As unique as each installation is, all pools require the same basic calculations before a pump can be chosen. First, you must determine factors such as the pool’s volume and capacity, the required water flow rate for proper water turnover and the system’s total dynamic head. Then, you must ensure that the suction pipe size can handle the flow at a safe velocity.
Even if you’re replacing a pump in an existing system, it’s best to go through the simple calculations provided here to be sure you’re installing the right pump for that pool.
Pool volume: To determine pool volume (the amount of space measured in cubic feet), you’ll need the basic pool dimensions. If you don’t have the specs handy, measure the pool. Don’t be too concerned about absolute accuracy because close estimates will be adequate for these equations. (If you must estimate, be sure not to underestimate.) For simplicity, make your measurements in feet and tenths of feet to keep the calculations in easy decimal form.Naturally, the pool’s shape will determine which equation to use to calculate the volume in total cubic feet. To figure the volume of oval, circular, square or rectangular, and irregular or free-form pools, see “Formulas” (below) or use a smartphone app.
Pool capacity: Next, determine the pool capacity (the number of gallons it will hold), keeping in mind that there are 7.48 gallons in 1 cubic foot of water. Simply multiply the volume of the pool by 7.48.
Flow rate: Now it’s time to determine the flow rate needed to circulate the pool water in a timely manner. The flow rate is simply how many gallons of water per minute the pump moves. It is tied into the turnover rate, that is, the amount of time it takes to move the entire capacity of the pool through the system. The recommended turnover time varies depending upon region and individual preferences, but eight hours can be considered an adequate turnover time. (To calculate the flow rate, see “Formulas.”)
Total dynamic head: Total dynamic head (TDH) is the sum of the resistance encountered by water flowing through the pool’s circulation system. Resistance is applied throughout the system, such as 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. Match the TDH of the system with the TDH of the 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 the circulation system blueprints, list every component and dimension, including the total length and size of PVC pipe and copper pipe, the skimmer and drains, 45-degree elbows, 90-degree elbows, gate valves, backwash valves, and the size and type of filter and heater.
Consult the pump manufacturer’s head-loss charts to find the appropriate value for each component. Most manufacturers have developed their own methods for interpreting a pool’s needs, so contact them to obtain charts showing their preferred techniques (see sample charts on this page). Once your list of values is com plete, add them together; their sum is the total dynamic head of the system.
If you do not know what’s under the deck, there is another option to calculate the TDH. Just use a pressure meter and this simple formula:
1. Check the influent pressure of the filter tank and multiply that number by 2.31.2. Next, use the meter to take a vacuum reading on the pump suction line and multiply that number by 1.13.
3. Add these two numbers together to get the system’s TDH. For example, if the pressure reading is 12 psi, multiply that by 2.31 to get 27.72. If the vacuum reading is 5” mercury, multiply that by 1.13 and you’ll get 5.65.
4. Add the two results together: 27.72 + 5.65 = 33.37 total feet of head.
Consult the charts
After making these calculations, the rest is easy. Consult the pump manufacturer’s performance curve chart (see “System Head Curve” below). Using the flow rate and TDH numbers, determine the pump model with the appropriate horsepower.
Keep in mind, however, that these charts vary from manufacturer to manufacturer. You cannot consult a pump curve chart from Company A and then use that information to buy the same horsepower pump from Company B. It’s unlikely they’ll be the same.
While you’ll 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 harm the pump, or strip the metal from copper pipes, producing leaks and copper stains.
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 8 feet per second for discharge (pressure) piping or 6 feet per second for suction piping when PVC piping is used. (This complies with the new California Standards, which went into effect in January 2008.) In the case of copper piping, the velocity should in no case exceed 8 feet per second, according to APSP’s residential pool standard.
Up to speed
Just a few more points to ponder before deciding which pump brand you want.
Two-speed pumps, which are popular in the spa sector, also are being used on residential pool systems for increased energy efficiency. Such pumps typically are run on low speed for normal circulation and filtration, then switched to high speed to operate an automatic cleaner or when a heavy bather load dictates increased filtration is necessary. New on the market are variable-speed pumps. These pumps have three main components: a wet end, a motor and a drive. The drive separates this pump from standard pump models. The drive is engineered to find the optimum flow for a given task — whether it requires high flow such as when operating spa jets, or a lower level such as what’s needed to perform basic circulation. Using the optimal flow minimizes energy consumption.
Regardless of the pump type used, the pump and electric motor typically are sold as a single unit. If you need to replace just the pump — or just the motor — 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).
Acknowledgements: Technical information was provided by Baker Hydro, Hayward Pool Products Inc., Jacuzzi, Pentair Aquatic Systems, and Premier Spring Water Inc.
