When homeowners decide to add a pool and/or spa to their property, they can typically rely on professionals to handle the technical specifications involved in making their designs reality. This includes choosing the proper equipment –from pumps to heaters to chlorinators - to make everything run smoothly and efficiently.
But over time, of course, all machines develop problems. Pumps, which perform the vast majority of work involved in a pool’s daily upkeep, are no different. They break down, develop leaks, and are made obsolete by newer, more energy-efficient models.
Is Change Really Necessary?
Some owners assume that replacing their existing pump (regardless of its age) with an identical new model is the safest move, but manufacturers are constantly improving their products and there can be a host of options that didn’t exist at the time of the previous purchase. And many who inherited pools in moves can’t even be certain how old their equipment is in the first place.
This can leave a pool owner in an uncomfortable position when the pump first shows problems, trying to assess individual needs while taking stock of all available options in the marketplace. But there are a number of steps you can take to make shopping for a pump less daunting, and to help ensure that you make the right purchase for your own particular situation.
How Do I Know What I Need?
First, you'll need to determine how powerful a pump you need:
Then measure the length and width of the pool in feet, and use this formula:
Length x Width x Average Depth x 7.5 = Pool capacity in gallons
So a rectangular pool that measures 40-feet by 20-feet with an average depth of 6-feet would have a capacity of 36,000 gallons (40 x 20 x 6 x 7.5).
According to industry professionals, a circular pool (like
an above ground model shown in the middle below) would use a multiplier of 5.9 instead of 7.5, and an
oval pool would use 6.7.
Pool Capacity / Turnover in Minutes = Flow Rate
36000 / 480 = 75 Gallons per Minute (gpm)
Next, you need to find the maximum flow rate possible for your pool given the number and size of intake lines which feed the filter and pump. Again, industry standards are a big help here. For lines 1.5-inches in diameter, the maximum flow rate is 42 gallons per minute, and 2-inch lines can accommodate 73 gpm.
So if your pool has one 2-inch intake line, your maximum flow rate is 73 gpm. This is useful information because the pump should never be operating at a higher gpm than your filter can handle. Thus in the example above, the desired flow rate of 75 gpm should be lowered slightly to avoid overworking the filter in the case of one 2-inch intake line. This is not a problem – in fact, operating a pump at slower speeds saves both money and wear and tear on valuable equipment while still cycling pool water sufficiently.
Step Three - Our final step in the math class phase of this process is determining your pool’s resistance to flow, measured in feet of head. This is made up of pipe length plus logistical factors which make circulating water more difficult, such as 90-degree turns in pipes, the use of heaters as well as pipes that narrow in diameter.
A generally reliable estimate of head is the distance in feet from the pool’s main drain to the pump itself (though more elaborate setups involving landscaping and spas could increase this number).
Matching Need to Speed
Thus, if the rectangular pool in the example above has 30-feet of head and one 2-inch intake line for a maximum flow rate of 73 gpm, its owner is now ready to shop for a pump with a much better idea how strong a model (measured in horsepower) is needed.
Pump manufacturers generally provide charts showing where their various models perform optimally along the relationship between flow rate and feet of head, as shown in this example of Pentair pumps:
Using the graph above, the one-horsepower Pentair model would seem the best option for handling 73 gpm and 30 feet of head. So once again, by using readily available information a pool owner can save substantial money (and strain on other system components), in this case by identifying a smaller pump as ideal for this particular situation.
Is Variable-speed Right for Me?
Variable-speed pumps in many ways have revolutionized the industry in recent years. The user can change the desired rate of flow as needed, taking a lot of the guesswork (and math) involved in such research out of the equation. Owners like the flexibility of operating their pump in proportion to the pool’s level of use for a given period of time (the busy summer months as opposed to the fall, for instance).
And more importantly to most consumers, these newer models generally are dramatically quieter and more energy efficient than the older single- and two-speed pumps. A federal government study, in fact, concluded that variable-speed pumps can reduce energy usage by 50 to 75 percent over the single-speed variety. They’re also much more amenable to a change in the pool system (changing a filter size, adding a spa, etc.) due to their programmability.
However, single-speed pumps (chosen correctly) are often completely sufficient for a given pool and generally are far more economical – at least up front - than their variable speed counterparts. Their simplicity of design also makes them inherently less susceptible to mechanical issues and operator error. The problem, of course, is that they’re more of an energy drain than the other types since they only run at one speed, which tends to show up every month in the power bill.
Two-speed pumps tend to split the difference between these two approaches. They typically have a toggle switch on the unit which allows the pool owner to set the pump for higher performance when needed or for a lower, more energy-efficient mode. Users report that this flexibility makes a major difference in reducing energy bills very similar to that provided by variable-speed pumps at a substantially lower cost up front.
With so many options from reputable suppliers (and a wealth of customer feedback available online), there’s no reason not to make an educated decision about a pump purchase.
Replacing a pump is generally the costliest naturally-recurring expense of operating a pool, but with the proper amount of planning and research there’s no question you can find the ideal solution for your own needs.
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