Choosing the Perfect Pump

Published on March 1, 2011

Two Asynchronous pumps  through 2˝ provide 3000gph at 4´ of head for only 250 Watts, plus Advantages
Two Asynchronous pumps through 2˝ provide 3000gph at 4´ of head for only 250 Watts, plus Advantages

Hello again! Last issue we talked about how much actually running a water feature can cost, and how to set up the plumbing system to get every drop of flow paid for, regardless of the pump installed. This issue we’ll talk about the different types of pumps in common usage, when to use which and how to get the most out of them. We’ll discuss submersible pumps because of the ease of plumbing and hiding them has allowed them to dominate the market and power the vast majority of water features out there.

First we have to generalize

Obviously all water features are not created equal – no one would expect to power a 10´ tall waterfall with the same pump they’d use for an overflowing vase. Let’s make some assumptions to allow us to compare the types of submersible pumps commonly used in waterscaping.

First, let’s limit our discussion to flows **between 1000 and 10,000 gallons per hour**, because under 1000gph operating costs aren’t as significant and features over 10,000gph are typically less common. Second, let’s assume we’re dealing with features with 5 to 10´ of static head, the actual height of the water feature. Finally, we want to keep the friction losses of the plumbing system to a minimum to keep the Total Dynamic Head (static head plus friction losses) **under 15´ TDH** – see Tubing Chart. (Check out last issue’s article for complete plumbing recommendations.) The vast majority of the features I’ve built fall into this range, and I’ll bet yours do too. In this range, the type of pump you choose can make a huge difference in the long-term cost to run and operate.

Direct Drive vs. Magnetic Induction Pumps

The common choice for the professional for many years, muscular Direct Drive submersible pumps, are descendants of sewage and effluent pumps. They’re tough, easy to hide in a skimmer and last for years when installed properly. They’re called Direct Drives because the impeller is attached directly to the motor, with compound seals on the drive shaft to keep water out of the motor. They’re hard to clog, capable of handling ½˝ solids and develop the pressure needed to push water to high head heights. But high head means high wattage, for reasons we’ll get into later. You do need a Direct Drive high head pump if you have a waterfall taller than 10´, if you have to locate your pump farther than 25´ away, and if you can’t pump clean water through the right size tubing. If not, you have a couple of other options.

First, there are the lower head, higher efficiency Magnetic Induction pumps, which use a replaceable ferrite magnet and simple, bi-directional “synchronous” impellers that can rotate in either direction. The pumps are reliable and easily repaired, but the simple flat-bladed impellers are noisy and inefficient, since they have to work equally well both clockwise and counterclockwise.

Mag Drives can be less costly to run than Direct Drives but can’t handle large solids and are only really effective up to about 3000gph and 10´ of head because of that inefficient ‘synchronous’ impeller design. If the impeller could be made to move in only one direction – “asynchronously” – then a very efficient curved-bladed impeller like those of the Direct Drives could be used, increasing flow capacity and head height. Asynchronous Hybrid Magnetic Drive pumps do just that. They use a copper and steel electromagnet in place of the cheaper ferrite; although no longer cheap to replace, the direction of the windings of the coil-within-a-coil allows efficient, quiet one-way rotation.

The term “Hybrid” refers to the use of Direct Drive-style impellers with these magnetic induction motors, allowing almost incredible efficiencies at low to medium head heights. A Direct Drive might consume over 1000 Watts to push 5000gph at 15´ THD; if the Total Dynamic Head can be reduced to half that, an Asynchronous Hybrid Mag Drive can move the same amount of water for less than 500 watts, which at $0.14/kW/hr adds up to $50 in savings per month. Over the average 3-year life of a pump that’s $1800 back in the homeowners’ pocket. The catch is, of course, that ‘Mag Drive’ and ‘Hybrid’ pumps can only deliver these amazing efficiencies pumping clean water through the proper size tubing at relatively low head heights. Still, if you can install a decent pre-filter and keep the THD low, the energy savings alone will easily pay for new high efficiency pumps over their three year lifespan.

What’s going on?

One reason for the difference in energy consumption lies in the way the pumps are designed. Direct Drive pumps have impellers 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 not, 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 Induction Pumps were developed by NASA in the late 60’s for the Space Program. These pumps suspend a magnet or electromagnet in the center of a copper coil; juice the coil and the attached impeller spins by magnetic attraction, or ’induction’. The trade-off, of course, is the much lower pressure/head height that these pumps are capable of in comparison to the Direct Drives, since the impeller is only coupled to the motor by magnetic induction, but that lower head capacity is precisely why they use so much less electricity.

The $64,000 Question (and the math)

Why should lower head mean lower wattage? The answer lies with the Laws of Affinity. The specific Law we’re interested in states that:

  • Flow (GPH) is directly proportional to impeller diameter;
  • Pressure (Head) increases by the square of the impeller diameter;
  • Power consumption (Wattage) increases by the cube of the diameter.

So, if you increase the size of an impeller by 20%, to 120% of its original size, it’ll push 120% more GPH, provide (1.20) 2 or 44% more head pressure, and consume (1.20 x 1.20 x 1.20) or 73% more wattage. Turn that around and you’ve got the tool needed to save lots of energy. Reduce the impeller to 80% of its original size and it will give 80% of the original flow, .80 squared or 64% of its original head, but it will consume only .8 x .8 x .8 = 51%, **about half the Wattage.** (For you techies out there, find the real scoop at, Pump Ed 101, by a genius of communication, Joe Evans, Ph.D – it’s a fantastic resource.) So now let’s take it all the way home. If reducing the head drops the wattage, and reducing the impeller size drops the wattage, what’s the best way to REALLY drop the wattage? Drum roll please…Using two half-size pumps instead of one.

The Advantages of Redundancy

Simply put, when it comes to pumps, Bigger is NOT Better; and Two IS Better Than One. Whether you have a Direct Drive, Mag Drive or Hy-drive, using two half-sized pumps of the same head capacity will significantly reduce the costs of running your feature, often by 50%. If you can also keep your THD low and pump clean water, then using two Asynchronous Hybrid Magnetic Drive pumps can *really* lower running costs. The generic examples below are typical of the savings that can be realized:

1–5000gph Direct Drives = 1000 Watts;
2–2500gph Direct Drives = 660 Watts;
2–2500gph Hy-drives = 330 Watts

If saving all that energy weren’t enough, there are lots of other situations where a dual pump system provides real advantages:

• One pump out won’t mean a total loss of circulation, especially critical on warm nights
• One can be removed for maintenance or repair while the other maintains circulation
• Running only one at a time, alternating with timers, doubles the life of the units
• Running only one pump at night or while on vacation saves on operating costs
• Running only one pump during winter economically keeps an area free of ice
• Running only one pump constantly leaves the other for higher flow on demand
• Running only one pump during outdoor parties, when two pumps would be too noisy

In conclusion

OK, so we’ve chosen the most efficient pump for our feature, taken advantage of the Advantages, saved all that money in operating costs and don’t forget, we’re doing our part to be “Green” – what could possibly be better? How about doubling profits too! Whether contractor, dealer, distributor or manufacturer, two pumps for every feature means double the profit per installation, so literally everybody wins, right down to the homeowner who’s paying less and getting much more out of their water feature. How’s that for a happy ending? Happy Ponding to All!

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