Short answer: a pressure tank stores usable water, called drawdown, between the pump’s cut-in and cut-out pressures. Tank size is based on pump delivery, minimum pump run time, and pressure settings—not faucet count alone. Correct sizing reduces short cycling, steadies pressure, and protects the pump.
5 Important Things About Pressure Tanks and Their Uses
What is a Pressure Tank?
A pressure tank is a pressurized vessel that stores water and compressed air in a pump system. The air cushion pushes water into the distribution line when a fixture opens, so the pump does not start for every small draw. A Pentair WellMate composite pressure tank is one option for well, booster, and hydropneumatic systems.
A pump pressure tank is different from an FRP filter-media tank. The first supplies drawdown and controls pump cycling; the second holds activated carbon, ion-exchange resin, or iron-manganese removal media.
How Does a Pressure Tank Work?
As the pump fills the tank, water compresses the air until pressure reaches the cut-out setting and the pressure switch stops the pump. Water use then lets the compressed air push water out. The pump restarts at cut-in pressure. The water delivered between these two pressures is the drawdown.
Precharge is the air pressure measured with the water side empty. It must follow the model instructions. The Pentair WellMate UT/HP/SP manual, for example, specifies 4 psi (0.28 bar) below cut-in; a 30/50 psi system therefore uses 26 psi precharge. Other series or applications may specify another differential, so the unit label and manual remain controlling.
Main Functions of Pressure Tank
-
Stabilize Water Pressure
- Supplies water between cut-in and cut-out for more controlled pressure changes.
- Reduces pump-related surges, but it does not replace a water-hammer assessment when rapid valve closure is the cause.
-
Pump Protection
- Reduces frequent starts and stops, or short cycling.
- Helps the pump achieve its manufacturer-specified minimum run time.
-
Energy Saving
- Avoids starting the pump for small water draws.
- Actual savings depend on the pump curve, controls, system pressure, leakage, and demand pattern.
-
Continuity of Water Supply
- Supplies the drawdown volume before the pump restarts.
- It is not a large emergency-storage tank: nominal tank volume is not the same as deliverable water volume.
-
Water Use Efficiency
- Matches usable storage to pump delivery and required run time.
- Makes frequent pump cycling easier to diagnose.
How to Size a Pressure Tank from Drawdown
Pentair identifies three core inputs: pump delivery rate, minimum pump run time, and cut-in/cut-out pressure. The preliminary method is:
- Minimum drawdown = pump delivery × minimum run time.
- Find the drawdown factor for the pressure pair in the manufacturer’s table.
- Minimum nominal tank volume = minimum drawdown ÷ drawdown factor.
- Select the next model at or above that volume, then verify pressure, temperature, connection, and installation-space limits.
Pentair’s catalog example uses an 8 gpm pump, one-minute minimum run time, and a 30/50 psi pressure range. Required drawdown is 8 gallons; the catalog factor is 0.30, so minimum nominal volume is 8 ÷ 0.30 = 26.7 gallons. This demonstrates the method and does not replace checking the pump curve and the table for the exact tank model.
| Design input | Why it matters | Mistake to avoid |
|---|---|---|
| Pump delivery at operating pressure | Determines water entering per minute | Using brochure maximum flow without system head |
| Minimum pump run time | Limits motor starts | Assuming one minute suits every motor |
| Cut-in and cut-out | Determines drawdown factor | Sizing from nominal tank volume alone |
| Precharge with water side empty | Protects drawdown and bladder/air-cell operation | Measuring or adjusting a water-filled tank |
| Maximum pressure and temperature | Keeps operation within rating | Setting relief pressure above tank rating |
Installation and Commissioning Checklist
- Isolate pump power and release water pressure before measuring precharge.
- Place the tank on a stable surface and support piping independently of the tank connection.
- Install the pressure switch, gauge, serviceable isolation/drain points, and pressure-relief valve required by the manual and local rules.
- Do not place a check valve where it isolates the relief valve from the tank.
- Cycle the system several times; record cut-in, cut-out, pump run time, leakage, and pressure stability.
- Never exceed the label pressure or temperature, and do not install where the tank can freeze.
Excellence of Wellmate Pressure Tank Classic Series
Pentair WellMate uses composite construction with a polyethylene liner and fiberglass winding on applicable series. Non-corrosive construction and lower handling weight can be useful compared with steel, but model selection must still follow drawdown, pressure rating, connection, and system configuration.
Prepare pump flow, minimum run time, cut-in/cut-out, water temperature, connection size, orientation, and available space before selecting a series. Nominal capacity alone is not enough because deliverable water is the drawdown, not the entire tank volume.
Pressure Tank Maintenance and Fault Symptoms
| Symptom | First check | Safe response |
|---|---|---|
| Pump starts and stops rapidly | Leaks, switch settings, precharge, and bladder/air cell | Isolate power, release water pressure, then measure precharge per manual |
| Pressure drops sharply when a fixture opens | Actual drawdown versus pump delivery | Record cycle duration and recalculate tank size |
| Water comes from the air valve | Possible bladder or air-cell failure | Stop operation and have the unit inspected |
| Gauge does not return consistently | Gauge, switch, and blocked sensing line | Service only after making the system safe |
| Fitting corrosion or leakage | Fitting, seal, and pipe support | Never tighten a pressurized connection |
Review pressure and cycling regularly, and measure precharge only when the water side is empty. The correct interval depends on the manual, duty cycle, and installation conditions.
So What’s the Point of Watermart?
PT Watermart Perkasa supplies Pentair WellMate pressure tanks and water-storage and distribution components for Indonesian projects. For a calculated selection, send pump delivery, pump model or curve, minimum run time, pressure settings, demand, temperature, and layout photos through the Watermart contact page.
Technical sources
- Pentair WellMate Composite Pressure Tank Catalog, sizing method and drawdown factors.
- Pentair WellMate UT, HP & SP Installation Instructions, precharge and safe installation.