Two-Speed pool pump motor: Worth it?

With electricity prices set to skyrocket in the Baltimore area this summer, I’m once again looking at ways to cut down on our consumption. And in the summer, one of our biggest consumers is the pool pump. I’ve read that a two-speed pool pump motor can cut down quite a bit on energy usage. A standard swimming pool pump runs at 3450 RPM. A two-speed pump can also run at half speed, or 1725 RPM. The interesting thing is, although it’s running at 50% of normal speed, a typical model draws less than half (about 30%) of the current that it would draw running at full speed. So in theory, you could run the pump at half speed for twice the length of a normal-speed run cycle, do the same amount of work, and use less power. Now, this might be wrong. I’m not well-versed enough in fluid dynamics to say whether, for instance, 10 hours at 1725 RPM would turn over the same amount of water as 5 hours at 3450 RPM. But, for the sake of argument, let’s assume it would. Running at low speed would certainly consume less power, and longer pump run cycles are good for the pool water because they keep it from stagnating (I’ve also heard anecdotal reports that sand filters “work better” with lower flow rates — take that for what it’s worth). Of course, that’s only one side of the story. There are also some caveats..

  1. I can’t run the pump on low speed 100% of the time. I have a pressure-side pool cleaner (Polaris) with a booster pump, and I’d need to run at full speed while the cleaner was on (3 hours a day, 3 days a week or so). And, I’ll want to run on full speed for 3 hours or so after adding chemicals, to get them distributed as quickly as possible. Any time the pump is on high speed, it cuts into my potential savings.
  2. The pump motors I’ve looked at all consume somewhat more current (1 amp or so on models I’ve looked at) on high speed, than equivalent single-speed motors. Not quite sure why this is, but it seems to be a fact of life.
  3. Initial cost for a two-speed setup is high, even assuming I’m only replacing the motor and not the entire pump. The pump would need special wiring, and a new timer/control to control high vs low speed operation. Plus, I’d need a new shaft seal. I’m probably looking somewhere in the $350-$400 range when all is said and done.

A couple years back I made a spreadsheet to estimate potential savings using a two-speed pump, and concluded that the payback period was too long to justify the initial effort and expense, given what I was paying for electricity at the time. However, with the pending rate hike, it’s probably worthwhile to crunch those numbers again..