Aquatics Unlimited: Articles: Selecting a Pond Pump
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Selecting a Pond Pump

Maximum Output. This is the feature we are all most familiar with, and is usually listed in quaint old gallons per hour (gph) here in the US. In order to achieve the biggest number possible, the pump rating is generally given as the maximum output that pump could have with no restrictions, and without lifting water above it’s surface level. This means that the pump will not ever actually push that much water in real use, but it’s still a good start for comparison.

Maximum Head. This rating basically reveals just how high a pump can push water, and is measured (in feet, in the US) as the vertical distance from the surface of the body of water to the point at which water is released. If one pictured slowly lifting a hose connected to a running pump, maximum head is the point at which the pump’s output first becomes zero. The manufacturer again wants to make this number as large as possible, so maximum head is calculated using a large diameter tubing projecting straight up from the pump. Since the pondkeeper presumably wants the pump to push at least some water, maximum head is again a comparison figure, rather than a practical one.

Head Ratings. These are far more useful than either the maximum output or maximum head ratings, and are often displayed as a graph somewhere on or in the pump packaging. Head readings are given along one axis of the graph, and flow ratings along the other. For example, a larger pump might show 1400 gph at 2 feet of head, 1100 at 40 feet, and 800 at 8 feet. Even these values are not entirely practical, since things like long or narrow tubing and sharp bends can also affect head considerably.

Submersible vs. External. While there are a few pond pumps available that are designed to be used outside the pond, virtually all the common, affordable units are submersible. Submersible pumps offer ease of installation, energy efficiency, reliability and low initial cost, although they are by definition more visible and less accessible than external pumps.

Standard vs. Low Voltage. For many years, low voltage water pumps were commonly available, and their use was widely recommended as a safety measure. The pump itself would operate on perhaps 24 or 36 volts, supplied by a separate transformer outside the water. If a pump housing would crack or wire split, there would be much less shock hazard than if full house current were entering the pond. Over the years the pump housings and wire casings of good quality pumps have become much more durable, and government electrical wiring codes often require the use of Ground Fault Interrupter Circuits on outside outlets to reduce shock risk. Low voltage pumps have become much less necessary, and due to the added expense of the transformer, poor sellers.

Flow Pumps vs. Pressure Pumps. Some pumps are designed to pump a lot of water per hour, but output decreases sharply when "head" increases. These "flow" pumps are often used to supply waterfalls or waterstreams, where large output is needed, and filtration is not often employed. "Pressure" pumps, on the other hand, usually have less rated output than a similar size flow pump, but the output doesn’t drop off so quickly as head increases. This makes them more suitable for use in operating filters and fountain heads.

Wet Rotor vs. Magnetic Drive vs. Direct Drive. Not too many years ago, almost all pond pumps had a direct drive system: an electric motor turned a drive shaft which turned an impeller. The motor housing was often filled with oil, mostly to keep water from coming in through the shaft seal and ruining the motor. Eventually, the seal could fail or the housing leak, destroying the pump and often leaving a huge, smothering oil slick in the pond. Direct drive pumps also used a lot of electricity, forcing many pondkeepers to consider turning pumps off regularly to keep the bills down. And while it might be possible to replace a chipped impeller in some models, direct drive pumps were generally not easily repaired by the user.

The next step up in the evolution of pond pumps were true "magnetic drive" pumps, in which the motor shaft turns a magnet, which is sealed in the same chamber. A second magnet was attached to the impeller in a second chamber, and turning on the pump caused the first magnet to spin the second and thus pump water. This eliminated the seal failure and oil slick problems of direct drive pumps, and made replacement of damaged impellers somewhat easier, but did not address the energy cost problem, and were still quite prone to destruction by water leakage into the motor housing.

Many modern pumps use the "floating rotor" (also commonly, but confusingly called "magnetic drive") design, in which the electromagnet part of the motor is sealed in a plastic chamber, which is also often filled with epoxy. The permanent magnet and shaft part of the motor are actually part of the impeller, which slides into a recess in the motor housing. There is no oil or shaft seal, the impeller is usually easy to replace, and the epoxy-embedded electrical coil is protected somewhat even if the motor housing becomes damaged. In addition, this method generally uses 1/2 or less of the electricity of either of the others.

Energy Efficiency. Many pondkeepers are rightly concerned with operating expenses, and careful selection of pumps can make a big difference. In some cases, investing $20 or $30 more initially on a pump could save $10 each month in electricity. Most pumps now have wattage ratings on their packaging, and consumers can easily compare costs: twice the wattage means twice the cost to operate.

Selecting the Right Size Pump. For small ornamental features like fountain heads and spitters, refer to the packaging or accompanying material. There is often a chart that shows the size of the spray pattern at various flow rates. Purchasing a pump slightly larger than recommended will allow for adequate flow even as the pump begins to slow due to plugging prefilter.

ImageFor waterfalls and water runs, the best method to determine an adequate flow is to run a garden hose into the header (water source area), and adjust the water flow until the water feature looks right. Leaving the hose running at that same rate, move and hold it over a standard 5 gallon bucket, and time how many seconds it takes to fill. Divide 18,000 by the number of seconds to determine the gallons per hour the hose was running (for example, if it took 30 seconds to fill the bucket, then 18,000 / 30 = 600 gph). Next, measure the "head" - how much higher the top of the water source is than the surface of the pond. Using the Head Rating Charts, select a pump that pushes at least the gallons per hour you calculated at the head height you measured. Oversizing by 20% or so gives a little margin for error and slowing pumps, and oversizing by 40% may be required if there is a long run of tubing or a few bends in its routing.

Speaking of tubing: to maintain adequate flow, use a wider diameter tubing - 1/2" ID may be suitable for pumps up to 300 gph, 3/4" ID for pumps up to 800 gph, and 1"+ would be recommended for more powerful pumps. Make the tubing as short and straight as practical, and be sure it doesn’t kink during installation. Heavier gauge, "kink-free" tubing is excellent for any purpose, and required if it is to be buried or snaked through rockwork.

Multiple Items from One Pump - sounds like a good idea, but may not result in either startup or operational savings. There also is something to be said for putting your eggs in more than one basket, so that a whole system is not dependent on a single pump. It is particularly difficult to attempt to operate a pressure device such as a fountain head along with a flow feature like a waterfall. To get a substantial fountain effect, the entire system must run at a higher pressure ( = "head" ), and the higher head means a slower waterfall. A small fountain might run off a 250 gallon per hour pump, and a small waterfall off a 700 gph pump, but a pump that can operate both features might need to be 1200 or even 1800 gph.


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