How to Select a Pump For Your Water Feature
The pump is the lifeblood of most water features. In a water garden or koi pond the pump provides the circulation, provides oxygen, and operates the filter. It also provides the sights and sound of moving water in these and other types of water features. Choosing the right pump for the job ensures your best chance for maximum enjoyment of the feature for yourself as well as for any aquatic inhabitants. But with hundreds of different pond pump models on the market it can be daunting. This article will step you through the various factors in pump selection to make a more informed decision.
Step 1. Determine Desired Flow Rate
There are many factors that help determine the proper flow rate for your water feature.
- Pond Size. For water gardens (ponds with plants and goldfish) you want to circulate at least half of your total volume every hour. This means for a 2000 gallon pond you should be pumping AT LEAST 1000 Gallons Per Hour (GPH). Koi ponds need a higher turnover rate and the minimum is the full volume every hour. Larger ponds (over 5000 gallons) can start to decrease the turnover rate. In other words, according to the prior rule of thumb an 8000 gallon water garden needs at least 4000 GPH, but in fact you may be able to get away with less. However, in these situations you should strongly consider additional aeration in the form of an air pump.
- Visual Effect. For the typical waterfall effect you will want 100-150 GPH per inch of waterfall width. This means that even if the pond is only 1000 gallons if you want a 12” waterfall spillway then the pump would need to provide around 1200-1800 GPH. For a trickle effect you can use closer to 50 GPH per inch or for a heavy flow closer to 200 GPH per inch.
- Other Equipment. Most pond equipment such as the filter, skimmer, or ultraviolet clarifier may have a minimum and/or maximum rating provided by the manufacturer. It is very important to abide by these ratings.
Step 2. Determine What Type of Pump is Most Appropriate
All pumps are not created equal. There are four main types of pumps used in water features. Each one has its pros and cons and one may be better suited for your application than the next. Read more about submersible pumps versus external pumps.
- Direct Drive Pump. A direct drive pump is simply a pump with the impeller connected directly to the shaft of the motor. Direct drive pumps most distinct advantage is power. In a high head pressure situation (detailed in Step 3 below) a direct drive pump can still provide good flows. You can often also find “dirty water” or “solids handling” pumps that are direct drive. These require less maintenance of the pump itself as small debris will be pushed through without clogging the pump. The most notable disadvantage of direct drive pumps is the higher cost of operation. They also tend to not last as long because the seals keeping water out of the motor wear out, this happens even quicker if the pump is not properly sized for the application.
- Magnetic Drive Pump. A mag-drive pump uses an impeller attached to a magnet. When the pump operates, the magnet rotates causing the impeller to spin. Mag-drive pumps can be a great choice for low-head situations. They do not have the power to overcome high head pressures. The biggest benefits of magnetic drive pumps are the low energy consumption and the long life-span.
- Asynchronous Pump. This type of pump also uses a magnetic field to spin the impeller. The main difference is that the impeller is not directly connected to a magnet. Unlike a magnetic drive pump, this design ensures that each time the pump starts the rotor and impeller spins the same direction every time. This allows the blades to be curved instead of straight. That enables asynchronous pumps to be more efficient and provide much greater flow rates than mag-drive pumps. Electrical consumption is on par with the mag-drive pump. The greatest disadvantage is that the design requires a little more maintenance to ensure that the rotor stays clean, otherwise the pump has a higher likelihood to overheat or seize up.
- External Centrifugal Pump. These pumps come in a wide range of styles and flow rates. An external pump may involve a little more plumbing and planning and they are often more costly to purchase but they certainly have their advantages. In general you are able to get more power at a much greater energy efficiency. Additionally these pumps tend to have a very long life-span.
Step 3. Calculating Head Pressure
Head pressure is the resistance against the pump that decreases the overall flow. It is measured in “feet of head”. To calculate the Total Dynamic Head (TDH) we must look at several factors and utilize a flow chart provided by the pump manufacturer as well as a Friction Loss Chart.
The first factor is the easiest. Static Head is simply the rise in elevation that the pump needs to push the water. For this calculation we are not looking at distance, only the elevation differential from the pond level to the high point in the waterfall or filtration system. For this every foot of rise is one foot of static head. For the next step we first need to utilize the flow chart for the pump we are looking at. Using the Static Head total see what the estimated flow rate is at that amount of head. Then take that figure to the friction loss chart to calculate the Friction Head. To make this calculation you will need the length of pipe and pipe diameter. You will note extreme differences here depending on what size pipe is being used. You can maximize your flow potential by using a larger size pipe. Once you have determined the Friction Head (don’t forget to include elbows and other plumbing as referenced on the chart) you add this to the Static Head. If there is no extra equipment on the line then you now have your TDH. However, if there is other equipment such as a pressurized filter or ultraviolet clarifier then you will need to add additional head to the total. You may need to contact the equipment manufacturer to get this figure. Now that you have your TDH (or at least a reasonable estimate of the TDH) use this figure on the pump’s flow chart to see if this pump will give you the flow you need for the application.
Step 4. Factor in the Operating Expense When comparing multiple pumps that will work in your application your first inclination may be to just go with the one with the lowest sticker price. However, it may be short-sighted to not also look at the total cost of operation over a period of time. You can determine the operating cost of any pump by using this formula: amps x volts divided by 1000 x KWH cost x 24 hours-a-day x 30.4 days-per-month = cost per month. If the pump is rated in watts instead of amps use this formula: watts divided by 1000 x kWh x 24 hours-a-day x 30.4 days-per-month. KWH is the kilowatt-hour cost, which you can get from an electric bill or by calling your local electric company. Pump Operation Cost Calculator
Step 5. Consider the Possibility of using Two Pumps
Using two smaller pumps instead of one large pump is an often overlooked option that has many advantages. Electrical consumption. Again, check that detail. Example: If your application needs a single pump that uses 1000 watts, but you could also accomplish the same thing with two pumps that use 300 watts each then this is an option that could mean significant savings in the long run. Balancing visual needs with equipment needs. Example: If your waterfall needs 3000 GPH but that is too much for your ultraviolet light then running one pump past the UV and both to the waterfall solves this without complicated plumbing or needing to oversize the UV unit. Temporary savings. With two pumps running it is possible to put one on a timer where it only operates it when you are there to enjoy the aesthetics. This allows you to keep the second pump operating the filter to run 24/7 as needed. Redundancy. Your pump will fail eventually. And even if you have a backup pump on hand this failure could occur at an inopportune time. The odds of both pumps failing simultaneously are very low. With two pumps in use, when one fails, the other keeps going. This could save the lives of your fish and prevent other disasters.
Step 6. Choose Your Pump(s)
Now that we know the process for selecting a pump let’s run through an example. For this example say this is for a water garden 12’x5’ that averages 2’ deep. That comes to about 900 gallons which means that it should have a pump moving at least 450 GPH. It will have a waterfall 18” wide. So, now we will try to size a pump to provide close to 2700 GPH (150 GPH per inch of waterfall) to get a nice effect. The waterfall will be only about 1 foot above water level and will have 18’ of 2" PVC with three 90 degree elbows. Now take this information and look at a few pump options. Since most charts the manufacturer provides will not show data at every possible amount of head pressure it will be necessary to extrapolate a little to come up with close estimates.
(NOTE: the information used below is current as of the writing of this article, pump specifics are subject to change)
Pump #1.Savio WMS3600. With just one foot of elevation this puts our starting figure close to the max flow of 3600 GPH. Take that information to the friction loss chart. First, notice that each 2” elbow adds 6 feet of equivalent length of pipe. So, the three elbows will add 18 feet. Add this to the actual 18 ft length of pipe to get 36 feet. Now look at what 3600 GPH lists under 2” PVC and see this listed as 5.84 feet of friction head. But this is for 100 ft of pipe, so multiply this by 0.36 for the 36 feet to get 2.1. Add this to the 1 foot of static head for a TDH of 3.1 feet. Now refer back to the pump’s flow chart and extrapolate that this will give around 3400 GPH. This is a little more than the goal of 2700 GPH, but more importantly you will see that this amount of head pressure is in the “Do Not Operate” range. However there is still another option to use this pump. If a ball valve is installed on the plumbing line the flow can be reduced which will also increase the pressure. If the flow is reduced to the 2700 GPH target flow then this puts it in the “Best Efficiency Point” range for this pump. So, this one is still an option. It’s a direct drive pump with a 2 year warranty and an estimated energy draw of 575 watts.
Pump #2. Proline HyDrive 3200. Starting again with one foot of elevation puts the 1 foot flow at 3500 GPH. Take that to the friction loss chart and see this will be just slightly less that the 5.84 used above so estimate and use this figure again. Using the same calculations find a TDH of about 3.1 feet. Returning to the flow chart for the pump notice this one will provide 2700 GPH at 3 feet of head. That’s right at the target flow. It is also an option to go to the next size up and use the ball valve to tweak the flow to exactly what is desired. This asynchronous pump has a 3 year warranty and uses only around 180 watts.
Pump #3. Tidal Wave TT4000. At the 1 foot of elevation this pump can provide around 4400 GPH. Taking this figure to the friction loss chart estimate 8.5 feet of head per hundred feet and multiply that by .36 to get 3.06. Round this to 3 and add to the static head to get a TDH of 4 feet. Back to the flow chart and extrapolate to find that this pump would give us around 3400 GPH. Again a valve can be used to tweak this. This asynchronous pump has a 3 year warranty and would use 238 watts.
Pump #4. Sequence 3600SEQ12. The static head puts it at around 3500 GPH. Back to the friction chart with the same numbers as Pump #2 again yields a TDH of around 3.1 feet. The flow chart for the pump shows that the flow would be around 2900 GPH. This external pump has a 5 year warranty and uses only 160 watts.
Now with the information learned about pumps earlier in this article you may have weighed the pros and cons of each style pump and have narrowed this down to 1 or 2 options. If you still have not completely decided which pump you can then refer to user reviews or the opinion of experts to finalize your decision.
Making sure you have chosen the right pump(s) ensures fewer headaches and more enjoyment of your water feature. The right pump for the right application will last longer while providing the needed benefits. Still confused or just don’t want to try to figure it out on your own? We are more than happy to help.
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