Pool Pump Speed Reduction at Constant Turnover

The affinity laws give flow proportional to speed and power to the CUBE of speed, so half speed is an eighth of the power -- and that is the figure a variable speed pump is sold on. IT IS NOT THE SAVING, BECAUSE THE TURNOVER IS FIXED. A pool needs its volume moved through the filter regardless of how fast the pump runs, so halving the speed doubles the run time. An eighth of the power for twice as long is a QUARTER of the energy: the cube divided by the speed, which is the square. That is still an excellent saving, and it is half again less than the number the power ratio alone suggests, so quoting the cube law to a pool owner overstates the result. THE EQUIPMENT MINIMUM IS THE REAL CONSTRAINT and it is what stops the answer being 'run as slow as possible'. A gas heater will not fire below its minimum flow, a salt chlorine generator will not produce, a pressure or suction cleaner will not drive, and a solar collector will not lift water to the roof. The usual resolution is a schedule rather than a single speed: a short high-speed period for whatever needs flow, and the balance of the turnover at the lowest speed the filter and the plumbing allow. AND FILTRATION IMPROVES AT LOW FLOW, which surprises people. Slower water through the medium gives fine particles time to be intercepted instead of being driven through, so a long slow cycle filters better than a short fast one as well as costing less. The instinct that a pump idling on low speed is not doing real work is exactly backwards, and it is the reason single-speed pumps ran short and hard for decades. This computes energy from entered figures. It does not model the pump curve or the system curve (the affinity relations assume the pump follows its curve on a fixed system, and a real installation has static head from the solar or the spa spillway that flattens it), determine the turnover a pool needs or the flow its filter is rated for, size a pump or a filter, account for motor and drive efficiency changing with speed, or evaluate what any code requires. The pump manufacturer's curve, the filter's rated flow, and the pool professional govern.

the affinity laws -- flow proportional to speed, power to the CUBE of speed -- with the TURNOVER held constant, so run time extends inversely with flow and the energy per turnover is speed^3 / speed, which is speed SQUARED.

Centrifugal pump affinity relations applied at constant turnover volume. The equipment minimum flow is ENTERED because heaters, salt cells, cleaners and solar all have one.

The published affinity relations and one exact horsepower conversion.

Estimate. AHJ and licensed professional govern.

Field names used by the API: pump_hp, full_speed_hours, speed_fraction, electricity_rate_per_kwh, days_per_year, minimum_flow_fraction, full_kw, reduced_kw, reduced_hours, energy_fraction, full_kwh_day, reduced_kwh_day, saving_kwh_day, annual_saving_cost

• Turnover held constant which is what makes the exponent 2 rather than 3standard pool practice
• Equipment minimum flow is entered heater, salt cell, cleaner and solar each have onethe equipment manufacturers
• Fixed system curve assumed static head from solar or a spillway flattens the affinity relationthe pump manufacturer's curve