Comparing The Flow Families

The flow family is the first decision; the model within it sets how deep a well it holds that flow in.

Sizing & Best Practices

How we size an SP pump, and what to check before installing.

1. Calculate the total head

Total head = depth to pumping water level at your maximum draw + the pressure you want in feet (psi × 2.31) + above-grade lift + friction loss in the drop pipe and discharge line. Size the pump so its head at your flow clears that number.

2. Estimate peak flow honestly

Use WSFU / Hunter’s curve (IPC Table 604.3) for residential demand and the IAPMO Water Demand Calculator for light commercial. A single-family home usually peaks at 8–12 GPM — match that to the 5S, 7S or 10S family rather than the flow on the label.

3. Choose 2-wire or 3-wire

2-wire single-phase needs no control box and is the simplest install. 3-wire single-phase uses a surface control box (included) and covers more horsepower. Three-phase needs a starter or a CUE drive and is the choice for the largest motors.

4. Check the well and the limits

Minimum well casing or sleeve is 4 in. Set the motor at least a few feet below maximum drawdown, above the well screen and clear of the bottom. Liquid temperature above 86 °F requires flow past the motor, which usually means a flow sleeve.

What Makes A Grundfos SP Different

Why the SP range is the reference for stainless submersibles.

Grundfos SP vs A.Y. McDonald

We sell both lines. Here is the honest head-to-head at matched flow families, all-stainless build against all-stainless build, so you can choose on the merits.

10 GPM — Grundfos 10S vs A.Y. McDonald 24000 K

Verdict. Even at 10 GPM the two are close on paper. The Grundfos 10S carries the documented NSF 61 and 372 listing and the fully stainless build with sand-channel bearings and priming screw; the A.Y. McDonald 24000 ships with the motor starter kit included on three-phase and is typically the lower-cost complete package. If first cost drives the job, the A.Y. McDonald wins; if the specification calls out NSF listing and long service life in sandy or aggressive water, the Grundfos does.

15 GPM — Grundfos 16S vs A.Y. McDonald 24000 L

Verdict. The Grundfos 16S rates a nominal 16 GPM against the A.Y. McDonald 15 GPM and reaches 10 HP and far deeper settings, so on a deep or high-head 15 GPM well the 16S has real headroom the 24000 L does not. For a shallow-to-moderate 15 GPM domestic well the A.Y. McDonald covers the duty at lower cost.

25 GPM — Grundfos 25S vs A.Y. McDonald 24000 M

Verdict. At 25 GPM the Grundfos 25S pulls ahead on head and horsepower and offers a factory constant-pressure package (CUE 100) the A.Y. McDonald line does not match. The A.Y. McDonald 24000 M remains the value pick for a straightforward fixed-speed 25 GPM well within its head range.

35 GPM — Grundfos 35S vs A.Y. McDonald 24000 R

Verdict. On high-flow, high-head duty the Grundfos 35S reaches 20 HP and holds a far wider range of duty points; the A.Y. McDonald 24000 R covers the mainstream 35 GPM band at lower cost. A.Y. McDonald also offers the AutoPAC pre-assembled system (pump, motor, tank and control), which has no direct Grundfos equivalent in this range.