Variable-speed pump: less energy with slower flow

What changes compared with a conventional pump

A fixed-speed pump has a motor that always turns at the same speed. Whether a lot of flow is needed or a little, it runs at the same point. A variable-speed pump has electronic motor control (an inverter) and usually a permanent magnet motor, more efficient than the conventional kind. You or the automation set the speed and schedule for each task.

Its other features, such as flow rate, head and pre-filter, are the same as for any pool pump. Here we look at what you gain from adjusting the speed.

Why low speed uses less energy

For centrifugal pumps, the following rule applies approximately:

• flow changes in proportion to speed;
• the power drawn by the motor changes much more steeply, roughly with the cube of the speed.

If the speed drops by half, the flow drops to about half, but the power in theory drops to about one eighth. To pass the same volume of water through the filter, the pump runs twice as many hours, and the total energy works out in theory at around one quarter. In practice the saving is smaller, because there are minimum flows that have to be maintained, but it remains one of the most effective measures for your pool's electricity consumption.

Slow flow also helps filtration: the water passes through the filter more gently, and the filter holds back fine particles better.

Less noise

At low speed the pump is barely audible. This matters when the plant room is close to bedrooms or to the boundary with your neighbour. Filtration can also run in the evening without disturbing anyone, while high speeds are kept for a few minutes when they are really needed.

How to set it up in practice

Most pumps have a built-in schedule with three or four stored speeds. A typical setup:

Points to watch:

• Skimmers need enough flow to "pull" the surface. If leaves are left floating at very low speed, raise the low speed a little or add a period at a higher speed.
• The heat pump and the salt chlorinator have a flow switch; below a certain limit they stop working.
• In an overflow pool, the flow must keep the water spilling evenly into the channel.
• In summer the hours go up, in spring and autumn they come down. You will find sample schedules under pump running hours.

Who it is worth it for

For a new pool that will run all summer, or longer with heating, a variable-speed pump is usually the sensible choice. In an existing pool, replacing the old pump is one of the simplest upgrades in a renovation, as long as it is checked to match the filter and pipework. How it affects the cost of your pool is something we look at with you during the site survey.

Questions

See also