Grundfos SCALA1 Self-Priming Booster Pump

SCALA1 is the self-priming member of the SCALA family — an all-in-one booster with the pump, motor, tank, non-return valve and sensors in one housing, drawing from a source up to 26 ft below the pump. That suits a rainwater tank, a shallow well or a cistern, where a non-priming booster simply will not start.

Two models. The SCALA1 3-45 carries its nominal 15.8 GPM at 82 ft and reaches a 150 ft maximum head; the SCALA1 5-55 carries 23.5 GPM at the same 82 ft and reaches 180 ft. Each comes on 115V with a NEMA 5-15 plug or 230V with a NEMA 6-15, giving four part numbers.

Two SCALA1 units run as one system on the Grundfos twin installation set, part number 99725168. The set supplies the base plate, inlet and outlet manifolds with isolating valves, the communication cable and the fixing screws. The pair alternates automatically on runtime or on number of starts, so wear is shared, and either pump can be taken out of service with the building still in water. In Duty/Assist both pumps run together for more flow; in Duty/Standby one runs and the other waits.

Two pumps hold more pressure at the flow the building actually draws. Each carries half the water, so each sits higher on its own curve. Grundfos publishes both cases on one chart, and the gain is large: on the 3-45, 22 GPM is 17.5 psi added from a single pump and 46.7 psi from the pair; on the 5-55, 26.4 GPM is 26.8 psi from one and 57.5 psi from two. The shut-off ceiling at zero flow is the one figure that does not rise, because it is fixed by the pump — 62.6 psi on the 3-45 and 73.5 psi on the 5-55 as that chart draws it. Maximum two units.

The twin set and the 250 micron inlet filter are on the SCALA accessories page.

Maximum inlet pressure differs between the two models and is the limit that rules a selection out first. On a strong main the 5-55 is the tighter of the two at 29 psi. Nominal duty and maximum head are Grundfos’ printed 60 Hz figures. Source: Grundfos SCALA Data Booklet 99735562 (07.2026), operating conditions, printed page 16.

The figures below are the pressure the pump adds on top of what arrives at its inlet, read from Grundfos’ published 60 Hz curves.

SCALA1 3-45 — 60 Hz

SCALA1 5-55 — 60 Hz

Drawing from a tank, cistern or shallow well — nothing arrives with pressure, so what the pump adds is what you get, and SCALA1 will lift 26 ft to reach the source. Read the tables straight through: 13.2 GPM comes out at 46.0 psi on the 3-45 and at 62.9 psi on the 5-55.

Fed by a city main — the pump adds on top of what already arrives, and incoming plus added has to stay under the 116 psi system limit. On a 25 psi main set to hold 60 psi at the tap, a 3-45 only has to add 35 — and at 35 psi added the table gives you roughly 17 GPM, against the 6 GPM the same 60 psi would cost you drawing from a tank. Incoming pressure buys you more flow, not more pressure. Check the inlet limit as well: 43.5 psi on the 3-45, 29 psi on the 5-55.

Best operating range: 11 to 16 GPM on the 3-45, and 13 to 21 GPM on the 5-55. This is where the pump turns the most electricity into water pressure. Ask for more flow than it can deliver at your setting and it runs at full speed continuously, which is where motors and seals wear out. Sizing a booster to the building rather than to the headline numbers is what makes it last.

Zero flow is the shut-off point — a ceiling, not a working point. The tables end at the last point Grundfos draws, 21.5 GPM on the 3-45 and 31.2 GPM on the 5-55; nothing is extrapolated or transposed. Grundfos’ printed operating-conditions table gives maximum head at 60 Hz as 150 ft for the 3-45 and 180 ft for the 5-55; the plotted curves read 150 ft and 182 ft at zero flow, and these tables print the curve. Best operating ranges are derived by us from the published head and input-power curves — wire-to-water efficiency peaks near 31 percent on the 3-45 and 34 percent on the 5-55 — and are not stated by Grundfos. Source: Grundfos SCALA Data Booklet 99735562 (07.2026), 60 Hz performance curves TM075369 and TM075368, printed pages 14 and 15.

The free Grundfos GO app gives you setup, status overviews, technical product information and live operating parameters from an iOS or Android device. SCALA1 has Bluetooth built in — pair the pump directly with your phone, no extra module needed.