All About Pond Pumps
How do pumps work?
Most pumps that are in use for Ponds and Water Gardens are known as centrifugal pumps. These use an impeller which has spinning blades that rotate causing the water to be pushed through the outlet of the pump. The design of the impeller and the speed of the motor varies but technically all centrifugal pumps basically work the same way. As the water enters the front (input) of the pump it comes in contact with the spinning blades or vanes on the impeller which pushes the water through the output or volute and through the tubing or pipe. Pond pumps are measured by the volume of water they produce in gallons per hour (GPH) while pool pumps typically are rated in horse power (HP)or the size of the motor itself. The pump produces pressure which is usually expressed in feet of Head which is the maximum height in feet that a pump can push a column of water. Naturally, it is easier for a pump to push water one foot high than to push it ten feet high. The flow varies depending on the height. Pump flow is measured in gph at a given Head height, such as 1000 gph at 1’of Head but as the Head increases it may drop to only 750 gph at 5' of Head.
Although a pump may produce 1000 gph you will need to know both the gph your pond requires and the head height of your waterfall in order to choose the right pump. In that way you will be able to select the pump that gives you the correct flow you want at the Head Height you will need. This information is typically supplied by the manufacturer.
What Does Head Height (shutoff) Mean
All pumps produce water flow and pressure. The water flow is easy to see when you plug in your pump especially as it flows over your waterfall. On the other hand, pressure cannot be seen and is sometimes difficult to visualize. Pumps are rated in Head Height which is an easy way to visualize the pressure a pump produces. One pound per square inch of pressure will lift a column of water 2.3 feet in the air so that a pump that has a shutoff height of 34.5 feet produces 10 psi of pressure. This is important to understand because the higher a pump has to lift water, the less water it can deliver. Naturally, a pump will deliver more water to a lower waterfall than a higher one. Head Height charts or graphs tell you exactly how much volume a pump will deliver at any given Head height. You may see 2000 gph @ 5' of Head.
In designing your pond you will need to know the head height of your own system. It is not enough to know just the highest point above the pond as there are other variables to consider. The water from the pump is carried to the waterfall via tubing, and the size of the tubing and the number of fittings will come into play in determinging what pump you will need. Both tubing and fittings cause friction resulting in a reduction of water flow which in turn can cause the pump to work much harder producing less flow. In order to keep this friction loss to a minimum, always increase the tubing at least one size larger(if possible)than the outlet of the pump. If the outlet is 3/4", use 1" or larger plumbing. If it's 1-1/2", go up to 2" tubing. There are several shortcuts in computing the total head required.
- For every 10' of Tubing, add 1 foot of Head
- For every Check Valve, add 1' of Head
- For every fitting, add ˝ foot of Head
So, if your waterfall is 5 feet tall, with 20 feet of tubing and two elbows and a check valve, your system would have a total head of 5 + 2 + 1 + 1 = 9'. with this information you can check the manufacturers charts to see which pump gives the flow you need at 9' Head. Keep in mind that in this example we did not calculate the addition of a pond filter which in itself can reduce the flow as much as 20%. In most cases it is better to opt for a larger pump as you can generally reduce the flow but if your pump is not large enough there is nothing you can do to increase the flow.
What Size Pump Do I Need
It is generally accepted that Koi ponds should turn over all the water in the pond each hour. That would mean that a pond with 1000 gallons of water should be using at a pump that produces at a minimum of 1000 gph. Keep in mind that other factors such as head height, friction loss and pond filtration can reduce the flow sometimes up to 40%.
Another factor to consider when choosing your pump is if you have a waterfall you may need an even larger pump depending on the visual effect you want to achieve. The rule of thumb for waterfalls is that for a normal flow you will need approximately 100 gph for each inch of width of the initial waterfall. In the example above if your 1000 gallon pond has a 12” wide waterfall then you would need 1200 gph at the top of the first spillway ( 100 gph X 12 = 1200 gph) If however, you wanted a stronger flow such as sheeting or more noise you would need a minimum of 200 gph for each inch of width or 2400 gph using the example above. The flip side of this is if you wanted only a zen like trickle it could be as low as 50 gph per inch. In most cases you can reduce the water flow from your pump if it is to strong using a ball valve but there is nothing you can do if it is not strong enough. For this reason it is best to purchase a larger pump if you are in doubt.
What Kind of Pumps Are Available
Pond pumps come in many shapes and sizes. Although they all do basically the same thing (pump water) they differ in the way they go about the job and how they actually work. With a little basic understanding you will have a better idea of which one will suit your specific needs. Some pumps are designed to work only submerged in water while others are designed to work exclusively out of the pond (external) while still others can be used in either application.
The first pumps available were out of pond (external) pumps and submersible sump pumps. These were direct drive pumps because the impeller was directly connected to the electric motor via a shaft. They are cooled by the water and the impellers are somewhat heavy so the motor needs to be strong enough to sufficiently rotate the impeller. The motors were wound with copper and protected by seals that kept the water away from the pump motor. They produce large volumes of water and are still used in applications where high head heights are required. Because of there design, they are not generally energy efficient.
Many years later the first magnetic pumps became available. These pumps featured lighter impellers and no seals and the small motor spins a magnet via magnetic induction and is fully encapsulated in epoxy to prevent damage. The result is a more compact design and much more energy efficient. They can also be used either submerged or out of pond. They do not produce the high head height of a direct drive pump. When the pump is energized, the magnet spins the impeller but you never know for sure what direction it will spin so the impeller is designed to work in either direction. Newer Hybrid pumps are able to operate the magnet to spin the impeller in one direction which has benefited in more efficient impellers which can produce additional volume and higher head heights. These Hybrids are known as Asynchronous pumps. These newer designed pumps are gaining favor as they are available in larger sizes, even above 6000 gph. Like a mag pump, they can generally be used submerged or out of the pond. The only caveat is that the high efficiency requires very close tolerances inside the pump which can make them susceptible to scale buildup and clogging in hard water locations. They need to have good prefilters to keep out debris, and regular cleaning to remove any lime scale or other water deposits.
Maintaining My Pump
Your pump is the heart of your pond and as such needs periodic attention in order to keep it operating safely and efficiently for many years. Below are a few tips to help you.
Direct drive pumps are the work horses of pond pumps but they can overheat which will shorten their lifespan. If your direct drive pump is an external one, be sure that you have a leaf/debris basket installed in front of the pump. This device will trap leaves and debris prior to it entering the pump. If the basket becomes clogged it will reduce the flow of water going to the pump which will cause overheating. Many direct dirve pumps have a thermal overload switch which will shut the pump off but as it cools down the pump will restart and overheat again. This constant overheating and recycling will eventually cause the switch to fail resulting in permanent damage to the pump. The same is true for submersible direct drive pumps used in a skimmer. Always check your skimmer for debris and remove it. If you have mats, pads or pump socks they need to be cleaned on a regular basis for the same reason.
The good news is that Mag Drive pumps are generally maintenance free (if there is such a thing). They tend not to overheat as long as they are submerged, and can run for years if they are kept clean. The most common problem is that debris can bypass the prefilter and cause the impeller to slow down or even stop. This is easily remedied by unplugging the pump and removing the volute and cleaning the impeller. Be careful when removing it as the ceramic shaft can be easily broken. If the shaft that the impeller resides in is not scored or damaged a replacement impeller will generally return the pump back to operating condition.
It is also a good idea to place your submerged pump on a stepping stone raising it a couple of inches from the bottom of the pond where the dirt and debris accumulates. Otherwise your pump will be pumping abrasive grit from the bottom of the pond which can wear the shaft that the impeller rides in. Once worn, the shaft will wobble causing the pump to fail. This condition is not repairable and you will need to replace your pump.
The popularity of Asynchronous Hyprid pumps is well-deserved. They are quiet, efficient and powerful and will give you many years of enjoyment. Because of their design and tight tolerances debris or scale deposits that build up can eventually cause the pump to fail by wearing off the stainless steel cladding surrounding the copper coils, or by cementing the rotor into the can because of excessive heat. Deposits around the rotor are the single greatest cause of Hybrid pump failure, however, there is a simple, effective way to avoid an early failure. You should check the pump initially after a couple of months to determine if your water is causing scale. A simple wash of white vinegar will remove it. Check again in a couple of months to see if it needs further maintenance. If so, you will know that cleaning will be necessary on regular intervals. If it is not a problem it is still a good idea to check at least twice a year in order to avoid any potential problems.