initial priming...? Can anyone who understands physics properly explain? Or is it something manufacturers just write on pumps for no reason?


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## Tom (31 Aug 2011)

I think it's simply because the higher the tank compared to the filter, the less flow you will get. If you build a waterfall, you will need to get a more powerful pump to get the water to the top. 

If you go high enough, the flow will more or less stop.


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## greenink (31 Aug 2011)

But you're not taking water from the bottom of the waterfall to the top... In a tank you're taking it from the top to the top - I think!


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## Tom (31 Aug 2011)

Yes, but it still has to be pumped back up again  Take an external filter's rated power. Then when you put tubes on/media in, it drops. Then lift those tubes to the height of the tank and measure again. It will drop more. 

If you tried syphoning water down into the filter and you had the outlet at the same level as the inlet, it would not run out the other end - it would stop. With the pump running, there is still a point where it will struggle to deliver the water back up to the highest point.


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## greenink (31 Aug 2011)

OK - but why? (Sorry to sounds like a small child)


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## foxfish (31 Aug 2011)

You have a good point - water is in effect being moved around a closed circuit so in theory, once the siphon is started, there is no head of pressure!
I guesse there is lots of restriction within the pipe work that would limit flow?
How about a centre heating pump in your house, that can circulate water two storeys high, although that is a true closed circuit, I don't know how that is so different?


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## ShaunWhite (31 Aug 2011)

Mike, you're correct in your assumptions, if you take and return water to the same body of water, the distance the pump is below the surface of the water is irrelevant to the pumping head.

However, the more pipework in the system the greater the frictional loss. The greater the distance below water level the greater the static pressure on the pump, perhaps it's to do with the pump seals?

Cheers

Shaun


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## clonitza (31 Aug 2011)

Google is your best friend:
http://www.aquariumpros.com/articles/headpress.shtml


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## ShaunWhite (31 Aug 2011)

I design pumping systems for a living and the pumps we use have a max rated pressure, not to do with the head they produce but associated with the pressure the seals etc can withstand, so i guess it's the same with external canister filters etc?


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## foxfish (31 Aug 2011)

I built a four foot deep tank a few years back, the tank was on a two foot six tall plinth so there was a 6'.6'' head involved. I could not find any filter on the market that would guarantee the 6'.6'' of water pressure would not cause leaks from the canister.
So it was not the pumps flow rate but the integrity of the canister itself that restricted the filters use.


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## spyder (31 Aug 2011)

To get an idea get your garden hose pipe full of water trailing on the ground,now blow the water out of it. Easy huh.

Phase 2. Fill hose but have an assistant hold the pipe out of an upstairs window. Now blow hard. It's more than likely you cant do it and end up getting soaked.

The higher the pump has to pump the water the harder it gets. Water is quite heavy and gravity is a hmmmm


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## greenink (31 Aug 2011)

foxfish said:
			
		

> I built a four foot deep tank a few years back, the tank was on a two foot six tall plinth so there was a 6'.6'' head involved. I could not find any filter on the market that would guarantee the 6'.6'' of water pressure would not cause leaks from the canister.
> So it was not the pumps flow rate but the integrity of the canister itself that restricted the filters use.



Aha! That makes sense


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## greenink (31 Aug 2011)

Have the official answer from Eheim coming in a second...


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## greenink (1 Sep 2011)

And here it is:



> -----Ursprüngliche Nachricht-----
> Von: Formular Internet
> Gesendet: Mittwoch, 31. August 2011 21:51
> An: technik
> ...



Very impressed with Eheim, have to say. But still don't understand the physics!


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## greenink (1 Sep 2011)

Here's where I move everything and get soaked...


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## greenink (18 Sep 2011)

Have put in a new jet 1200 pump well below (6ft) the water level, with quite long pipes and pipe clips. Flow is much improved and no sign of any leak issues at all.


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## cheebs (18 Sep 2011)

^^ Love home experimenting 

I wish I had seen this thread earlier. but, as you have found out, head pressure (AKA lifting height) relates the the ability of the pump to lift water in an open loop (where the output of the pump is not below the surface of the water that the input is drawing from). In a closed loop situation, it can be thought of as its ability to push past obstructions. You are absolutely correct, once a siphon is active, it doesn't need to "lift" that amount of water, as its being pushed up to the same level as the tank water by atmospheric pressure.

I'm sure that the seals/pump integrity are pressure rated, but that is not what the head rating describes.


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