Anyone on a well knows the pattern. The shower is fine until someone starts the dishwasher, then the pressure drops, then it comes back. That is a conventional pump cycling between its cut-in and cut-out settings, doing exactly what it was designed to do.
A constant pressure system removes that swing, and the way it does it has a second benefit that matters more than the comfort.
What Is a Constant Pressure Pump?
A constant pressure pump is a pump paired with a variable frequency drive that changes motor speed to hold a single pressure, whatever the demand. Open a second tap and the pump speeds up instead of letting pressure sag. The larger gain is that it almost never stops and starts, and starts are what wear a motor out.
Key Takeaways
- A VFD varies motor speed by varying supply frequency.
- Steady pressure is the visible benefit. Fewer starts is the valuable one.
- Power falls with the cube of speed, so part-load running is cheap.
- These systems use a small tank rather than a large one.
- A drive cannot fix a weak well or a badly sized pump.
Conventional or Constant Pressure?
| Conventional | Constant pressure | |
|---|---|---|
| Pressure at the tap | Swings between cut-in and cut-out | Held at one setpoint |
| Motor starts | Every cycle, many per day | Few, then variable speed |
| Pressure tank | Large, 40 to 80 gallons typical | Small, often under 20 |
| Energy at part load | Full power or nothing | Falls sharply with speed |
| Complexity | Switch and tank | Electronic drive |
| Cost | Lower | Higher up front |
The tank row surprises people. A conventional system needs a big tank precisely because the pump is either on or off, and the tank is what stops it starting on every cup of water. A variable speed pump can simply run slowly, so the tank shrinks to a small buffer.
Why Speed Control Saves So Much Energy
Pump power does not fall in line with speed. It falls with the cube of it, so a pump running at 80 percent speed draws roughly half the power. That relationship is what makes part-load operation so much cheaper than throttling a full-speed pump.
It also explains why oversizing hurts less on a variable speed system than a fixed one: the drive simply runs slower. Water efficiency and energy efficiency tend to move together here, which is the case the EPA makes through its WaterSense program.
The underlying electronics, converting AC to DC and back at a chosen frequency, are described in this overview of the variable frequency drive.
What a Drive Will Not Fix
A constant pressure system holds pressure. It does not create water. If the well yields 8 gallons per minute, no drive extracts more, and running the pump harder simply draws the level down toward the intake.
Nor does it rescue a badly sized pump. Sizing still starts from the pumping water level, the flow needed and the well’s yield, as set out in our guide to sizing a submersible well pump.
The drive is also an electronic component in a wet, rural environment. It needs clean power and sensible surge protection, and when it fails it is a board rather than a mechanical part. Conventional systems fail more often but more cheaply.
Where They Earn Their Keep
- Homes with several bathrooms where simultaneous demand is normal.
- Irrigation running zones of different sizes off one pump.
- Wells with limited yield, where gentle continuous draw beats hard cycling.
- Long pipe runs where friction makes pressure swings worse.
- Any system already killing pumps through short cycling.
That last case is the strongest argument. If a system has worn out more than one pump, the cause is usually cycling rather than the pumps, and the reasons are covered in our notes on what actually shortens pump life. Where the cycling traces to a waterlogged tank instead, that is a far cheaper fix.
Frequently Asked Questions
What is a constant pressure well pump?
A constant pressure well pump is a pump paired with a drive that varies its speed to hold one pressure regardless of how much water is being drawn. Open a second tap and the pump speeds up rather than letting pressure sag. Conventional systems cycle between a cut-in and a cut-out pressure, which is why the shower changes when someone flushes.
What is a VFD pump?
A VFD pump is a pump driven by a variable frequency drive, an electronic controller that changes motor speed by changing the frequency of the supply. Instead of running flat out or not at all, the motor matches demand. On a water system that means steady pressure, and on any system it means far fewer starts.
How does a VFD work on a pump?
A VFD works on a pump by converting incoming AC power to DC and back to AC at whatever frequency the controller calls for. Motor speed follows frequency, so the drive can hold a target pressure by adjusting speed continuously. It also ramps the motor up gently, which removes the hard inrush of a direct start.
Is a constant pressure system better than a conventional one?
Usually yes, a constant pressure system is better where pressure quality matters, because it holds a steady figure and starts far less often. Fewer starts is the real gain, since starts rather than running hours wear a motor out. It costs more and adds electronics, so a simple system on a good well may not need one.
Constant Pressure Systems at Ken’s Distributing
Ken’s Distributing supplies Franklin Electric multi-stage pressure packages and water pressure boosting systems for constant pressure duty.
Tell us the well yield, the number of fixtures and the pressure you want held, and we will confirm whether a variable speed system earns its cost on your setup or whether a conventional one is the better buy.
Drives sit across several pump families, all mapped in our overview of the Franklin Electric range explained.