Pond Tips  Buying a Pond Pump
When choosing a new pump for your pond or water feature, it is important to look at all factors. Obviously, we want to make sure the pump will provide the needed flow. Then we look at pump construction. Demand a stainless steel shaft and long life seals. You can buy cheap pumps at Harbor Freight, but these have low grade steel shafts that rust immediately. These also have plain rubber seals. The rusty shaft will wear the rubber seal quickly and water will enter and ruin the pump. Typical life is six months, sometimes less. Two cheap pumps a year cost far more than investing in one long life pump. Quality seals made of ceramic last ten years or more and cost very little extra when buying a pump.
There are two styles of pumps. One sits outside the pond and can be very accessible and usually more efficient. The other is called a submersible and sits under the water. With an exterior pump you may have to deal with the inability of the pump to prime itself. Self priming pumps are available, but not as efficient. Some designers will place the pump in a covered pit adjacent to the pond. This can get the pump below the water line so it will prime by gravity flow. The downside of pits is that leaks can fill the pit and ruin the pump and they don't lend themselves well to retrofitting.
Sumbersible pumps are very popular, but tend to wear out faster than exterior pumps. Submersible pumps are very easy to design into a system, and can be installed in a way that makes removal very easy. In either case, all inlets leading to a pump must be screened. A simple window screen type bag enclosing the intake can be enough, but larger screened devices make cleaing a less frequent job. Debris, snails, and anything else that is untethered in the pond can migrate into pump inlets.
An often overlooked factor is the cost of operating the pump. The newer designs are less expensive to operate as they use fewer watts of electricity. You can easily spend several times the cost of a pump in electricity consumption, if you select an inefficient design like a sump pump or most older swimming pool pumps.
Do the math. Look at the number of gallons that a pump moves and the watts of power it consumes. Volts x amps = watts. i.e. 120 volts times 2.4 amps = 288 watts. If this pump moves 1500 gph, then your cost is 288/1500, or .192 watts per gallon moved. This is a quick method of comparing two pumps.
More exacting cost calculations can be determined by using these formulae:
amps x volts divided by 1000 x KWH cost x 24 hoursaday x 30.4 dayspermonth = cost per month.
If the pump is rated in watts instead of amps use this formula: watts divided by 1000 x kWh x 24 hoursaday x 30.4 dayspermonth.
KWH is the kilowatthour cost, which you can get from an electric bill or by calling your local electric company.
Also consider the warranty. If you buy online, who pays the freight on returns. Pumps are large and heavy. Will you be forced to send it back to the manufacturer? If so, be sure to register the pump to start your warrantee.
Buying a quality pump is a VERY SMART decision.

The next thing that you want to consider is the amount of head that your system will have so that the correct pump size can be determined. Head is the resistance the water is exposed to as it flows. More resistance = more head. This is shown as a number representing the height that the water is raised.
The three main sources of head are STATIC HEAD this is the vertical distance you raise the water above the pond surface;
FRICTION HEAD this is the resistance from pipe and fittings as the water flows through, shown as a number equivalent to a vertical head with similar resistance;
PRESSURE HEAD the additional pressure required by some filters, venturis, and other devices. Just know that closed (canister) filter types are less efficient than gravity flow types and will decrease flow significantly as they get dirty.

First determine the vertical distance in feet above the pond water surface. This would be the height to the top of the waterfall or gravity flow container/box/drum filter. This is the STATIC HEAD. Next, use a Friction Loss Calculator to determine the FRICTION HEAD. Add this to your PRESSURE HEAD to get total head. Use this number to enter a pumps chart and determine the pumps output. i.e. Adding 6 foot rise to get the water up to the waterfall plus a 4 foot equivalent resistance to flow in the pipe, gives you an approximately 10 foot head. The pump must overcome all of this resistance before any water will come out at the highest end of the pipe. This can significantly reduce flow. You can use this figure to size the pump needed by looking at the pumps output chart at a 10 foot head.
Most pond designers shoot for one pass thru the pump for all of the water each hour. If you have a 2000 gallon pond, you want a flow of 2000 gallons per hour out of the top pipe. A pump rated at 3600 gph at zero head should push 2000 gph thru a pipe with a ten foot head. Obviously, you need to use the numbers that represent your pond for all calculations. Lastly, if using a gravity return system, be sure to size all return lines much larger than the pressure side lines as water returning by gravity flows much slower thru a pipe. Where we would typically design a system with 2 inch pressure lines, we would use 3 inch gravity return lines.
We highly recommend bending rigid PVC pipe by heating it with a grass torch or using flexible PVC pipe to eliminate elbows. This provides higher flow rates by reducing friction loss. There is no problem in using pipe larger than required but smaller pipe will cut your flow rate. Schedule 40 Rigid PVC can be assembled, then carefully heated until it bends like a noodle. You should have several minutes in which to drag the pipe into position. Use a wet rag to "freeze" the PVC back into a rigid state as needed. I recommend buying a Propane grass torch from some place like Harbor Freight (less than $20) and practicing on a length of pipe before assembly. Walking the torch rapidly back and forth will keep you from burning the pipe. Be patient. This process is very predictable, you just need to practice a bit.
