Variable speed pool pumps and energy use: how they work and what to ask before switching

How a variable speed pump differs

A single speed pump runs at one speed all the time. The US Department of Energy's measure guideline on pool pumps explains that a pump is called on for several jobs: circulating water through the filter, running cleaners, backwashing, feeding a chlorinator or heater and supplying water features. The demanding jobs take only a small share of the running time, but a single speed pump has to be sized for the most demanding one, so for most of its hours it moves more water than filtration needs.

A variable speed pump can be programmed to run at different speeds for different jobs. The DOE guide notes that many variable speed pumps use permanent magnet motors, which it rates at around 90 percent efficiency, compared with 30 to 70 percent for an average single speed pump.

The physics: the affinity law

Most of the saving comes from a simple relationship. The DOE guide explains that when motor speed is halved, the flow halves too, but the power drawn falls to about one eighth. ENERGY STAR makes the same point, and adds that filtration needs about half the flow of running a pool cleaner.

The DOE guide's worked example for one pump shows how steep the curve is: about 2,000 watts at 3,450 rpm, about 250 watts at 1,800 rpm, and about 74 watts at 1,200 rpm. Your pump will differ, but the shape of the curve does not.

The guide sums up the principle as "run it slower and run it longer". Lower speed saves power, but the pump has to run for more hours to move the same volume of water.

ENERGY STAR reports that its certified in ground pool pumps use 20 percent less energy than standard models. That is a US figure across certified pumps, not a promise for a UAE villa.

The UAE constraint: turnover time

This is where UAE pools differ from the examples in American guidance. Dubai Municipality's technical guidelines for pool plans, which apply to public and private pools in Dubai, say pool water turnover must not exceed four hours. Its guideline for private villa pools says the retention time must not exceed four hours.

The DOE guide's example pool is turned over once in 24 hours, with a variable speed pump taking around 19 hours to do it. That schedule would not meet the Dubai figure. A pump in Dubai can only be slowed to the point where it still circulates the whole pool volume within four hours.

Savings are still possible. The DOE guide notes that single speed pumps are often oversized, and even a modest speed reduction cuts power sharply because of the affinity law. Expect smaller savings than headline examples, and be wary of any proposal that does not mention turnover at all. Other emirates set their own rules, so check the figure that applies to your pool.

Other equipment that needs flow

A pump does not work alone, and slowing it down affects everything downstream.

• Chillers and heat pumps. Jandy's manual for its VersaTemp heat pump range, for example, lists a minimum and a maximum water flow for each model and uses a water pressure switch to protect the unit. Too little flow and the unit will not run properly.
• Skimmers. The DOE guide notes that most skimmers need roughly 25 gallons per minute to work well, and that the skimmers should supply most of the flow.
• Overflow and infinity edges, cleaners and water features. These usually need higher speeds, which should be programmed as separate scheduled periods.
• Safety. The DOE guide warns that some variable speed pumps can produce more flow than the old pump, which can create suction entrapment hazards with older equipment, and that scheduled programmes can start the pump unexpectedly. Power must be isolated before anyone opens the pump.

Questions to ask before switching

1. What is my pool volume, and what speed does the new pump need to meet the four hour turnover?
2. What flow do my filter, chiller or heat pump, chlorinator and skimmers need, and will the programme provide it?
3. Who will programme the pump, and will they show me how to change the schedule and use a temporary boost after heavy use?
4. Will the existing pipework, valves and filter suit the new pump? The DOE guide notes that smaller pipes and sharp bends add resistance and reduce savings.
5. Who will check the electrical supply and make the connections? The DOE guide tells installers to verify the supply voltage, wire and breaker size and to reconnect according to local codes. This work must be done by a qualified electrician.
6. Can the old and new pumps' energy use be measured, so the saving is confirmed rather than assumed? The DOE guide recommends exactly that comparison.

After installation

Maintenance is much the same as for any pump. Keep the strainer basket clean, give the motor shade and ventilation, keep chemicals away from it, and watch the water. If clarity slips after the change, the speed or run time may be too low, and the programme needs adjusting.