How it works
How a pressure tank stores water in squeezed air
A 20-gallon tank gives back about 5 gallons between pump starts. Squeeze the air yourself and see where the other 15 went.
A pressure tank is a spring made of air. Water goes in only by squeezing a fixed charge of air into a smaller space, and it comes back out only as that air expands again. How much water a tank can give back between pump starts, its drawdown, is decided by Boyle’s law, the switch settings and the precharge. For the most common setup, a 20-gallon tank on a 40/60 switch with a 38 psi precharge, the answer is about 5.2 gallons [1].
Water in, air squeezed
Inside a modern well tank, a rubber bladder or diaphragm keeps the water and the air apart. When the tank is new and empty of water, the air fills the whole tank at the precharge pressure, 38 psi from the factory on Amtrol’s Well-X-Trol line [1]. As the pump pushes water in, the bladder swells and the air is crowded into the top; when a tap opens, the squeezed air pushes the water back out [2].
The air obeys Boyle’s law: at constant temperature, a fixed amount of gas has a volume inversely proportional to its pressure [3]. The catch is that the law uses absolute pressure. A tank gauge reads pressure above the atmosphere (psig); to get absolute pressure (psia) you add the atmosphere, 14.7 psi at sea level [4]. So a 38 psi precharge is really 52.7 psi absolute, and filling the tank to 60 psi takes the air to 74.7 psi absolute. That ratio, not 38 to 60, sets how far the air shrinks.
Here Vtank is the tank’s total volume, P0 the precharge and Pa the atmosphere, all pressures in psi as a gauge reads them. Whatever space the air does not use is water. For the 20-gallon tank at 60 psi the air takes 20 × 52.7 / 74.7 = 14.1 gallons, leaving 5.9 gallons of water. At 40 psi the air takes 19.3 gallons and 0.7 gallons of water remain. The water that leaves between the two, 5.9 − 0.7 = 5.2 gallons, is the drawdown. Drag the water level in the tank at the top of the page and you can feel the curve: the first gallons in barely raise the pressure, the last gallons raise it fast.
Manufacturers publish exactly this calculation. Pentair’s WellMate data sheet says its drawdown figures are “based on Boyle’s Law” and gives them as a multiplier of tank volume: 0.25 for 40/60 [5]. Amtrol lists 5.4 gallons for its 20-gallon WX-202 at 40/60 [1]; equation (2) gives 5.4 when the precharge is set equal to the cut-in, and 5.2 with the precharge 2 psi lower. The tables and the physics agree to a tenth of a gallon. So when a homeowner asks why the “20-gallon” tank empties after five gallons, the answer is that most of the tank is always full of air, and it has to be.
Precharge: why 2 psi below cut-in
Equation (2) has the precharge in it, so the precharge is a knob on the drawdown. Figure 1 turns it: one 20-gallon tank on a 40/60 switch, with the precharge set anywhere from 0 to 70 psi.
Raise P0 from zero and there is more air to push with, so the drawdown grows. Once the precharge passes the cut-in pressure, the tank runs empty while the pressure is still above cut-in, and every psi of precharge above cut-in is a psi of the switch band in which the tank has nothing left to give. The peak sits exactly at the cut-in. Tank makers still say 2 psi below it: Amtrol and Goulds both instruct installers to set the precharge 2 psi below the switch’s cut-in, 38 psi for a 40/60 switch [2][6]. The 2 psi costs about 4 percent of the drawdown (5.2 instead of 5.4 gallons) and buys a small cushion of water, about 0.7 gallons in this tank, that is still there when the pump starts. Without it the tank would run dry at the same moment the switch closes, and any lag in the switch or the pump could show up as a dip at the taps.
The same figure explains why a tank that has lost its charge cycles so hard. At 0 psi precharge (air at atmospheric pressure), this 20-gallon tank gives 1.4 gallons. Real waterlogging is worse: when a bladder fails, the air leaves altogether and the drawdown falls toward nothing [7]. And it explains the instruction that the precharge be checked with the tank drained and the system off [2]: with water in the tank, the air valve reads the water’s pressure, not the charge.
A note on doing this yourself. Amtrol’s manual says only licensed professionals should check, adjust or re-charge the precharge, and that a tank which has lost air may be corroded inside and must not be re-pressurized, because it can rupture [2]. The pressure switch beside it carries line voltage: power off at the breaker before anything is opened, and call a licensed well or pump professional if you are unsure. Rules on who may do this work vary by state and county.
Drawdown, tank size and run time
Drawdown is proportional to tank volume: double the tank, double the drawdown, at the same settings. What changes across switch settings is the share of the tank you can use. With the precharge 2 psi below cut-in, it is about 35 percent at 20/40, 30 percent at 30/50 and 26 percent at 40/60, within about a hundredth of the multipliers Pentair prints (0.35, 0.29 and 0.25) [5]. Higher pressure costs storage, because the air is already squeezed hard before the band begins.
Drawdown matters because it sets how long the pump runs. While the pump refills the tank it delivers its flow Q, and the house uses d at the same time, so the tank gains Q − d gallons a minute. Then the pump rests while the house alone drains it:
rest = drawdown / d(3)
The shortest run happens when nothing is being used: the pump fills the drawdown alone in drawdown ÷ Q. That is why the sizing rule is written as a minimum drawdown. Franklin Electric asks for motors to run at least one minute per start, to clear the heat of starting, and limits starts per day (300 for its encapsulated motors up to ¾ hp) [8]. Amtrol builds its tank table on one minute of running for pumps up to ¾ hp and two minutes above [1]; Goulds calls one minute “adequate” and two minutes “recommended” protection [6]. So a 10 gpm pump needs at least 10 gallons of drawdown. Our 20-gallon tank gives 5.2, and its pump would run 31 seconds with the taps shut. Amtrol’s own table puts a 10 gpm pump at 40/60 on a 34-gallon tank [1].
The cycle rate depends on use, and it has a worst case. Starts per hour are 60 ÷ (run + rest); with equation (3) this is largest when the house draws exactly half the pump’s flow, where it reaches 15 × Q ÷ drawdown. For the 10 gpm pump on the 20-gallon tank that is 29 starts an hour; on a 40-gallon tank, 15.
Constant-pressure pumps: the other answer
Everything above follows from one design choice: a pump that is either fully on or off. A constant-pressure system changes that. A variable-frequency drive slows or speeds the pump to hold one pressure, so the pump runs at whatever flow the house is using instead of filling and emptying a tank. Grundfos’s SQE, for example, varies motor speed between 65 and 100 percent from a pressure sensor and soft-starts over two seconds [9]. The tank’s job shrinks to smoothing small demands: Franklin’s guidance is that such systems need only a small tank, about 10 to 20 percent of the pump’s rated flow in gallons, and it still asks that starts stay below 200 a day [8].
That is a trade, not a free fix. The drive is electronics in a basement or well house, and a constant-pressure system still needs a tank, a check valve and a sound well. For an on/off system, the lessons are the ones the figures show: keep the air in the tank, set the precharge to match the switch, and give the pump enough drawdown to run for a minute every time it starts.