Water flow, can the pump be too big?

[Charlotte Seibert]

Hi all, I dabbled in aquaponics a bit a few years ago., even though I went against some of the rules (as in using 2 pumps and 2 timers) I did pretty well. Some things came up and I had to pull my system down, Well I am back at it and have some questions. I still have my grow media and 200gal fish pond. I am putting up 2 grow beds for a total of 24 sq ft of grow area. I will be pumping a height of 5 ft., flood and drain using an indexing valve. I have to get a larger pump, question is can your pump be too big? Can you cycle too much water. I found a fountain pump that pumps 1200 gph @ a 5 ft lift. Given that I will only be pumping 30 min out of each hour, that's 600gph or 3 times my tank. Some of the water being pumped out will be sent  back for aeration.  

[Alex Veidel]

I think the excess power is more of a concern; smaller pump is likely going to use less electricity. I can't think of a reason why moving your water too fast would be a problem in a timer based system. If for some reason it seems to be too much, you can always divert some of your water back to your fish tank through an aeration bar to get some extra oxygen in your water.

[Charlotte Seibert]

Thanks that is what I was thinking.

[David - WI]

Is that the "rated" flow or did you actually measure it?

If that's the rated output you might be surprised at what the "actual" flow is once you have it all hooked up.

Five feet of "head height" includes the lift plus any restrictions from the length of pipes, fittings, elbows & valves especially, and any screens or filters on the inlet or outlet... what is it rated to flow at 7' or 8' of head?

[Charlotte Seibert]

I think it said 950 @ 8ft. My main concern here is if there can be too much flow for the fish. Can you have too much air? too much current?

David - WI said:

Is that the "rated" flow or did you actually measure it?

If that's the rated output you might be surprised at what the "actual" flow is once you have it all hooked up.

Five feet of "head height" includes the lift plus any restrictions from the length of pipes, fittings, elbows & valves especially, and any screens or filters on the inlet or outlet... what is it rated to flow at 7' or 8' of head?

[Glenn]

Actually, there CAN be instances of too high of a flow rate/pressure and too much oxygen. While too much oxygen is rare, some fish can die if water is over saturated. I think in AP, we only need to worry if we use concentrated/bottled/pure oxygen.

I know for trout in a stream bed, some resting areas are required. And too much flow in a channel can squash them against the down stream end. I've read where flow faster than 7 times the length of a trout in captivity per second should be avoided and that 3 times for most species is quite normal.

[Alex Veidel]

Yes, technically you can have too much oxygen. You pretty much have to be injecting pure oxygen into your water to start causing damage though.

Glenn said:

Actually, there CAN be instances of too high of a flow rate/pressure and too much oxygen. While too much oxygen is rare, some fish can die if water is over saturated. I think in AP, we only need to worry if we use concentrated/bottled/pure oxygen.

I know for trout in a stream bed, some resting areas are required. And too much flow in a channel can squash them against the down stream end. I've read where flow faster than 7 times the length of a trout in captivity per second should be avoided and that 3 times for most species is quite normal.

[Darin Tito Schrock]

I have an oversized pump and I just wired in a dimmer switch and run it at half speed or so.

[Robert C. Wood]

I would be careful and wary of using a dimmer switch to reduce the power to the pump because of the increased heat being generated. All electric motors are designed for the rated power that is written on it and not less. It will reduce the life of the pump drastically. this is something that we covered in detail in trade school.

[Hank Paloci]

I had a bit too much flow with my new fountain pump. It came with a very rudimentary valve attachment that closed up about 20%. That did the trick. Maybe if you put a ball valve somewhere in your lines you could do the same.

[Johan Prins]

yes, be wary about dimmers; inductive loads like pump motors don't like anything else than sinewave current. Dimmers chop the sinewave at a certain point, and not only make the pump motor unhappy, but also create a lot of parasitic signals (radio frequencies) on your house's grid, also referred to as "electro-smog" by some.

A variac (variable transformer) can be a good solution, but isn't as cheap.

My fountain pump has a valve which can restrict the flow, but of course the pump will draw just as much current, or worse, the impeller starts cavitating if you restrict the flow too much.

A standard (isolation or other) transformer with + / - 10% taps can be used to reduce the pump power by 10%, but just that.

From then on (further down), you'd have to re-wind your transformer if you want less voltage.

(like Rob from the bigelow brook farm I'm an electronics guy who stumbled into aquaponics)

[David - WI]

I have a couple of these in the shop (for other things) and they work fantastic.  We use them on vacuum/blower motors, but most pumps should be fine with them, too:  http://www.harborfreight.com/router-speed-control-43060.html#.UyoS1...

[1EyedTuna]

I prefer to have a slightly larger pump than required. You can always cut back on the flow rate... Here is how I do it:

Many pumps do not come with a valve to regulate how much it is pumping, its either on or off. So, you need to put a ball valve in your plumbing after the pump. Here you can cut back and adjust the flow. However, if all you do is put a valve in the line and you cut way back on the flow, you will put a lot of back pressure on the pump. This could burn up the pump and or reduce its life. In order to prevent this back pressure you want to plumb in a valve on a return line to the tank your pump is located in. This valve is where you will adjust your flow. Sorry if that is confusing... I have attached a photo illustrating this a little better!