Water gardens range from small container gardens with a fountain or small sculpture to ponds with large waterfalls and streams for a dramatic sound and visual effect. The common element for any garden pond with moving water is that there is a pump making it happen. Which pump is best is the real challenge we face. Before even thinking about buying a pump, it's important to define what the pump is supposed to do. This article is not about any specific pump, but covers some things about all pumps. The goal is to have a water garden that fits your needs and wishes. The right pump will go a long way toward adding the desired visual effects and eliminating headaches, especially when the pump will be used for filtration purposes.

How will the pump be used?
While there is a seemingly endless array of pumps available, and more are being developed and marketed all the time, the end application still determines the ultimate pump choice. Decide what the pump is supposed to do. Is it for mechanical or biological filtration, a fountain, a stream or waterfall, skimming, or a combined use with multiple flow directions? A filtration pump should run continuously. A fountain or waterfall pump used only for effects may be used only when people are there to enjoy the water feature. Many people are now using multiple pumps to match the exact needs they have. This provides increased reliability and the opportunity to match the pump better to the specific needs of the water feature.

Pumping rates for waterfalls, biological filters, and skimming Next, determine the desired flow rate for the application. For example, a waterfall or stream should have a minimum of 1,000 gallons per hour per foot of width. A fountain head will have a specified gallons per hour for a spray height or spread. A prefabricated waterfall or sculpture will usually have a recommended flow rate. We recommend that biological filters should operate 24 hours a day at their maximum specified flow. Ponds under 400 gallons could pass through a filter two to four times per hour, (i.e., 800-1600 gph), while ponds approaching 2000 gallons could move from the one-half to one pond volume per hour, (1000-2000 gph), and still keep the pond clean biologically. Skimming action, on the other hand, is not volume sensitive, but is pond area sensitive. Skimming pumps are most effective at ten to fifteen times the pond area in gallons per hour. A 10'x10' pond, for example, with an area of 100 square feet, should be skimmed with a pump of at least 1000 to 1500 gph rating.

Once the flow rates are known, the choice will be limited to a few possible pumps and the options become more manageable. Some elements are your choice, like purchase price and warranty, but the sales representative should guide and explain any additional choices.

The next step
Unfortunately for many pond applications an old statement about pumps is all too true: A pump is a mechanical device for moving water that never produces enough flow. Keep in mind that a pump cannot produce more flow than it was designed for. You can always reduce the flow with valves if needed, but it is best to operate the pump at its specified flow rate. Simply stated, a pump creates flow. It is the resistance to flow that creates pressure in the system. Identifying the sources and evaluating the effects of this resistance are where pump selection and installation challenges arise. Ideally, the only resistance would come from gravity, but other factors add resistance as well. Every pond will have elements that help you determine what it will take to get the flow you want.

Fighting the two-headed monster
There are two types of head or resistance to flow. The first is static head or gravity head. This is the difference between the lower water level, which is the surface of the pond, and the level at the discharge -- whether it is the end of the pipe, the surface of an upper pool, or the tow of an upflow filter. This vertical height is the static head of the system. The static head is not affected by the depth of the pump in the pond or its height about the pond for external pumps. The gravity head represents the work the pump must do to lift the water from one level to the other. Think of lifting a bucket of water out of your pond. You don't feel the weight of the water in the bucket until it begins to leave the pond surface. The pump feels the same resistance regardless of how deep it is in the pond.

The harder part to determine is the dynamic or friction head. Dynamic head results from restrictions to flow and friction in the piping and other components like filter, UV lights, and fountain heads. Dynamic head is usually expressed as an equivalent static head for a given flow velocity or flow rate. The fact that changes in flow rates will change the dynamic head explains why the manufacturer's specifications often appear to be incorrect. A 5' static head may become an equivalent 10' head when the effects of pipe size and length, inlet filtration, and discharge filtration or fountains at the desired flow rate are added to the static head to find the total effective head.

The design will also show the pipe lengths that will be needed for the installed components. Combine the static head with the estimated dynamic head to determine the total head, and then check the pump specifications for that head to determine flow rate for a given pump. For example, if a 600 gph flow is desired at a 5' gravity head, a pump using 100' of 1/2" pipe would see a friction head of 80.5' for a total head of 85.5'. If the pump is only capable of pumping to a 20' head, the flow many drop to about 200 gph at the 5' gravity head. For that same application, a 1" pipe would offer only 6.6' dynamic head and the pump would see and 11.6' total head. many pumps could provide this flow at the lower total head. This shows the benefit of using larger pipe sizes in general, and whenever pipe length increases.

Many pumps that perform very well in fountain applications with small pipes and fittings will not perform in a water garden with a waterfall and stream where the pipe length is much greater. It is best to use adapters to increase pipe size for these applications. Elements that contribute to dynamic resistance include the number of elbows or fittings, pipe size, pipe or hose inside surface condition, suction strainer or filter, and discharge orifice size for fountains. Many of these elements can be controlled by the design; others are dictated by the system and must be factored into the pump selection. In addition to the pond components, the consistency of maintenance will have a significant effect on performance. The common effect of a clogged liner screen reducing flow to a trickle is one that is familiar to most pond owners. So in addition to selecting to the right pump, be sure to install it so it is protected form debris and easy to service. We always use external skimmer boxes. This approach satisfies both of these needs for waterfall and skimmer pumps. Skimmer boxes also work well to protect small fountain pumps that can be plumbed to a fountain head located in the pond. In-pond filter cages and screens can also provide additional mechanical filtration.

Pump selection and application
The elements above need to be defined before looking for a pump. In most cases, a careful look at your pond and a sketch of the system will help with the selection process. Now it's time to look at different pumps that will produce the desired flow at your total head. Several additional questions are likely to come up. The following sections cover some details that will help you get the pump that best fits all your needs.

Types of pumps and impellers
Without looking at specific manufacturers we can classify pumps according to categories and capabilities. The following generic types will define virtually all pumps. Garden pond pumps operate with electric motors and any installation must meet local codes and safety rules. Most will operate with 110 volt house power, but 12 volt pumps and some low voltage solar pumps are available. Consider alternate energy if your application fits. Certain situations may also be suitable for 220 volt power, but these are usually very specialized applications.

* External (dry) or submersible
Two obvious pump categories are dry, or open air pumps and submersible pumps. Submersibles generally have the advantages of being easier to install, cost less to purchase, are quieter in operation, and don't require separate housings or covers for weather protection. Virtually all smaller fountain and filtration applications use submersible pumps. Energy efficient pumps up to about 4000 gph are available, and the pump manufacturers are introducing better pumps all the time.

Depending on the size of the pond and pump, waste heat from submersible pumps could be a problem. A very large pump in a relatively small pond in a warm climate could contribute to fish stress by increasing water temperature in the summer, but in the northern part of the country this should not be a factor. Additionally, efficient submersible pumps decrease the amount of waste heat generated. External pumps normally require more complex plumbing systems. These pumps are air cooled so will not contribute to pond heating. They may require a separate housing or shelter.

These pumps may be virtually silent while in operation, but it is best to listen to a demonstration pump to know exactly what the sound may be. With most moving water applications, the sound of an open pump is masked by the sounds of the water. However, some homeowners find that any mechanical sounds reduce the appeal. Always resolve this issue before deciding on a pump. External pumps have the additional benefits of a very long life and keeping any electrical connections away from the water. Bearings and seals can easily be replaced if needed.

*Oil-filled, oil-sealed, or oil-free?
Perhaps the fastest growing area of pump design and availability is in oil-free submersible pumps. Some are among the most efficient available in terms of energy cost per thousand gallons per hour of pumping capacity. Where oil filled pumps are a concern, definitely install an oil-free pump. We have rarely experienced oil leaks with oil-filled pumps, but it is a possibility. Oil leaks may create a hazard for fish and plants, and will require clean up. The oil-filled and oil-free question applies to pumps using motors that require shaft seals for best operation. This category generally applies to the larger pond motors and pumps starting at around 1000 gph. Oil-sealed pumps have a motor that runs in air, but the shaft is sealed with an oil chamber. The volume of oil is very small and if there is a leak, the oil normally leaks into the motor and not the pond.

In addition to the sealed submersible pumps for pond applications, mag drive pumps are expanding in size and application ranges. These pumps use permanent magnets with an encapsulated electrical system. They offer maintenance benefits and are now available with flow rates over 2000 gallons per hour, with larger sizes on the way. The simplicity of these pumps makes them ideal for lower flow and lower head applications. In height flows, efficiency and head performance do suffer.

*Impellers
Impeller design is a major factor in service performance in the pond application. Many pond pumps offer non-clogging impellers. These typically are known as vortex or open impellers and are used in trash pumps, ejector pumps, and many sump pumps. These pumps will pass solids, (possibly over an inch), and fibers without clogging the impeller. If the downstream components are properly designed, the debris will be captured in some type of filter or screen. These pumps will also work well with most skimmers and inlet screens. A disadvantage of this impeller type is that its performance drops rather dramatically with increasing heads, and most are not suitable for heads over 12'. The next style of impeller is the semi-open. These have tighter clearances but will still pass small solids, (about 1/8" depending on design), and fibers. The head performance is better, and these pumps will perform well with higher pumping heights. The vortex and semi-open impellers are suitable for most pond applications and offer the benefit of minimal clogging of the impellers. Most will require inlet screening or filtration to protect any additional components in the system. These screens must be kept clean for best performance.

Closed impeller pumps can handle the most demanding flow and head conditions. The impeller design does make them more prone to clogging, so inlet screens or filters are very important and the pumps can be installed outside the pond in a skimmer box. Another important consideration for many centrifugal pumps is that they are more efficient at certain heads. Lowered heads may cause excessive energy use and may actually damage the pump motor. Be sure to check the manufacturer's specifications to verify the best range of operation.

As applications and flows vary, other pumps or designs may be appropriate. For very high flows at low heads, axial propeller pumps may offer energy savings.

*Multiple pumps
We are also finding a growing popularity for multiple pump installations. For example, a small, high-efficiency pump may run continuous for skimming and/or biological filtration, and a second pump is on a timer or switch to provide a dramatic flow to a waterfall or fountain when the pond owners are at home. Two pumps are also valuable to run extra skimmers or increase the total waterfalls or pond filtration capacity. In general operation costs for two pumps to produce a certain water flow are less than a single, larger pump.

Pump purchase and operating costs
Cost, efficiency and warranty are three common concerns to the customer, Many pumps offer low initial cost, but the operating cost quickly surpasses the savings in initial cost. An effective approach for grouping pumps is to use "Standard Efficiency" and "High efficiency" categories. In cost cases, the high efficiency pumps will have a higher initial cost, but energy savings should soon make up the difference. For pond filtration applications, where the pump should run continuously, this efficiency difference will soon have an impact on total cost. The difference in energy costs between two similarly sized pumps can be significant. A $20 to $30 difference in energy cost per month for continuous operation is not unusual. Today's high-efficiency pumps provide about 1000 gph flow per 100 watts or 1 amp of power consumption.

Pump warranties typically vary from one to three years with longer warranties usually related in the purchase cost. A longer warranty should give the buyer a higher degree of confidence and is often accompanied by more sophisticated pump appearance and features like improved pump sales.

Getting the most from your pump
Pumps will use electricity at a very consistent rate while in operation. Since you will be paying for the power anyway, you might as well get as much water for your dollar as possible. There are many things you can do like sizing mechanical filters and pipe or hose to reduce friction and installing the pump outside the pond with a skimmer so most pond dirt cannot reach the pump. Clean and maintain all components like filter boxes to keep problems to a minimum, and look at upgrading the capacity of undersized components.

Troubleshooting pump problems
Every pond owner will look out at a pond and see that something isn't right. If a pump doesn't perform to expectation, or performance deteriorates over time, it is probably due to flow restrictions rather than pump problems. When a pump fails, it usually stops completely. If flows diminish, check the water flow path from inlet to the end of the discharge piping to look for problems. Chances are the inlet screen or filter is clogged. In some cases, the impeller may be jammed or partially blocked. If a downstream filter is installed, it may need cleaning or replacement of filter elements. For safety, always unplug the pump before servicing the system.

To verify the operation of a pump, put it in a bucket of water or laundry tub to check the flow characteristics. It's a good idea to do this with a new pump to see how it performs for comparison later if a problem arises.

Conclusion
The most important part in selecting a pump is knowing the application and operating conditions. The pump manufacturers have produced a wide variety of pumps to fit any pond. Their tables and charts will guide you to the right choice. Be sure to set your priorities for purchase price, efficiency, flow, and head. For example, a more efficient pump may save you $15 per month but have 20% less flow. Is that a trade-off you are willing to make? Anticipate some of the questions that will come up and be ready to ask some of your own.