Pond Design | When Two Pumps are Better than One

My first pond project was a disaster. I had built two large, shallow bowls, one 50´ x 25´ and the second 30´ x 40´, connected by a 20´ stream, in a new subdivision on Long Island, NY, and I was in trouble. Not only had I gone over board and over budget on 11 1/2˝ thick concrete shells poured over massive rebar and wire reinforcement, I had designed what might have been the worst functioning ponds on the Island. First, they were shallow; they went from nothing at the edges to only 18˝ deep in the center, and the water disappeared out of them so fast that first August I had to ask a Physics professor at the local University if the losses could really be due to evaporation. (After only a moment’s deliberation he solemnly congratulated me on designing the most efficient evaporation pans possible. Thanks, Doc.)

Next, they were an open invitation to every raccoon, heron and egret in the county – which waded into perfect fishing shallows every dawn for three weeks to feast in what was certainly the biggest Sushi Buffet in the state at the time. 4000 Goldfish Maki, anyone? But the kicker was the waterfall. I had picked up a powerful looking irrigation pump at a local farm auction. (How many fields did it water? “Oh, all of them.”) I went to the local industrial supply and bought the biggest electric motor I could find (“Biggest we got’s this here 15 horse, should do it.”) The display of about 40,000 gallons per hour was indeed impressive, but it paled, as my clients’ faces did, in comparison to the display they put on when they opened their electric bill that first month, and found it $700 more than the month before. Ooops.

I planted to the south and west to block both sun and wind; I dropped storm-killed cedars in the ponds to shelter the fish, but there was no remediating that monster pump – it had to go. I ate a little crow, put in nice, reasonable 2 h.p. pump, and just barely kept my job, but I walked away with a really good education. You can make lemonade out of lemons ‘til the cows come home, but nobody’s gonna pay $25 a cup for it. That’s why Energy Conservation and Thinking Green are so critical, especially now with oil at almost $100 a barrel and rising.

Let’s look at a “typical” 11´ x 16´ pond, just the feature itself, not the surrounding landscaping. What’s the largest cost your customer is going to face? The initial installation, right? Probably not. The single greatest cost of the feature over its 15-20 year lifespan may very well be its operating cost. A common, solids handling Direct Drive pump providing 4800 gph at 5´ of head might draw 1200 watts, over $50 per month even at the US Census Bureau’s 2003 average electric rate of $0.08/kilowatt hour (it would average almost double that here in NY). Add to that the cost of purchasing the pump and you might reasonably expect to saddle your customer with a $2,500 expense every three years or so for 15 to 20 years, so $20,000 in energy costs over the life of the pond is hardly an exaggeration, especially with energy going up constantly. How many ponds would you sell if the first thing you told your client was that it could cost $20,000 more than the installation just to run the pond?

What if, instead of 1200 watts a month, you were able to provide the same water flow for 1/4 to 1/6 the electricity? What if you were able to guarantee that water would always circulate even if a pump broke down? What if you were able to double your profits on the pump installation at the same time? Welcome, my friend, to the Advantages of Redundancy. Simply put, when it comes to pumps, Bigger is NOT Better; and Two is Better Than One.

First let’s talk pumps from a pondbuilder’s perspective (I apologize to the engineers in advance. For the real scoop check out Pump Ed 101 by a genius of communication, Joe Evans, Ph.D – “He is passionate about the sharing of knowledge and its ability to replace memorization with understanding.” It’s a fantastic resource, check it out at www.pumped101.com.) The common choice for the professional for many years has been the stainless, muscular Direct Drive submersible pump, descendant of sewage and effluent pumps and still capable of handling 1/2˝ solids and accelerating water to high head heights. They’re popular because they’re tough, easy to hide in a skimmer and they last when installed properly. But these high head Direct Drives are often as guzzling as the tanks they’re built like. That’s because of the physics that govern pumps, the Laws of Affinity. From my (limited) perspective, it seems it takes twice the energy to double the volume of water pumped, but twice squared (2×2) or four times the power to double the pressure. High head is high pressure, so these pumps draw a lot of power. Unless the application requires that high head height, you don’t want to be paying for it. Even more importantly, these pumps need a minimum head height to avoid cavitation (see www.pumped101.com) and premature motor failure. If they don’t have enough work to do, they won’t last.

In my experience, the majority of the residential pond and commercial pondless features these days have a waterfall that’s under 10´ high, usually 25´ or less from the pump, with 5-15´ TDH, or Total Dynamic Head (the vertical head height of the water feature from the water surface to the top of the waterfall, plus the friction losses of the plumbing system). In this range, lower-head, high-efficiency Magnetic Induction and Hybrid Magnetic pumps hold a significant advantage over Direct Drive pumps, typically consuming one third the electric for the same volume of water at the lower head heights where they excel. The explanation lies in the way the pumps are designed.

A Direct Drive pump has an impeller directly attached to the motor; the motor needs to stay dry, the impeller needs to be in the water, so shaft seals (typically 2 or 3) between motor and impeller have to work perfectly whether the pump is running or off, hot or cool. Direct Drives have to overcome the resistance of these shaft seals clamped down tight on the spinning impeller shaft. Add to that a heavy impeller and you’ll need the torque and power of a hefty motor. Eliminate the seals and heavy impeller and you’ll realize some major energy savings, precisely why Magnetic Drives were developed.

Magnetic Drive pumps have a free-spinning cylindrical magnet suspended on a shaft at the center of a sealed coil. Juice the coil and the magnet spins. Attach an impeller and you have a leakproof pump with no seals, since the outer coil is basically an epoxy-filled doughnut. Efficiencies at low heads effectively doubled with the introduction of Mag-Drives, with a 5000gph pump drawing 500 watts rather than a thousand. The catch is, the magnet can spin in either direction at startup, so the impeller blades have to be flat to work equally well in both directions.

The Hybrid Magnetic Drives replace the inner spinning magnet with another, smaller coil; because the direction of the windings can be controlled in both the inner and outer coils, the impeller can be made to spin in only one direction. This allows for a very efficient curved impeller designed to work very well in one direction, and this breakthrough allowed efficiencies to double again. If the installer can keep friction losses to an absolute minimum by upsizing pipe diameter and eliminating restrictive fittings, these pumps will pay back handsomely in electric savings. A Danner 4800 Hy-Drive can push 4500 gph at 5´ of head for only 260 watts, a third or less than comparable Direct Drives.

That $50 electric bill just shrunk to $15, and if you pay more than $0.08/kWh, you save even more.

On the other hand, to get these efficiencies requires 3˝ pipe for 5000gph, either rigid PVC, with lots of fittings, or 3˝ flex, which is beastly to work with in comparison to industry standard 2˝ Flex PVC. That not only raises the cost of materials, but labor as well – 3˝ just takes longer to install. Not coincidentally, the Direct Drive prefers the restrictive 2˝, precisely to get the friction head high enough to satisfy its minimum head requirements, and so is easier and cheaper to install. OK, one pump costs less to run but more to install versus another pump that’s easier install but costs as much as triple to run – it can be argued either way. What about using two pumps instead? Let’s examine some really good reasons to consider this approach.

Whether you have an easy to install guzzler Direct Drive or a light-sipping HY-Drive that needs massive plumbing, you share the same problem when it comes to failure – one strike and you’re out. Let me present a scenario – it’s August, and the fish in the warmest pondwater of the year are experiencing the lowest oxygen levels of the season. It’s night; plants and algae undergoing reverse photosynthesis pull oxygen out and exhale carbon dioxide back into the water. There’s no breeze to help aerate the water so it all depends on the pump – and it fails. Jean Womack of Nevada Water Gardens says her clients have 3 hours to get the pump running in their koi ponds before fish start dying. But what if you had 2 pumps? What if each one pumped half of the total flow, through 2 separate lines and 2 sets of fittings? You wouldn’t mind that one pump went, you might not even notice until morning, and then you could take your time fixing or replacing it while the other pump kept aerating the pond. There are lots of other situations where a dual pump system provides real advantages:

Having two pumps instead of one allows:

• One to be removed for maintenance or repair while the other maintains circulation

• Running only one at a time, alternating with timers, to double the life of the units

• Running only one pump at night or while on vacation to save on operating costs

• Running only one pump during winter, to economically keep an area free of ice for gas exchange

• Running only one pump constantly, with the other switched for higher flow/greater show on demand

• Running only one pump during outdoor parties, where two pumps would be too noisy

This might be enough of an argument for some contractors, but wouldn’t it be tough to sell the 2-pump system since it would have to cost so much more? What if the two smaller pumps draw so much less electric that over time the two pumps cost less than one! How can that be?

Here’s an example of the efficiency of 2 smaller pumps over a single larger one. You’ve contracted for a vigorous fall 4´ high by 30˝ wide; you’ll need 25´ of flex hose, a 90ľ elbow, a male/female adapter and a check valve to get from the pump to the falls. The customer definitely wants action, so you want to put about 200 gallons a minute per inch of width over the 30˝ wide falls, or 6000gph. Let’s see what a single 6000gph pump will cost to run and compare that to 2 smaller pumps, each supplying 3000gph to our water feature. We’ll assume a total head for our system about 7´ to choose a pump. Checking the flow at that head we might find a Direct Drive at 5400gph drawing 1000 watts, another at 6200 drawing 1200 watts, even a Hybrid Magnetic Drive that draws 520 watts to push 5400gph, or two smaller pumps each pushing half the desired flow:

TWO Danner HY-Drive 4000 pumps will provide 5900gph at 440 watts (through 2-2˝ lines.)

Is this Danner Magic? Unfortunately not. It has more to do with that exponential increase of energy required to do more pumping. This technique works with most pumps and many different situations, but really shines in water gardening, where we have many low head, high volume applications. Is it worth going to the trouble of plumbing 2 pumps in your project? Let’s look at the Table of Costs over Three Years for the pumps above. The Direct Drives cost a great deal more than the small savings they realize with that single 2˝ line to run. The Hybrid 1 pump will require a single 3˝ line or a manifold to split the output into 2-2˝ lines to get the reported flow, so plumbing costs are the same or less with 2 pumps. The two smaller pumps, each pumping half the flow, can efficiently use inexpensive, readily available 2˝ Flex PVC straight up to the falls, one line each. Even though the end user will have to pay for the purchase and installation of 2 pumps and 2 lines, the savings in operating costs will pay back the difference and then some in well under the typical 3 year lifespan of the pumps. The contractor benefits twice: he gets paid to purchase and install 2 pumps up front, while the customer saves money every month thereafter.