· by Michael Blair, Nashville's Pool & Spa
For most of the history of residential pools, the pump had one speed: on. It ran at full power for eight or more hours a day, turning the water over whether the pool needed that flow or not, and it was the biggest item on the summer electric bill after air conditioning. Variable-speed pumps changed that, and federal efficiency rules have since made them the standard for new pumps above a certain size. If your pool still has a single-speed pump, the question is not whether the newer type is better, but whether the switch is worth doing before the old one fails.
Why a slower pump costs so much less
Pumping water follows a rule called the affinity law: the power a pump draws rises with the cube of its speed. Run a pump at half speed and it moves half the water but uses roughly an eighth of the power. So a pump that runs slowly for a long time moves the same volume of water through the filter as one that runs fast for a short time, for a fraction of the electricity.
Filtration and chemical distribution are better at low flow as well. Water moving slowly through the filter is cleaned more thoroughly, and a pool that circulates gently all day stays clearer than one blasted for eight hours and left still for sixteen.
What a variable-speed pump actually does
It has a permanent-magnet motor with a controller that lets you set speeds and a schedule: low speed for most of the day for filtration, a higher speed for a couple of hours to run the skimmer and the chlorinator effectively, full speed when the vacuum runs or the spa jets are on. It is quieter at low speed, to the point of being hard to hear, and the motor runs cooler and tends to last longer than a single-speed induction motor.
The controller is the part people underuse. A variable-speed pump left at full speed all day saves nothing. The savings come from a schedule set for the pool, which a pool technician can do in a visit, and which is worth revisiting when the season changes or an automatic cleaner is added.
- Low speed most of the day: filtration and chemical circulation
- Medium speed a few hours: skimming and salt cell or chlorinator flow
- High speed on demand: cleaner, spa jets, backwash
- Off at night if the pool allows, or lowest speed for freeze protection in winter
The rules, and what that means for a replacement
Federal efficiency standards for dedicated-purpose pool pumps took effect in 2021 and effectively require most new residential inground pool pumps above a small size to be variable-speed. Single-speed replacement motors for existing pumps are still available in some cases, but the market has moved, and a new pump for an inground pool is going to be variable-speed whether or not you set out to buy one.
That changes the math on an aging single-speed pump. Replacing the motor alone on the old pump keeps the old technology and the old running cost; replacing the whole pump gets the variable-speed unit that a new install would have anyway. Some utilities have offered rebates for the switch; check with NES or your provider for current programs.
When to switch and what to check
If the single-speed pump is running well and the pool is small, waiting until it fails is defensible. If the pump is loud, hot, more than a decade old, or the bill is high, replacing it now starts the savings sooner and avoids an emergency replacement in July when every pool company is booked. The savings are largest on pools that run long hours, on pools with spas and water features, and on a pump that was oversized to begin with, which most were.
Match the new pump to the plumbing and the filter. A variable-speed pump on undersized pipe still has to push hard, and a cartridge or DE filter has a flow range it works best in. Get the plumbing looked at, get the schedule set, and check the first month’s bill against the last.