In theory 2 x 1200 = 2400.

Flow of water going thru a pipe is a function of pump output and pipe diameter and fittings. So two pipes vs one pipe is apple and oranges.

In general a 2400 GPH pump is going to use fewer watts than two 1200 GPH.

Some keepers like two pumps in case one goes out. This is more for high fish loads where a lack of water movement could kill fish in a short time.

Some important variables come into play. The head height which can be very different depending on the diameter and length of the tubing, the actual power of the pump which changes if there is no pre-filter or if it is clogged, maintenance of the pump and filters. Even the shape of the pond can affect the efficient movement of water. If you plan to run the pumps when there is ice, is there enough outflow in one spot to keep a hole open?

At first glance theory always seems great but the devil is in the details. Sometimes you don't know until you try.

The term "head height" is often misused to describe friction loss inside pipes. I guess it sounds more experty to novices. They're two different issues.

Shape of the pond does effect water movement, but that wasn't the question. Shape of the pond would effect water movement whether 1, 2 or 5 pumps were used. If pumps were expertly positions movement could be improved with multiple pumps or better yet a well designed outflow system.

Sometimes you can just look at the number of watts used, add the amounts of two lower flow pumps and compare to the watts used by the higher flow pump to see there is an absolute difference.

I happen to have several pumps/sizes, so maybe I'll just try and see, and report back....

Otherwise.... can you use one single pump to draw from both a skimmer filter and one on the bottom of the deepest part of the pond. NOt sure how this would work, since doesnt the pump generally go inside the skimmer?

The difference in the length of the tubing would make me think the intake on the bottom would be less efficient just because of the friction. It might be possible to adjust the flow if you use an adjustable valve on both tubes. You would need to keep an eye on it because the balance could be upset by the slightest thing, even a dirty prefilter. Adjustable valves are expensive, BTW. The loss of head height would be a problem if the pump is rated too low.

I would prefer to go with two pumps to cut down the need for maintenance or adjustments and in the case of pump failure the second would continue working and reduce the possibility of harm to the fish. That is just my opinion though. I have tried both ways when I used a floating skimmer or a UV light. There was much less fiddling around with two pumps.

If the discharge of a pump in the skimmer is directed to the pond bottom, or near the bottom, the effect is the same as having 2 pumps with one on the bottom. This is basically called "Tangential Pond Returns" if you want to Google for more info.

Placing a pump on the bottom is a really bad idea imo because it normally clogs. But more importantly because if there's any problem with the pipe and there's a leak the pond can be emptied in an hour or two killing the fish.

One good reason for 2 pumps, not in this case but in general, is when a pond has both a skimmer and bottom drain. Genrally 2 pumps are used.

Two half size pumps will ALWAYS use less electric than one "double" size pump of the same type. This is a direct result of the Laws of Affinity, which govern impeller/flow relationships and basically state that power requirements increase exponentially while flows increase geometrically. This is an oversimplification, you can get the real formula on Google, but basically it means you need much more than double the power to double the flow. On the other hand, cutting the flow in half means that the two pumps will each require LESS THAN HALF of the electric required by the larger pump (of the same type). Check it out on any website that shows flows and watts. For example, take two Danner 1200 pumps at 110Watts vs. one 2400 at 265W. Same flow, almost 20% less electric, and lots of other advantages, as the other posters pointed out - pull one pump for service while the other runs, run one at night and both during the day (or vice versa), a pump failure doesn't leave the pond without circulation, cycle pumps to increase life, less flow while on vacation or at work, more when you get home or for parties, etc. Look up Advantages of Redundancy in Pond Trade Magazine for further details. Good Ponding! D

Pumps in bold moved less water per watt than the next smaller size pump. It wasn't easy to find pumps that were exactly double, but there are some. Laws of Affinity isn't specific to doubling so I think this survey is OK.

From first place I found via Google and I didn't cherry pick.
Alpine Cyclone 4000 Pump - 4000 gph 310 watts, 12.9 GPH/watt
Alpine Cyclone 8000 Pump - 8000 gph 600 watts, 13.3 GPH/watt

Fish Mate 600 Submersible Pump - 580 GPH 40 watts, 14.5 GPH/watt
Fish Mate 800 Submersible Pump - 790 GPH 42 watts, 18.8 GPH/watt
Fish Mate 1000 Submersible Pump - 1000 GPH 44 watts, 22.7 GPH/watt
Fish Mate 2000 Submersible Pump - 2000 GPH 95 watts, 21.1 GPH/watt

Laguna 400 Power Jet Fountain Waterfall Pump - 400 GPH 19 watts. 21.1 GPH/watt
Laguna 600 Power Jet Fountain Waterfall Pump - 600 GPH 32 watts, 18.8 GPH/watts
Laguna 2400 Power Jet Fountain Waterfall Pump - 2400 GPH 100 watts, 24.0 GPH/watt
Laguna 2900 Power Jet Fountain Waterfall Pump - 2900 GPH 130 watts, 22.3 GPH/watt

Danner pumps from their web site:
Model FP 2: 250 GPH 21 watts, 11.9 GPH/watt
Model FP 3: 350 GPH 37 watts, 9.5 GPH/watt
Model FP 5: 500 GPH 49 watts, 10.2 GPH/watt
Model FP 7: 700 GPH 49 watts, 14.3 GPH/watt *** Seems like a mistake on their web site
Model FP 9.5: 950 GPH 93 watts, 10.2 GPH/watt
Model FP 12: 1200 GPH 110 watts, 10.9 GPH/watt
Model FP 18: 1800 GPH 150 watts, 12.0 GPH/watt
*** 2400 GHP 265 watt, 9.1 GPH/watt *** No spec on web site

Some externals
Sequence 750: 3600 GPH 139 watts, 25.9 GPH/watt
Sequence 750: 4200 GPH 160 watts, 26.3 GPH/watt

Sequence 4000: 3600 gph 227 watts, 15.9 GPH/watt
Sequence 4000: 5000 gph 263 watts, 19.0 GPH/watt
Sequence 4000: 5800 gph 296 watts, 19.6 GPH/watt
Sequence 4000: 6900 gph 376 watts, 18.4 GPH/watt
Sequence 4000: 8200 gph 450 watts, 18.2 GPH/watt

Sequence Self Priming: 6800 gph 369 watts, 18.4 GPH/watt
Sequence Self Priming: 7800 gph 504 watts, 15.5 GPH/watt

These are all, I think, what the maker says is the max flow. I assume there would be differences at different heads. Maybe it varies more by technology.

So in my limited survey and hopefully no math errors, to my way of thinking it seems to me GENERALLY smaller pumps aren't as efficient. I'm happy to be taught different. But for now I can't see where "Two half size pumps will ALWAYS use less electric than one "double" size pump of the same type".

I'm not saying the Laws of Affinity are false, just seems to be more complex in actual practice.

Waterbug Guy,

The problems with your examples are that you are assuming that either the power consumption is constant or that the highest usage is at max flow. Your example of Squence 750 pumps if you look at the pump curve and power consumption curve you would find at 3600 gph pump will get just more than 28 gpw and the 4200 gph will get less than 28 gpw. This is assuming that the manufacturer is giving good information.

Hi Demi

I have known Demi for years and I can say that Demi is one of the most knowledgeable people in the pond industry. He is the author of Atlantic's online pond college courses. He is very well known and respected in the industry. Demi I will see you in November.
Mike

Hi Mike, thanks for the vote of confidence. Looking forward to seeing you at Infotanza!
Dan, you're absolutely right, it is complicated. There are going to be huge variations in the published literature for a number of reasons. Many mfg's "tweak" the numbers, to adapt what they have in stock to fit a perceived need in the marketplace. Pumps in the same apparent series may seem to be similar, yet vary in magnet type as they get larger. This happens a lot with mags where the smaller sizes can be made with cheap, less-efficient ferrite, while larger pumps require very efficient neodymium to achieve performance goals. The pumps being compared almost always vary in the pressure they produce as well, and that accounts for huge differences in power consumption. Because it takes the cube of the power to increase pressure, but "only" the square of the power to increase volume, higher head heights require much more power than higher volumes. Apples to apples, the Laws govern. It's just tough finding the apples to compare. My mistake.

To get back to the original question, are 2 1200's better than 1 2400, the answer is still yes. You will be able to get more flow for less Watts by using the Laws of Affinity to your advantage, and the best part is, the savings will typically pay for the cost of the second pump, and more, during its lifetime. That is, you will find multiple smaller size pumps of the same or a more efficient type that will provide a portion of the flow for a smaller portion of the power required. This works especially well for our industry, where 'typical' water features have flows under 10000GPH and waterfalls less than 10' tall, and the savings can be substantial and very welcome. You will find them when you look for them, and that's the point - you want pumps that allow you to take advantage of this neat side effect of a physical law for the benefit of your pond -- and your wallet. I never really thought to look for the apparent exceptions.

Mike,
I couldn't find a "pump curve and power consumption curve", so not really sure what you're telling me. I assume "pump curve" is flow rate? Is this the chart?

So if I pick 10' head:
Sequence 750 3600 GPH moves 1400 GPH for 139 watts, 10.1 gpw.
Sequence 750 4200 GPH moves 1750 GPH for 160 watts, 10.9 gpw.

What am I doing wrong? Wrong chart? Bad math? Where do I look to see the 28 gpw for example?

Demi,

Taking the best case from my survey, the Danner Model FP 2 at 250 GPH and 21 watts. Ten of these are cheaper to run than one 2400 GPH model.

Taking the worst case...
Alpine Cyclone 4000 Pump - 4000 gph 310 watts, 12.9 GPH/watt
Alpine Cyclone 8000 Pump - 8000 gph 600 watts, 13.3 GPH/watt
How would these pumps be run to make the smaller pump more efficient than the larger pump? Taking pipe friction out of the equation to make things simpler, that only leaves head correct? You and Mike are saying at some head the smaller Alpine would be more efficient than the larger? How does one figure that out?

I understand manufacturers play games, and I understand not every pump off the line is going to perform exactly the same, but I have to use something to make a decision. Is there a better way short of actually testing each pump myself?

Not trying to be a knob, just trying to understand what you're saying.

Waterbug guy,

This two pumps are a prefect example. The power consumption for 3600 at 10 ft is about 127 watts and the 4200 is about 147 watts. With these flow rates that your pump curve shows the 3600 would have 11 gpw and the 4200 would be 12.3 gpw. But looking at the power consumption curve that you will find on the same web page as your chart but under the pump specs for each pump the output of the 4200 pump at 10 ft is 1500 gph or 10.6 gpw. So what makes this a perfect example is that you found a mistake on the published pump curve for the 4200 pump. This was probably a honest mistake by the manufacturer but it still is wrong information.

Hi Demi,

I am looking forward to seeing you at Infotanza.

Mike

Mike, that's what I was looking for, thanks. Unfortunately I had not been looking on Sequence's web site and instead was getting info from retailer sites which didn't have this info. Here are the charts (I hope).
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I don't feel I'm reading these right as my number are so different from yours. At 10' of head...

I see (anally counting pixels)
3600: 1206 GPH 108 watts 11.2 gpw
4200: 1500 GPH 129 watts 11.6 gpw

Not saying you're wrong, I'm saying I don't know where I'm making a mistake.

You say the power consumption for the 3600 at 10' is about 127 watts...I see 107.5 watts.
You say the power consumption for the 4200 at 10' is about 147 watts...I see 128.8 watts.

The only thing I can figure is you're using the flow curve instead of the power curve to get the watts??

I'm also not understanding the mistake in the chart you referred to.

Trying to understand. The reason this all is important to me is because I'm thinking of writing a pond design application and this is just the type of thing a computer can crunch for the user if I can figure what you're saying and how to read the data properly. So I thank you for your time.

Waterbug guy,

The way you read the curve is to take a vertical line up from where the intersection of the pump curve and 10 ft. The intersection of this vertical line and the power curve is how many watts are being used.

The mistake that I am talking about is on your first pump curve at 10 ft of head the 4200 pump pumped 1750 gph. Where on the power consumption/pump curve chart at 10 ft says 1500 gph. And if you look at the pump curve chart on sequence's web site it has 1300 gph for this pump. What numbers are correct I do not know but it shows how misleading information can be depending which chart you look at.

Mike

Thanks, I see now. If I'd tried to find the number of watts for 6' head my way I wouldn't have intersected the power curve at all.

And I see now what you're saying about the charts being off. Kind of makes this all silly if the data can't be trusted. That's disappointing. Sequence is a pretty good manufacturer imo and I guess if they can't provide reliable data there's really no point in trying to evaluate pumps at all, at least from my perspective.

Thanks very much for your time and patience.

Waterbug Guy,

If I had to guess I would say that the power curve/Pump curve chart is what the manufacturer says is correct. The little arrows on the curves are the points that they actually tested the pump at. But that only half the story. A few years ago I tested about a dozen pumps on the same test equipment. What I found was some manufacturers overstated what their pumps would do and others understated what their pumps do. There are no standards that manufacturers have to test their pumps to. As an example of what I found. This pump performance curve said that at 2 ft of head the pump would move 11,000 gph and use 1100 watts. At 7.14 ft of head the pump I tested did 10,984 gph and used 946 watts.The manufacturer said at 5 ft of head the output should have dropped to 10,100 gph and 1075 watts.
Mike

I agree the power curve would also be my guess to be more accurate...but I don't like guessing if I'm going to be doing an app other people depend on for accurate info. I guess I could just provide an estimate and give a range.

And of course how the pump is installed and perhaps even the density of the water would affect the result. And how individual pumps come off the line.

That was my original purely gut thinking, that all these little problems would accumulate in multiple pumps vs a single larger pump. So I'm still not convinced that smaller pumps are ALWAYS more efficient than larger pumps of the same model. Seems like Dmei backed off that too, but I'm not clear on that point. However it seems the differences might be less than I thought and instead of saying "generally a larger pump is more efficient" I would say something like "it's possible for smaller pumps to be more efficient but there's no good data to be able to tell".

Waterbug guy,

Are two smaller pumps more efficient than one larger pump? Absolutely no. If it was that easy all pumps would the same. In making a pump there are tradeoffs that must be made. Such as head height, volume pumped, efficiency of the motor, the quality of the parts used, and how many different pump models will use the same parts. In the Sequence 750 example both models use the exact same parts except for impeller. Because of this there are tradeoffs made in designing the two pumps. It is done to keep the cost down.

Yes the density of the water will affect the performance but a lot of other things will also affect the performance. The height above sea level, the TDS in the water, the temperature of the water, the line voltage, and many other things.

Mike

I'm new to ponds, but I can assure you that, in general, multiple small pumps are not more efficient than a bigger pump. Each pump has inefficiency, and you get a net gain using a single bigger pump, optimized for bigger flow. Multiple pumps multiply the inefficiency. You then have to upsize the max flow on a multi-pump system, since the added fittings and line length increase the effective head. You also have to buy two pumps.

The situation where this makes sense to do is if you have to have flow all the time; you could hot-swap one failed pump while the other is running, but is your pond really that mission critical? Besides, two pumps means twice the cost and twice the probability of failure. The only time that the redundancy makes sense is if you absolutely cannot have an interruption.

Mike is dead on that most of the small centrifugal pumps that are sold for our uses in water features (for a given company) are the same motors, housings, seals, with a different impeller. Comparisons within the same motor family will probably show a fluctuation in efficiency for comparisons with different impellers.I bought mine specifically for that reason. I was unsure where the flow rate for my waterfalls was going to be, so I got a pump that I can swap impellers on to tweak the performance. Also, being realistic about power consumption, we are talking about a few watts difference here, it's not going to break the bank. ;)

Mike