Franklin Electric BT4 Multi-Stage Booster Pump

The BT4 is Franklin’s high-performance booster, and it comes in two distinct families that share a name and little else.

The 5 to 25 GPM family is built on Franklin’s patented Tri-Seal floating stage system (U.S. patent 7,290,984), with acetal impellers, modified PPO diffusers and 300-series stainless hub, eye seals, shaft and shell. Stainless discharge and a stainless bracket with a rugged support foot for horizontal or vertical mounting, a carry handle, and 1 in NPT inlet and outlet.

The 35 to 45 GPM family has enhanced hydraulics for those higher nominal ratings, 1.5 in NPT connections to cut system friction loss, a bracket that rotates in 90° increments, multiple drain and pressure tappings for bypass and accessory plumbing, and a back pull-out design that lets the motor come off for service without disturbing the pump or the plumbing.

Both families carry the same three hard limits: 75 psi maximum inlet, 400 psi maximum discharge, 120°F maximum fluid.

The BT4 is a pump and motor assembly supplied as a single unit. Franklin publishes no paired or staged configuration for the series — multi-pump operation is a function of the control in front of it.

Where the job wants constant pressure or redundancy, Franklin packages that separately. The VersaBoost Pro links four units wirelessly in parallel, and the Inline SpecPAK stages two to four pumps on one manifold with lead/lag or parallel selectable from a touchscreen.

Tri-Seal floating stage system — Franklin’s patented design (U.S. patent 7,290,984) on the 5–25 GPM family, which Franklin publishes as giving maximum efficiency, longevity and performance.

Serviceable without breaking the plumbing — The two-compartment square-flange motors are serviced or replaced without dismantling the pump end or disturbing the plumbing connections. On the 35–45 GPM family the back pull-out design does the same thing.

400 psi maximum discharge — A discharge rating far above what a domestic booster needs, which is what makes this pump usable in pressure washing, misting and spray-cooling work.

Stainless bracket, rotatable — Rugged support foot for horizontal or vertical mounting with a carry handle on the 5–25 family; on the 35–45 family the bracket rotates in 90° increments for compact plumbing.

Drain and pressure tappings — On the 35–45 family, multiple bracket tappings allow draining in any position and provide for bypass and accessory plumbing.

Inverter duty rated, three phase — High service factor two-compartment dual-voltage motors, with the three-phase versions inverter duty rated.

The figures below are the pressure the pump adds. Franklin publishes tabulated boost for the 35 and 45 GPM families, so those tables are the manufacturer’s own.

35 GPM nominal — PSI boost at flow

45 GPM nominal — PSI boost at flow

5 GPM nominal — PSI boost at flow

7 GPM nominal — PSI boost at flow

10 GPM nominal — PSI boost at flow

15 GPM nominal — PSI boost at flow

20 GPM nominal — PSI boost at flow

25 GPM nominal — PSI boost at flow

5 to 25 GPM models. Franklin publishes these as plotted curves rather than tables, with the pump operating range and best efficiency range printed as bands on each chart. Rather than read points off a plot and publish them as data, we quote the published ranges below and will send the factory curve for any duty point you name.

Drawing from a tank or cistern — nothing arrives with pressure, so what the pump adds is what you get. Read the table straight through: a 3507FBT2S4 delivers 84 psi at 30 GPM.

Fed by a city main — the pump adds on top of what already arrives, and Franklin caps inlet pressure at 75 psi for the whole series. On a 50 psi main the 3509FBT3S4 puts 163 psi into the system at 30 GPM, which is well past what any domestic plumbing wants — a pressure reducing valve or a smaller stage count is the answer, not a bigger pump.

Best operating range: the ranges in the table above, and 25 to 45 GPM on the 35 GPM models and 35 to 55 GPM on the 45 GPM. These are Franklin’s own published best efficiency bands, not derived figures. Below this range the pump recirculates internally, which causes vibration and premature seal wear. Above it, it cavitates and the motor draws past its rating. Most booster failures are pumps that were oversized for the building.

Zero flow is the shut-off point — a ceiling, not a working point. Boost figures are tabulated by Franklin for the 35 and 45 GPM families and reproduced unchanged; best efficiency ranges are the bands Franklin prints on its own performance curves. Source: Franklin Electric RLC Surface catalog M5049, 60 Hz US data. Convert psi to head by dividing by 0.433.

This model has no app or remote connectivity. It runs from its own controls and needs no network connection.