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  • reasons to convert to a variable-speed pump

    March 4, 2016
  • There are plenty of reasons to convert to a variable-speed pump: they are quieter; they can be programmed to run at different speeds for specific tasks; and they are even required in many areas. But the NO. 1 reason to convert from a single-speed pump is the long-term energy savings. Simply put, variable-speed pumps are more energy efficient. By virtue of their design — a permanent magnet monitor instead of an induction motor– they are inherently more efficient than single- and dual-speed motors. But the main reason they are more efficient is their ability to be programmed to slower flow rates. Everyday circulation simply does not require the high-flow rates of single-speed pumps which are necessary for the pump to perform its most demanding job. And when it comes to everyday circulation needs, less is best  should be he motto. This is because of the Pump Affinity Law, one of several affinity laws that express the relationship between head, flow rate, shaft speed and power that are involved in pump performance.

    There are three relationships given by the law that are particularly relevant to the discussion of pumps. The first is that the shaft speed is proportional to the flow rate. The second is that the pressure or head is proportional to the square of the shaft speed. The third is that the power is proportional to the cube of the shaft speed. So if the speed is reduced by half, the power is reduced by an eighth, or 1/2 speed = 1/2 x 1/2 x 1/2 power = 1/8 kW.If the speed is reduced by a third, the power is reduced by a 27th. So when we slow the shaft speed, lowering the flow rate, power is dramatically reduced. Operating pumps at high-flow rates is a waste of energy for every day use. But single-speed pool pumps are needed to perform a variety of jobs, and have to be sized to perform the most demanding task. That could include running the filter or pool cleaner, operating jets or fountains, and running a solar heater.

    Because single-speed pumps cannot change their flow rates, they are providing far greater circulation than the filtration system needs. That is the advantage of variable-speed pumps, they can be operated at the lowest flow rate that provides adequate filtration most of the time , and then amped up only when the situation calls for it. That equals substantial financial savings. They will significantly lower the energy usage, thereby saving money, and it is interesting to determine the potential  financial savings possible. A standard, traditional single-speed pool pump is typically     1 1/2- to 2- horsepower, generating motor speeds of 3,450 revolutions per minute, resulting in a flow rate of 87 gallons per minute. The power necessary to accomplish that amount of flow is approximately 2 kilowatts.

    Using these values, for a 15,000 gallon pool, one turnover would require almost 3 hours. At this rate, the single-speed pump would consume 6 kilowatt hours. The price of electricity varies by region, but using the national average this year of about 13 cents per kWh, a single-speed pump costs about $284 per year. Because of the pump affinity law, lower flow rates lead to greater energy savings. For the same pool, set at 22 GPM, and drawing 84 watts of power for one turnover in 11.3 hours, one year’s use of a variable-speed pump would cost $52.30. But this is assuming that the pool owner is operating their single-speed pump for the required period of time, and it is a good idea to determine the real amount of of time that is being used to get a true idea of the real savings. Ask the pool owner or review the timer because in practice, pool owners may be operating their pumps much longer than in the example given here. The long and short of it is that the energy savings is real.

    The initial price of a variable-speed pump is greater than traditional pumps but many manufacturers advertise that the money can be recouped in 3 years ago or so. And plenty state utility companies offer rebates and incentives for converting to energy efficient, variable-speed pumps. Nationwide, incentives range from $100 to $600. Considering that an average variable-speed pump is priced at roughly $1,200, the incentives make these pumps even more attractive.

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