The July 19, 2021, pump regulation date has passed, meaning that most single-speed pumps will no longer pass muster with the Department of Energy. But so long as distributors continue to have stock on the shelves, single-speed pumps will still be available for purchase. That means pool owners will still have the option of replacing their existing pumps with single-speed models, a decision that might, at first glance, seem like a good idea given the lower initial sticker price.
But if pool owners understood the long-term monetary savings that can be realized with variable-speed technology, they might be convinced to make a different decision.
That’s because if there is one reason to convert from a single-speed pump to variable-speed technology, it is the energy savings over the long haul.
Manufacturers boast up to 90% financial savings which, for many residential pools, can translate close to $1,000 per year. This figure is based on a comparison of a single-speed pump running about 8 hours per day for a 20,000-gallon pool at a price of $0.16 per kilowatt hour.
How do they figure that?
First, it’s necessary to understand why variable-speed pumps are more energy efficient.
There is a single major reason for the enhanced energy efficiency, and the concept can be summed up as “less is best.”
Actually, it’s called the Pump Affinity Law, and in hydraulics, it’s
only one of several affinity laws that express the relationship between variables (head, flow rate, shaft speed, power) involved in pump performance.
There are three relationships described by the law particularly relevant to the discussion of pumps.
First,theshaftspeedinrevolutionsper minute is proportional to the flow rate.
Second, pressure or head is proportional to the square of the shaft speed.
Third, power is proportional to the cube of the shaft speed.
Speed (rpm) ≈ Flow (GPM) Speed (rpm) ≈ Pressure2 (ft of Head) Speed (rpm) ≈ Power3 (kW) Thus, if we reduce the speed by half, we reduce the power to an eighth, or
½ speed ≈ ½ x ½ x ½ power = 1/8 kW.
Similarly, if we reduce the speed by a third, we reduce the power to a 27th.
Since the shaft speed is proportional to the flow rate, the power can be significantly conserved by lowering the flow rate.
The fact is, operating pumps for normal filtration at high flow rates is an unnecessary waste of energy.
Because a pool pump must perform a variety of operations, it must be sized to perform the most demanding task, which may include running the filter or pool cleaner, operating jets or fountains, and running a solar heater.
Since single-speed pumps cannot change their flow rates, for normal filtration purposes (the primary purpose of the pump), they are providing far greater circulation than the filtration system needs.
Switching to a variable-speed pump, and operating it at the lowest flow rate while providing adequate filtration, will significantly lower the energy usage, thus saving money.
To convince pool owners of the merits of switching pumps, it’s useful to determine the potential financial savings possible.
A standard, traditional single-speed pool pump is typically 1½ to 2 horsepower and operates using a singlespeed induction motor to generate 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, we can calculate the savings for a 15,000 gallon pool running for 6 hours a day.
At this rate, the single-speed pump consumes 12 kilowatt hours. The price of electricity varies by region, but using a value of 16 cents per kWh, a singlespeed pump costs about $700 per year.
Because of the pump affinity law, lower flow rates lead to greater energy savings.
For the same pool, set at 30 gpm, and drawing 300 watts of power running
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at 10 hours per day, a years’ use of a variable-speed pump would cost $175. That’s a savings of $525 per