Home Well Pump or Pressure Tank? How to Tell Which One Is Failing
Short answer: If the water pressure surges and falls, the switch clicks rapidly, and the pump starts and stops in seconds, the pressure tank is the prime suspect in roughly that order of likelihood — a tank that has lost its air charge or ruptured its bladder produces exactly those symptoms, and it is far cheaper to replace than a pump. If instead the pump runs continuously without building pressure, or is completely silent, the pump or its power circuit is the problem. Four tests below (knock, air valve, ten-minute pressure hold, current draw) will tell you which side of the system you are on.
A well system is a loop. The pressure tank stores a cushion of compressed air that pushes water into the house between pump cycles; the pressure switch reads system pressure and tells the pump when to start and stop.
Break either half and the loop misbehaves the same way:
The difference is which component is doing the work. That is what the tests measure.
Knock on the side of the tank. Work from top to bottom.
| Where you knock | Healthy tank | Waterlogged tank |
|---|---|---|
| Upper third | Hollow, echoing sound (air) | Solid thud |
| Lower third | Solid thud (water) | Solid thud |
| Entire tank | Hollow top, solid bottom | Solid everywhere |
A tank that sounds solid from top to bottom has filled with water — the air charge is gone and the bladder has almost certainly failed. There is also a visual version of this test: a healthy tank is dry and uncondensed at the top, because that end is air, not water.
This is the test that settles the question.
Reading the result:
The “2 PSI below cut-in” rule is standard practice rather than a preference: the pre-charge must sit just under the point where the pump starts, so the tank is nearly empty of water exactly when the pump kicks in.
Sometimes neither the pump nor the tank is at fault — the check valve is draining the system backwards between cycles, which produces the same restart behaviour.
If the pressure holds, the check valve and drop pipe are sound. If the pressure drops, water is escaping back down the well through a leaking check valve — and that alone will make a healthy pump cycle itself to death.
If tests 1–3 are clean, look at the pump itself.
With a multimeter on its 10-amp range, placed in series with one leg of the circuit, watch the current as the pump starts and runs. A healthy pump shows a short start-up spike and then settles to a steady running figure. A pump that holds an elevated amperage is working against resistance — a clogged or worn impeller, sand in the stages, or a failing bearing.
Two more electrical clues:
| Symptom | Pressure tank | Well pump |
|---|---|---|
| Rapid clicking at the switch | Very likely | Possible (secondary) |
| Pressure surges strong → trickle | Very likely | Possible |
| Water spits and sputters at taps | Very likely | Uncommon |
| Tank sounds solid when knocked | Diagnostic | — |
| Water from the air valve | Diagnostic (replace tank) | — |
| Pressure falls when isolated for 10 min | Check valve | Check valve |
| Runs continuously, no pressure | Unlikely | Very likely |
| Completely silent | Unlikely | Very likely |
| High sustained current draw | Unlikely | Very likely |
Whichever component started it, short cycling is what finishes the job. Every start draws a heavy inrush current and heats the motor; a pump that should run for minutes at a time but instead runs for seconds never gets the cooling or the run-time it was designed around. Motors fail early, and the maintenance bill arrives twice.
The most common root cause is not a broken part at all — it is a tank that is too small for the pump.
Drawdown is the number that matters. Drawdown is the volume of water a tank can actually deliver between cycles, and it is far smaller than the tank’s nominal size. A standard 20-gallon tank typically yields only about 4–6 gallons of drawdown. A 10 GPM pump on a ½ HP motor wants a tank that can deliver at least 10 gallons of drawdown to give the motor a sensible run time. Put that pump on a 20-gallon tank and you have engineered short cycling into the system from day one.
A “waterlogged tank” and an “undersized tank” therefore produce the same symptom by different routes — and both are tank-side problems, which is why we usually recommend checking the tank first.
Replace the tank when:
Do not replace the tank when:
And if you are replacing a tank that failed early, size the new one on drawdown against the pump’s flow rate — not on the physical size of the old tank. Replacing like-for-like is how a system ends up short cycling again a year later.
Can a bad pressure tank damage my well pump? Yes — this is the main reason not to defer a tank replacement. A tank that has lost its air charge forces the pump into rapid cycling, and the repeated inrush current and lack of run-time cooling are what actually destroy the motor.
How long does a pressure tank last? Bladder tanks typically give many years of service, but the bladder is a wear item and chlorinated or aggressive water shortens its life. Annual pre-charge checks catch most failures long before the tank floods.
Why does my water pressure drop then recover every time I open a tap? That surge-and-fade pattern is the signature of lost air charge. The tank delivers a burst of stored pressure, then has nothing left, so the pump starts and rebuilds — then it repeats.
Should I set the pre-charge with the system pressurised? No. The pre-charge is measured with the tank drained and the power off. Measuring a charged tank tells you the system pressure, not the air charge.
Is a larger pressure tank always better? Not always, but undersizing is a much more common mistake than oversizing. Size on drawdown against the pump’s flow rate, and remember that a bigger tank also means longer, gentler pump cycles.
My pump clicks off and on constantly but the tank tested fine. What else could it be? Check the check valve with the ten-minute isolation test, then the pressure switch itself (contacts can stick or the cut-in/cut-out differential can drift), and finally confirm the pump is not oversized for the well’s recovery rate.
Most of the failures described here are sizing failures, not manufacturing failures. A pump and tank that are matched on drawdown, head and flow will run long, cool cycles for years; a mismatched pair will destroy itself regardless of build quality.
Elestar manufactures deep well and borehole pumps, household water supply sets and pressure stabilisation tanks, and we publish the drawdown curve for each tank model so you can match it to the pump rather than guess. Technical documentation and OEM options are available through our factory page or by contacting our team.
Sources and further reading