In our industry a great deal has been written about the features and advantages of magnetic and direct drive submersible pumps. Most professionals and consumers alike select a submersible pump for their pond and waterfall installations. This article will address a different type of pond pump used by installers that may have considerable advantages over submersible pumps in many specific applications. The external pond pump is an invaluable tool for our trade and one that is often overlooked as an installation option.
What is an external pond pump? While the types and features of external pond pumps may vary the definition remains constant. An external pond pump is one that sits outside of (or external to) the pond itself. Applications for these pumps include, but are not limited to, waterfalls, pulling from bottom drains, formal fountains, pond circulation, or any situation where a submersible pump is not practical or attractive.
One type of external pond pump is a magnetic coupled asynchronous pump. These pumps may be used both as submersible and as external in-line pumps and feature tubing ready intakes and outlets. These are often used on smaller ponds, tanks and wall fountains. New technologies are improving this type of pump and although they are not the focus of this article they may one day become a more widely recognized choice for certain external applications.
External pumps are divided into flooded suction or self-priming. They are also divided into high flow- low head or high flow – high head. Additionally they may be either 115 volt or 230 volt or, as many do, can be operated at either voltage. External pumps can be single speed or multiple speeds. With so many choices available it is little wonder that consumers and installers alike must take great care in choosing the correct pump for each specific application.
**Low Head and High Head External Pumps**
The selection of either a high head or low head external pump depends upon the height of the water feature it is intended for. If building a 3-4´ high waterfall or running to a pressurized filter it is very likely that a low head external pump will be perfect for the job. If supplying a 20´ tall waterfall or moving through a very long run of pipe you may be looking at the need for a high head external pump. The first step in determining the correct type of pump will be to calculate your total dynamic head. That includes calculating for the height of the water feature, the length of the run, the inside diameter of the pipe used, and the number and type of fittings such as elbows or Y’s that will be used in the plumbing. Several industry charts provide directions for calculating the total dynamic head and this article will not delve into those calculations. (See Dynamic Head Box)
Once you have your total dynamic head pressure (TDH) for your installation you can then determine the GPH flow required at the TDH and consult the flow charts of any of the external pumps on the market. According to Rick Smith of Easy Pro Pond Products, “Each external pump will have a chart with the GPH flow at various head heights. By matching up your requirements to these charts you can select the best external pump for your job.” Rick goes on to say that ” Using the wrong size or type of external pump can cost a tremendous amount in wasted electricity or could even damage the pump or reduce its lifespan.”
As an example, if your planned installation requires a total dynamic head of 15 foot and you would like a flow of 5000 GPH it would be physically possible to use an external high head pump. However, you would not want to for two reasons. First, the high head pump may consume double the electricity that the low head pump does. This means hundreds of dollars more per year in operational costs due to the selection of the wrong pump type. Second, the high head external pump may have a greatly reduced lifespan if not operated with enough backpressure. Lack of proper pressure can lead to cavitation in the impellor chamber and overheating of the pump motor.
With the example above let’s compare two different external pumps. Both are rated at just over 8000 gph maximum flow but one is a low head external pump and the other is a high head external pump. The Easy Pro LOW HEAD EX8800 has a maximum flow of 8200 GPH and at the 15 foot head will generate 5040 GPH. The Easy Pro HIGH HEAD EX8500 has a maximum flow of 8500 GPH and at 20 foot of head is still pumping 6960 GPH. If you were to use the High Head EX8500 you would be consuming 1385 watts versus 449 watts with the Low Head EX8800. For an average electrical rate this means that in the course of one year’s time the High Head pump will consume $616.85 MORE in electricity than the Low Head pump will.
Additionally the High Head pump when used in a low head application must have the water throttled back in order to put enough backpressure on the pump to prevent cavitation and premature pump failure.
Conversely if the TDH of the application was 30 foot then the Low Head pump would not work at all but the High Head EX8500 would still be generating a flow of up to 6000 GPH.
**115 and 230 Volt Pumps**
External Pond pumps are sold as either 115 volts or 230 volts with quite a few operable at either voltage with a simple “in field” modification. Many smaller external pumps will be 115 volts exclusively and many very large external pumps are 230 volts exclusively. Many people have been confused over using 115 or 230 volts. A lot of bad information exists telling consumers that 230 volts will save them electricity and save them hundreds of dollars in operating costs. That is not exactly true. If a pump running on 115 volts pulls 4 amps it will consume 460 watts of electricity (volts times amps equals watts). If the same pump runs at 230 volts the amps drawn will be half of what they were at 115 volts. Therefore 230 volts x 2 amps = 460 watts. The cost of operation will remain the same.
On the other hand, very large external pumps benefit from installation with 230 volt lines. While some larger external pumps may allow switching from the 230 volt preset to a 115 volt output it is often not in the best interest to do so. For example: A high head self priming external pump set to draw 11 amp at 230 volts will pull 20 amps at 115 volts. If this is compounded with a lengthy run of electrical wire it will be necessary to increase the SIZE of the wiring in the installation to cover the voltage drop and the high amperage draw. This also would require a certified electrician to install and negates any savings associated with the initial desire to run at 115 volts.
**Flooded Suction and Self-priming Pumps**
Like with the choices above, the decision of whether to purchase a self-priming pump should be made according to the job at hand. With either type of pump one must remember that external pumps must have water flowing through them in order to operate properly and prevent pump damage. If using a flooded suction pump this means that the pump must be placed at, or below the level of the pump in order to have a constant supply of water. This type of pump is often placed into a well or sump, which is excavated below the water level of the pond. If using in a well it is important to remember to leave enough space around the pump for air to circulate.
Andy Schoenberger, Outdoor Living Product and Marketing Group Manager at Franklin Electric (Little Giant), suggested, “Efforts should be made to keep external pumps cool to promote longer pump life. Even the use of a small fan inside a pump chamber can be an effective way to prevent overheating and help the pump last many additional years.” Andy continued, “External pumps, when properly cared for and operated, can last upwards of 10 years. They can last far longer than any type of submersible pump.”
In some cases a flooded suction pump can be set on the same level as the pond and still function properly. Installing a check valve in the tubing BELOW the surface of the pond will help keep the pump primed when the unit is shut off and will allow for restarting the pump without hand priming.
A self-priming external pump is a time and labor saving product that will often eliminate the need for below grade pump placement. A self-priming pump will pull water to the pump from short distances thereby eliminating the need to dig a well or sump for the external pump to sit in. External self-priming pumps are more expensive than their flooded suction counterparts but the extra expense may be worth it depending upon the design of the pond or water feature installation.
Please remember that both self-priming and flooded suction pumps must be primed prior to their first operation. The installation of a check valve and the filling of the tubing or leaf trap (also known as a priming pot) will prime the pump for its first use.
**Single Speed and Two Speed Pumps**
The final choice in External Pond pumps is one of speed. Some manufacturers of external pumps offer a simple switch to run the pump at either high or low speed. The speed setting of these pumps determines the GPH flow from the pump. One would want to consider this type of pump when running it at the lower speed will produce the needed flow but there will be times when a higher flow is needed. That could include the desire to have a higher waterfall flow during a party or public viewing. It could also provide the ability to backwash a filter at a higher flow for enhanced cleaning. The important thing to remember is that running at the higher speed is extremely costly. Demi Fortuna cautions buyers by stating, “Doubling the speed and flow of the pump often requires four times the electricity.”
**Variable Speed Control Panels**
The larger the external pump the more power it takes and the more it costs to operate. Like two- speed pumps, Control Drive panels are a way to reduce the speed of the pump and the energy it consumes. According to Art Hantla, Fielding Pumps national sales manager for the Shinmaywa pump line, “A variable speed control panel will not only allow the customer the option of substantial power savings and flow control but will extend the life of a pump with “soft” starts and stops.” Art goes on to say, “Control panels provide multiple speed settings for consumers to change the appearance of their water feature while providing instant power downs when a fault or short is detected in the system.”
Control panels are typically expensive but offer considerable benefits especially during the installation of very large pumps that may more than pay for themselves over time.
**General External Pond Notes and Tips**
1. Pool pumps are not a good choice for most pond applications. Pool pumps are designed to be low flow with extremely high pressure. Those type of pool systems have very small (usually 1 or 1.5 inch tubing) and are designed for low flows. Using a pool pump for a low head pond or waterfall application is both costly in terms of electrical consumption as well as the frequent cause of premature pump failure. An external pump must run with adequate backpressure to prevent cavitation and motor damage.
2. The installation of a check valve is a good idea with all external pump installations. The check valve makes the external pump easier to prime and is definitely a must for above grade pumps.
3. A leaf trap or priming pot is a good idea for external pump setups. External pumps have a much lower solids handling capability than some direct drive submersible pumps. The leaf trap is an easily cleanable pre- filter that both helps the pump maintain its prime as well as keeping large solids away from the pump.
4. Placing a ball valve in the tubing on both sides of the external pump makes removing the pump for maintenance much easier and prevents drainage of pond water during removal.
5. External pond pumps should be placed as close as possible to the water supply. Even self-priming pumps will have difficulty drawing water from a distance. If a long run of tubing is required you would want to make it on the discharge side of the pump rather than the intake. Likewise you would want to push water through a filter rather than drawing it through on the intake side.
6. The manufacturer built outlets on an external pump may not be the required tubing diameter size. Tubing requirements are determined by your height of feature, distance traveled and the amount of GPH flow desired. Consult a plumbing flow chart when planning your project to determine which size tubing your job requires. Typically the larger the better for any type of pond pump. The larger the tube the less friction on the water flowing through it and the more efficient the pump will operate.
7. On the intake side of a pump use tubing equal to or larger than the tubing on the discharge side.
8. A pond pump that sounds like it is chewing on gravel is cavitating. The sound you hear is the pump chewing on a froth of air and water. The pump and backpressure on the system need to be checked.
In conclusion the selection of an external pump can be a confusing prospect for a first time purchaser. With multiple options from more than a dozen external pump manufacturers it is important to select your pump according to the specific installation requirements. External pond pumps offer a wide range of features and benefits that submersible pumps do not. Wonderful results are possible with the correct pump and an abundance of information and assistance is available from manufacturers to assist you in the selection process. I encourage everyone to take time to learn more about external pond pumps and add this valuable resource to your company’s offerings.
For those who do not have their own system or chart for dynamic head I strongly recommend the one Demi Fortuna, owner of August Moon Designs and associate of Atlantic Water Gardens, has painstakingly put together over many years of experimenting. Please contact the IPPCA (International Professional Pond Companies Association) at www.ippca.com or call 770-592-9790 and your industry trade organization can make sure this information is provided to you.