Although the low speed isn't as efficient in watts per rpm the real gains are in lack of resistance to flow. 1/2 the rpm and flow but 1/4 the power usage. variable speed ecm motors are great but super pricey. Depending on what you pay for power the payback could be way out vs a cheap 2 speed. Though 2 speeds aren't necessarily perfect I'd rather make my money back sooner in case of failure.
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