The pump seems such a simple little piece of pool equipment, but its looks certainly are deceiving. In fact, the pump is unquestionably the most important single piece of equipment in the circulation system, no matter the size or type of pool.
Looks are similarly misleading when it comes to selecting the right pump for the job: A surprisingly long list of factors must be taken into account in order to choose the pump best suited to a given poolside task.
"Right size" is not an absolute, but rather depends on a number of variables that will change from pool to pool. In that light, pump selection becomes an act of balancing pool size and the pool's hydraulic system against customer satisfaction, including a pump's economic efficiency and quiet performance.
Down to business
To select the best pump for the job, you will need to determine the pool volume and the number of gallons it will hold; the required flow rate with respect to turnover time; and the system's total dynamic head (the friction loss due to piping, fittings and equipment.)
Once these variables are determined by simple mathematics, you will be ready to select the pump that has all the right stuff for any type of pool. Moreover, even if you are replacing a pump in an existing system, it's best to go through these simple calculations to make sure you're installing the right pump for the job.
Pool Volume:
To Calculate volume, you will need to use some basic geometry to estimate the number of cubic feet in the pool. Don't worry about the math - the equations are simple, and all you need to do is plug in the numbers and multiply.
First, you need the basic dimensions of the pool; if you don't have the specs handy, you'll need to measure it. Don't be too concerned about absolute accuracy, because close estimates will be adequate for these equations. (Although if you must estimate, be sure not to underestimate.) For simplicity, make your measurements in feet and tenths of feet to keep your calculations in easy decimal form.
Naturally, the shape of the pool will determine which equation to use to calculate the volume in total cubic feet. To find the number of cubic feet in a square or rectangular pool, multiply the length by the width, then multiply by the average depth. This tells you the number of cubic feet in the pool. See Figure 1.
To calculate the volume of an oval swimming pool, multiply the length by the width, them multiply by the constant, pi (3.14). That number in turn is multiplied by the average depth of the pool. See Figure 1.
If the pool is circular, you can calculate the number of cubic feet by multiplying the radius (one-half the diameter) of the pool by itself (that is, the radius squared) and multiplying that by pi (3.14). Again you multiply the total depth to find the volume. See Figure 1.
When working on an irregular-shaped or free-form pool, estimate the volume by using the equation for an oval or rectangle as needed. If you really want to get technical, you can measure the pool with a planimeter, but that is not necessary here: All you need is a close estimate.
Pool Capacity:
Once you've calculated the volume of the pool in cubic feet, you will want to calculate the number of gallons of water that the pool will hold. There are 7.5 gallons in 1 cubic foot of water, so simply multiply the number of cubic feet in the pool by 7.5. See Figure 1.
Flow Rate:
Now determine the flow rate needed to circulate the number of gallons of water contained in the pool by the preferred turnover time. The recommended turnover time varies between regions of the country and individual preferences, but eight hours can be considered an adequate turnover time.
Simply divide the number of gallons of water in the pool by the turnover time, eight hours, to find the flow rate in gallons per hour. Then convert this figure into gallons per minute, by dividing by 60. See Figure 1.
Total Dynamic Head:
Total dynamic head (TDH) is the sum of the resistance experienced by water flowing through the circulation system. Resistance is applied throughout the system - when water flows through elbows, fittings, valves, equipment and even through straight lengths of pipe. The TDH will affect flow rate and therefore is an important factor in selecting the right pump.
All components in the circulation system, from the skimmer and main drain to the return inlets, must be factored into TDH calculations. Referring to th circulation system blueprints, list all components and dimensions, including the total length and size of PVC pipe and copper pipe, the skimmer and drains, 45-degree elbows, gate valves, backwash valves, and the size and type of filter and heater.
The next step calls for consulting head-loss charts provided by pump manufacturers to find the appropriate valve for each component. Most manufacturers have developed their own methods for interpreting a pool's needs, so contact them to obtain charts that detail their preferred techniques. Once your list of valves is complete, add them together; their sum is the total dynamic head.
Choosing the Right Pump:
After making these calculations, the rest is simple: Just choose a pump model that provides the flow rate you need, taking into consideration the head loss in the system.
As an aid, pump manufacturers have developed pump curves that show the performance characteristics for pumps on the market. These curves are plotted on a grid to indicate the size of pump needed to produce the calculated flow rate with respect to the loss of a system.
To use the curve, just locate the required flow rate and head along the borders of the chart and extend lines to a point where the two valves intersect. The proximity of the point with the pump curves plotted on the graph will indicate which pump you should choose. If your specs fall between the curves of two pump models, choose the larger model.
While you will need a pump big enough to do the job, be careful not to exceed the maximum flow rate recommended for the plumbing, filter and heater. If water is pumped through the system at too high a velocity, it may damage the equipment, cause cavitation that may damage the pump, or strip the metal from copper pipes to produce copper stains on pool walls.
Pool piping must be of the correct size to handle the needed flow rate without causing the water to flow too fast. Water velocity should not exceed 10 feet per second for discharge piping or 8 feet per second for suction piping when PVD piping is used. In the case of copper piping, the velocity should in no case exceed 8 feet per second, according to the National Spa & Pool Institute Residential Pool Standard.
Now you've narrowed the selection process down to choosing your preferred brand of pump. You're probably already familiar with most of the pumps on the market from the pools you service on your route, so the decision won't be all that tough.
Other Pumping Points:
The spa market has led to the installation of many two-speed pumps (which is actually a slight misnomer, because it is the motor that creates the two-speed effect.) Such units have a low speed that circulates the water in the concert with filtering and heating and a high speed that powers hydrotherapy jets.
Increasingly, however, two-speed pumps are being used in residential swimming pool systems for increased energy efficiency. They are typically run on low speed for normal circulation and filtration, then switched to high speed when a heavy bather lad dictates increased filtration is necessary.
Whatever the pump type, the pump and electric motor typically are sold as a single unit. If you need to replace just the pump - or us the motor - you must be sure to keep three factors in mind: the mounting face, the shaft and the horsepower (which will be regulated by the energy draw needed by the pump impeller.)
Once you're certain you have a perfect match, it's time for installation.