Upgrade guide
Constant-Pressure Well Systems: How They Work and Whether They're Worth It
A variable-speed drive ends the 30-to-50 psi swing. It doesn't add water to a weak well, and it adds electronics to a system that used to have one switch.
On this page
A constant-pressure system replaces the on/off pressure switch with a variable-speed drive and a pressure sensor. The drive speeds the pump up or down to hold one setpoint — for example 50 psi — whether one faucet or four are open. You get steady pressure and a much smaller tank. You pay more up front (HomeGuide: $2,000–$5,000 installed[1]), you depend on electronics that need surge protection and a suitable location, and the tank precharge rule changes to about 70% of the setpoint.[2][3]
How it works
- 1SensorA pressure transducer near the tank reads system pressure continuously.
- 2DriveThe controller compares it with your setpoint and changes the motor speed.
- 3PumpRuns faster when demand rises, slower when it falls, and stops when there's no demand.
- 4Small tankCovers tiny draws (a glass of water) and smooths stops and starts.
Examples with published installation manuals include Franklin Electric's SubDrive and MonoDrive controllers and the Grundfos SQE pump with its CU 301 control box.[2][3]
The drive usually runs a standard submersible pump; the rest of the system is described in the private well pump guide.
Compared with a pressure-switch system
| Pressure-switch system | Constant-pressure system | |
|---|---|---|
| Pressure at the faucet | Swings between cut-in and cut-out (e.g., 30–50 psi) | Held near one setpoint |
| Tank size | Sized for 1–2 minutes of pump run per start[4] | Small; Franklin's generic VFD guidance is 10–20% of pump output in gallons[2] |
| Tank precharge | Cut-in − 2 psi[5] | ≈70% of setpoint (Franklin, Grundfos)[2][3] |
| Controls | Mechanical switch (plus control box on 3-wire) | Electronic drive, sensor, often built-in fault codes |
| Installed cost (HomeGuide) | Submersible $1,000–$2,500 | $2,000–$5,000 |
| Repairs | Cheap, widely stocked parts | Drive repairs or replacement; brand-specific parts |
Real benefits
- Steady pressure. The main reason people upgrade: no more shower pressure fading as the tank empties.
- A small tank. Useful where space is tight or a large tank is due for replacement anyway.
- Gentler starts. The drive ramps the motor up rather than starting at full voltage.
- Built-in protection on many drives — for example, fault codes and troubleshooting tables are part of Franklin's controller documentation.[2]
Drawbacks to weigh
- Up-front cost. Roughly double a conventional submersible replacement on HomeGuide's ranges.[1]
- Electronics. Surge protection and grounding matter; Franklin's installation checklist calls for a dedicated surge arrestor on submersible motors.[2] Drives also need a location within their temperature limits.
- Compatible components. Franklin notes that most standard spring-loaded check valves aren't designed for variable flow and recommends valves made for VFD systems.[2] Motors must be used within the maker's VFD guidelines.
- It doesn't fix a weak well. A drive controls speed; it can't make a low-yield well produce more water.
- Service. Fewer local contractors stock every brand's drive; ask who can service it before you buy.
Setup rules that change
From the manufacturers' manuals
- Tank precharge
- About 70% of the setpoint. Franklin's SubDrive/MonoDrive table lists 35 psi for the 50 psi factory setting; Grundfos says 70% and warns that a precharge above the setting prevents pressure control.[2][3]
- Tank size
- Follow the drive maker. Franklin's generic guidance for VFD systems: 10–20% of the pump's rated output (a 100 GPM system → a 10–20 gallon tank minimum).[2]
- Relief valve
- Grundfos calls for a pressure relief valve downstream of the wellhead set at least 30 psi above the pressure setting.[3]
- Sensor location
- Grundfos recommends placing the pressure sensor as close to the points of use as possible.[3]
Who benefits most
- Homes where several fixtures run at once and the pressure swing is noticeable.
- Homes needing a pump replacement anyway, where the extra cost is incremental.
- Tight mechanical rooms where a large tank doesn't fit.
Less compelling: a small household happy with its pressure, a well with limited yield, or a location prone to lightning without good surge protection. If your real problem is weak flow everywhere, check filters and piping first — see low water pressure.
Professional installation
Drive installation involves line-voltage wiring, motor compatibility checks and programming. Ask the installer for the drive's manual, the tank precharge they set, and the fault-code list — you'll want them the first time the drive shows an alarm.
Sources
- HomeGuide. How Much Does a Well Pump Cost To Replace or Install? (article dated Aug. 7, 2025). Accessed Sept. 28, 2026.
- Franklin Electric. Submersible Motors: Application, Installation & Maintenance (AIM) Manual, 60 Hz. Accessed Sept. 28, 2026.
- Grundfos. CU 301 Installer's Manual (SQE constant-pressure control). Accessed Sept. 28, 2026.
- Water Well Journal. Tank Selection by the Numbers — David L. Kill, PE (April 19, 2022). Accessed Sept. 28, 2026.
- Amtrol. Well-X-Trol Residential Pre-Pressurized Well Tanks — Installation & Operation Instructions (WX-101 through WX-350). Accessed Sept. 28, 2026.
We cite manufacturer documentation and public agencies first. Read how we choose and check sources.