Pond pumps are sized according to the number of gallons they can move in one hour. The best pond pumps are going to have a sizing chart to determine how many gallons of water per hour the pump can push at a particular height. One of the first steps in choosing the right size pump is to determine the volume of water in your system.
Start by measuring the pond’s length, width and average depth. Unless your pond is one continuous depth, using the deepest measurement can over-estimate the amount of water in your pond or water feature. There are about 7 ½ gallons of water that will occupy one cubic foot. Multiply the length, width and average depth then multiply that number by 7.48 to determine the number of gallons in your pond. So, if your pond is 10 feet wide, 15 feet long and has an average depth of 2 feet, your pond is 2,244 gallons.
For a more accurate water volume use the same formula to determine the volume of water in all of the components in your system including streams, waterfall basins, skimmers and filters then add them all together with your pond volume to get a total volume of water for your system when it is running.
The ideal for most pond systems is to have a pump capable of moving the entire volume of the system in one hour. Once you have determined the total volume of water in your pond you can determine the head height of your system.
The ideal gallon per hour (gph) capability of the pump may be adequate for proper circulation and filtration but may not be visually stimulating as it flows through your system. Most water gardens have a stream or waterfall in the system.
You can size your pump based on your personal preferences and the width of your waterfall by using simple formulas. It takes about 36 gallons per minute of water flowing to create a twelve inch wide waterfall with a Zen like gentle trickle. With 100 gpm flowing across a twelve inch width you would create a nice solid sheet appearance. 200 gpm flowing creates a nice little Niagara Falls effect which can be seen and heard from about 30 feet away. It’s possible to reduce the flow of a larger pump but not make a smaller pump push more water.
The head height of a water garden or water feature refers to how high the pump has to push water in feet above the surface of the pond. For example, if your pond includes a waterfall that is 4 feet high, the head height is 4 feet. Things get a little more complicated once you start talking about “Total Dynamic Head.” Total dynamic head or TDH refers to the height of the waterfall plus the friction loss created by your plumbing.
Small diameter piping, too many fittings and long plumbing runs all create friction loss which can dramatically reduce the flow of water. Each restriction adds feet to your TDH. Small diameter piping creates friction loss because only so much water can flow through a certain sized opening. No matter how large your pond pump is you will only be able to move 5,400 gallons of water per hour through a 2 inch inside diameter pipe. Every time a fitting is added water flow is reduced as it moves through that fitting.
Fortunately there are general rules we can follow to account for this friction loss. First choose a plumbing size that is 1 or 2 sizes larger than the outlet of the pump. If the outlet of the pump is 1 inch use plumbing that is 1 ¼ inch or 1 ½ inch. Next, for every 10 feet of piping add 1 foot of TDH. For each elbow add ½ foot of TDH as a general rule. For every check valve or ball valve used, add 1 foot. So, if your waterfall is 4 feet high, you have 20 feet of piping, two elbows and one check valve your TDH is 4 + 2 + 1 + 1 = 8 ft.
Now let’s put it all together. You determined that your water garden holds about 3,000 gallons. You have a waterfall that is 2 feet wide and you want to hear and see a nice amout of water flowing. You determined your TDH is 8 feet so you will need a pump that can deliver close to 4,800gph at 8 feet of head height. By selecting a pump which is a little larger you will have the option of controlling the water flow and changing the appearance of the waterfalls. It’s fun, enjoy it.