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.
Updated 8 August 2026: this guide now answers buyer questions about capacity, precharge, commissioning evidence, and the difference between a pressure tank and an atmospheric storage tank.
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.
WellMate Classic Pressure Tank Selection Data
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.
Diagnose Short Cycling Before Replacing the Pressure Tank
Rapid pump cycling does not always mean that the tank needs replacement. Use this sequence so that a leak, pressure switch, or gauge fault is not mistaken for a failed tank.
- Record cut-in, cut-out, pump run time, and cycling when no water is being used. Cycling without demand points to leakage or reverse flow that should be investigated first.
- Isolate electrical power and release water pressure. Measure precharge only after the water side is empty, then compare it with the model instructions and cut-in setting.
- Restart the system and measure the water delivered from cut-out to cut-in. Compare actual drawdown with the minimum required from pump delivery and run time.
- If water appears at the air valve, stop operation and investigate bladder or air-cell failure. If precharge and drawdown are correct, check the switch, gauge, sensing line, check valve, and leakage before ordering a replacement tank.
Data to Request a Pressure Tank Selection
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.
Selection Evidence Before Ordering
Direct answer: order a pressure tank only after the pump duty and pressure settings are known. If the only data available are pipe size and faucet count, collect cycle time, pump curve, cut-in/cut-out, and actual drawdown first; those records prevent a nominally large tank from being selected for the wrong operating point.
| Evidence available | Ordering decision |
|---|---|
| Pump model, curve, operating pressure, and minimum run time | Calculate required drawdown and shortlist WellMate pressure tank models |
| Cut-in/cut-out pressure and empty-tank precharge | Verify drawdown table, pressure-switch setting, and commissioning target |
| Frequent cycling with no demand | Check leakage, reverse flow, switch, gauge, and check valve before replacing the tank |
| Need for large emergency water reserve | Add or resize an atmospheric storage tank; do not treat pressure-tank nominal volume as reserve storage |
| Booster or apartment system with peak demand | Review parallel pumping, storage, controls, and pressure tank together rather than as one isolated component |
Buyer Questions about Pressure Tanks
Is a pressure tank the same as a water storage tank?
No. A pressure tank provides pressurized drawdown to control pump cycling, while an atmospheric tank stores a larger reserve of water. A system may need both: the atmospheric tank buffers the supply and the pressure tank sits after the distribution pump.
Is a larger pressure tank always better?
A larger tank usually provides more drawdown, but selection must still follow pump delivery, minimum run time, cut-in/cut-out, pressure rating, connections, and installation space. Oversizing also adds cost and space without correcting control faults or leakage.
When should pressure-tank precharge be checked?
Check it at the manual’s interval and when the pump begins short cycling, pressure becomes unstable, or the system has been serviced. Isolate electrical power, release water pressure, and empty the water side before measuring; the target value depends on the tank model and cut-in setting.
Technical sources
- Pentair WellMate Composite Pressure Tank Catalog, sizing method and drawdown factors.
- Pentair WellMate UT, HP & SP Installation Instructions, precharge and safe installation.
Pressure-tank drawdown and commissioning worksheet
Select a pressure tank from its available drawdown over the operating pressure range, not nominal volume alone. Start with pump delivery and the manufacturer’s minimum run time; an initial drawdown requirement is pump flow × minimum run time. Match that result to the model’s drawdown table at the actual cut-in/cut-out setting. This does not replace the pump curve or tank manual.
| Check | Record | If it is out of range |
|---|---|---|
| Pump cycle | Time from pump start to stop under stable demand | Check leakage, precharge, and drawdown size |
| Precharge | Measure with water side empty and power isolated | Set only to the manual/model and actual cut-in |
| Pressure | Record cut-in, cut-out, and outlet fluctuation | Check switch, sensor, and pressure loss |
For WellMate pressure-tank selection and distribution components, send pump flow, pressure settings, and cycle data to Watermart.
Test actual drawdown after installation
The sizing calculation should be proven on the installed system. Close all demand, let the pump reach cut-out, then collect and measure the water delivered until pressure reaches cut-in. That volume is the actual drawdown at those settings. Repeat after checking for leakage, and confirm that the pump runs for at least the minimum time specified by its pump or motor manufacturer.
| Field record | How to assess it | Action if it fails |
|---|---|---|
| Actual drawdown | Compare with pump flow × required minimum run time | Check precharge, pressure settings, and model selection |
| Pump run time | Time from cut-in to cut-out under unchanged demand | Check the pump curve, leakage, and tank volume |
| Cycling with no demand | It should not repeat while every outlet is closed | Find leakage, reverse flow, switch, or check-valve faults |
Do not alter cut-in, cut-out, precharge, or the relief setting merely to make the test pass. Every setting must remain within the tank, pump, and pressure-switch instructions.