The difference between a fixed speed pump and a variable frequency drive (VFD) booster is what happens between start and stop. A fixed speed pump has two states: running at full speed, or off. A VFD booster reads the pressure in the line and adjusts motor speed to hold a set point, so it can run slowly when demand is low.
How each type behaves
- Fixed speed: starts at full speed, fills the pressure vessel, then stops. Pressure rises and falls between the cut-in and cut-out points, and each start draws a current surge.
- VFD constant pressure: starts softly, then matches speed to the number of outlets in use. The set point is held while the family uses water, and speed falls as demand falls.
Where the difference is noticed
Single-storey houses with a steady mains supply often work acceptably with a fixed speed pump. The difference shows up where demand changes a lot, or where the pipe run is long.
- Multi-storey homes and villas, where a shower and a washing machine can run at the same time on different floors.
- Peak hours in small hotels and guesthouses, when several rooms draw water together.
- Irrigation and garden lines that are opened and closed tap by tap.
- Sites with unstable supply voltage, where a soft start avoids a hard current draw.
What stays the same
Both types still need correct flow and head selection, a clean water supply and a properly sized inlet. A VFD booster does not remove the need for a strainer, a non-return valve or dry-run protection on suction-lift installations.
Noise and running cost also depend on the duty point. A pump running near its design duty is quieter and draws closer to its rated input than a pump throttled far from the curve. Our technology page explains the permanent magnet motor and controller arrangement in more detail.