Well Water Pressure Tank: The Complete Guide (How It Works, Types, Air Charge, Lifespan)

People call me and say, "I just moved into a house on well water, and the only thing in my basement is a blue metal tank. What is it?" It is your pressure tank, and this page is everything I would tell you about it on the phone: what it does, what is inside it, where it goes, how to check the one thing that is actually checkable, how to tell when it has died, and what to buy when it does. Sizing, cost, the pressure switch and troubleshooting each have their own page; this one ties them together.

What a pressure tank does, and why every well needs one

A pressure tank turns a pump that is either on or off into a house with steady water pressure. The pump fills the tank until the pressure switch shuts it off at the cut-out pressure (50 psi on a 30/50 switch); the compressed air in the tank then pushes water out to your fixtures until the pressure falls to the cut-in point (30 psi) and the switch starts the pump again. Without the tank the pump would start the instant you cracked a faucet and stop the instant you closed it, and you would have no pressure between starts. Amtrol, which makes the tank we sell, puts it plainly in its installation manual: the tank exists to control pump cycling, save energy and extend pump life.

Here is how I describe the inside of one on the phone. Out in your yard there is a well casing, and say 200 feet down there is a submersible pump on a 1-inch black poly pipe. That pipe comes into the house and connects to the tank. On top of the tank is an air chamber. In the middle is a rubber barrier. In the bottom is the water, and not as much of it as you would think. The air presses on the rubber, the rubber presses on the water, and that is your house pressure. Without it you could not take a shower, run the dishwasher or do laundry.

Two things follow from that picture that surprise people. First, the tank does not decide your pressure. The pressure switch does; every tank on the market makes exactly the pressure the switch is set to, so a bigger tank does not give you a better shower. Second, the pump starting when you use water is normal, not a fault. Once the water in the bottom of the tank is used up, which happens in seconds when a toilet flushes, the pump kicks on and stays on for as long as you are using water. If the shower pressure is steady while it runs, nothing is wrong.

Why a 32 gallon tank holds 10 gallons of water

The gallon figure on a pressure tank is the total volume of the vessel, most of which is air. The water it actually delivers between pump starts is called drawdown, and on a residential tank it is roughly a quarter to a third of the label: 9.9 gallons for a 32 gallon WX-203 at a 30/50 switch, 6.2 for a 20 gallon WX-202. As I tell callers, they are not water storage tanks. They just create pressure.

The numbers below are Amtrol's published drawdown for the four Well-X-Trol sizes we ship, at a 30/50 switch. On a 40/60 switch each figure is about 13 percent lower (8.6 gallons for the WX-203), because the same air charge compresses less between 40 and 60 psi.

Drawdown is the number that sizing is built on: the tank should hold at least as much water as the pump moves in a minute, so the pump runs a full minute every time it starts. That decision, with the pump chart and what to do when you do not know your pump's flow, is on its own page: what size pressure tank do I need. The short answer for most houses is the WX-203.

The three kinds of tank, and the naming mess

There are three designs: a diaphragm tank, where a rubber sheet is clamped into the seam of a steel shell (the Well-X-Trol and most steel tanks); an air-cell or bladder tank, where a separate rubber bag hangs inside the shell (the fiberglass Pentair WellMate); and the old galvanized air-over-water tank with nothing between the air and the water. In the trade all of the first two get called "bladder tanks." A caller once asked me whether his Well-X-Trol had a bladder because the label said diaphragm. We call it a bladder. It is the diaphragm, bladder, it is all the same to me. What matters is that a rubber barrier keeps the air from dissolving into the water.

The Well-X-Trol is the diaphragm tank we sell, and two things about the current Pro Series matter to a homeowner: the connection at the bottom is stainless steel, and the warranty is 7 years. The stainless connection is the field detail. Older models did not have it, and over the years they would get a leak in the bottom and flood a basement; that is not something you have to worry about now. The rest of the spec sheet, and how it compares to the other brands, is in the best well pressure tank.

Fiberglass tanks (Pentair WellMate, Flexcon FlexLite) have no steel to rust from the outside, which makes them the right tank for a wet location: an outdoor well house, a damp dirt crawl space, a coastal shed. They are not the right tank for acidic, iron or sulfur water, because in a Well-X-Trol the water never touches the steel. An earlier version of this page said otherwise; it was wrong, and a Michigan caller with high-sulfur water quoted it back to me this summer. Sulfur is not going to affect a Well-X-Trol. The comparison is in fiberglass vs steel pressure tanks.

If you have a galvanized air-over-water tank, it is not broken because it needs air every few months; that is how the design works. But when it goes, replace it with a diaphragm tank, not another galvanized one.

Where the tank goes and what hangs off it

The pressure tank is the first thing the well line reaches inside the house, and every piece of water treatment installs after it. The tank connects to the plumbing through a tank tee, and the tee carries the pressure switch, the pressure gauge, the relief valve, a boiler drain and a shutoff. As I told a caller in South Carolina who asked where his filters would go: it all goes down after your well tank.

Two placement rules from Amtrol's manual are worth a homeowner knowing. There must be a relief valve at the tank, because if the switch ever sticks the pump will keep building pressure and the relief valve is what protects the house. And the tank should not sit where a leak would damage the floor below, where it can freeze, or in direct sun.

One more part usually sits at the tee: a check valve on the line coming in from the well, which installers put between the incoming water and the tank. A Georgia caller whose plumber had just added a thermal expansion tank at his water heater asked me whether his system even had a check valve; on a standard tank tee it usually does. That small basketball-sized tank on the water heater is a different device with a different job; see the FAQ if you have both and are not sure which is which.

The air charge: what it is and how to check it

The air charge is the pressure of the air sealed above the diaphragm when the tank is empty of water, and it should be 2 psi below the pressure your pump starts at: 28 psi on a 30/50 switch, 38 psi on a 40/60. You read it with a tire gauge on the small air stem on the tank's shoulder, with the pump off and the tank drained, and never with the tank full. Amtrol ships every Well-X-Trol at 38 psi because 40/60 is the most common switch; if yours is a 30/50, a new tank needs air let out before it is right.

Here is the check the way I walk callers through it:

1. Turn off power to the pump at the breaker.
2. Open a faucet, ideally the boiler drain on the tank tee or a hose bib nearby, and let it run until the water stops. The tank is now empty and the gauge on the tee reads zero.
3. Unscrew the cap on the small air stem and press a tire gauge on it. Air should come out, not water.
4. Compare the reading to your switch: 28 for 30/50, 38 for 40/60. If it is high, let air out in 20 or 30 second bursts and recheck; it takes more than you expect. If it is low, read the next paragraph before you add any.
5. Close the faucet, restore power, and watch the gauge climb to the cut-out and stop.

The mistake I hear most often is checking the air with the tank full. A Minnesota caller last July added air because his two-year-old tank's pump started on every flush (which is normal), then could not get his tire gauge to read anything but 50 no matter how much he added or released. That is because the water in the tank was at 50 psi and pushing on the diaphragm from the other side; the gauge was reading his system pressure, not his air charge. Drain it first. In his case he had overfilled it and needed to let a lot out to get back to 28.

Now the honest part. Amtrol's installation manual says pre-charge adjustments are for the time of installation, that only a licensed professional should check or adjust it, and, in capital letters, do not add air to a tank that has lost air, because a loss of air can mean internal corrosion and pressurizing a weakened tank can rupture it. We were a service company for 25 years and we kept them at 28, and I still tell callers to check theirs. But the manual's point stands: if a tank that held 28 for years is suddenly at 15, the question is not how to get it back to 28, it is why it lost the air. On a Well-X-Trol with a welded air stem that usually means a torn diaphragm, and the fix is a new tank, not a bicycle pump.

Ask Aidan

Got a reading you cannot make sense of?

Text me the number on your tire gauge, what the switch cover says, and a photo of the tank. I'll tell you whether it is the air, the switch, the gauge or the tank.

Text Aidan 443-277-2204 Call800-460-5810Aidan

8 AM to 5 PM Eastern, seven days a week. A real person, not a call center.

How to tell the tank is going bad

A failing pressure tank shows up as pressure that surges and drops while you are in the shower, a pressure switch that clicks on and off every few seconds while water runs, and a tank that is heavy from top to bottom when you try to rock it. A healthy tank feels light, because most of it is air. Those three signs together mean the diaphragm has torn and water has displaced the air on top; the trade word for it is waterlogged, and it is not repairable.

As a service company that was the biggest problem we saw, and a tank that should feel nearly empty weighs like 50 pounds. If yours is heavy, it is done. If it is light and the shower pressure is steady, it is probably fine even if the pump starts more than you expected, and the thing to look at is the gauge: when you open a faucet it should fall to the cut-in, the pump should start, and it should climb to the cut-out and stop. A gauge stuck on one number is often just a gauge port clogged with dirt.

A torn diaphragm is not an emergency. A Connecticut caller with a short-cycling 2012 WX-203 asked whether it was safe to use the water over the Fourth of July weekend until he could replace it. As long as you have pressure, you are good to go. What you are risking by waiting weeks is the pump, which is the expensive part; every start is wear on the motor. The full diagnosis, including low pressure, air in the lines and a pump that will not shut off, is on pressure tank troubleshooting; the switch itself has its own page at well pump pressure switch settings.

How long a pressure tank lasts

There is no honest single number. I have seen brand new pressure tanks fail in 4 years, and I have replaced Well-X-Trols that had run 30 to 40 years. Amtrol warrants the Pro Series for 7 years, which is the manufacturer telling you which end of that range it expects to be on the hook for. You will read "10 to 15 years" everywhere online, and an earlier version of this page contributed to that with figures I could not stand behind. The range above is what I have actually seen.

What ends a tank is the rubber, not the shell. I have seen a tank go up in four months because it had a defective diaphragm, and I have seen good ones run for decades. The shell on a Well-X-Trol is rated to 150 psig and the water never touches it, so corrosion from the water side is not what kills these tanks. What I tell someone with a 30-year-old tank that is holding 28 psi with steady pressure: you can replace it because the newer versions are much better, or you can run it until it dies. If it is not waterlogged and the pressure does not fluctuate, you are lucky.

The warranty matters more than the life expectancy. If a Well-X-Trol fails inside 7 years, you carry it to a plumbing supply house that stocks Amtrol and they give you a new one; if a plumber puts it in, you pay the labor and the tank is free. When a caller's two-year-old blue tank was misbehaving, the first thing I told him was that it was still under warranty.

Maintenance: what is actually worth doing

A pressure tank has no filter to change, no media to replace and nothing to lubricate. The useful maintenance is a once-a-year look: rock the tank to feel whether it is still light, watch the gauge through one pump cycle, and look at the base and the tee for rust or dampness. Amtrol's manual asks for a licensed professional to check the complete system yearly, and more often as it ages; that is the manufacturer's position, and the rock-and-gauge check above is what I ask callers to do before they call anyone.

What I would not do is treat the air charge as a monthly chore. On a diaphragm tank with a welded air stem the charge should not move; if it has, go back to the air charge section and read Amtrol's warning before you add any. The galvanized air-over-water tank is the exception: it loses air into the water by design and has to be re-aired, which is the main reason nobody builds them for houses anymore.

Two habits protect the tank from the rest of the system. Keep the relief valve on the tee; a stuck switch with no relief valve is how a tank sees pressure it was never built for. And if your water carries sand or iron sludge, deal with that after the tank with a sediment filter or iron filter rather than letting it settle in the tank; a caller with a 30-year-old tank drained rusty gunk out of his when he checked the air, and the answer was iron in the well, not a tank problem.

Replacing a pressure tank

Replace the whole assembly, not just the tank: the tank tee, pressure switch, gauge, relief valve and boiler drain have been cycling for as long as the tank has. For most houses the replacement is a Well-X-Trol WX-203, and we sell it as a complete package with a brass tee kit, switch and gauge for $1,295 shipped. We recommend a licensed plumber or well driller for this job; Aidan supports your installer by phone at 800-460-5810, and the tank is the same either way.

When a caller with a 2012 WX-203 asked what else to replace, my answer was everything from the tank tee up. A switch that has clicked on and off for 14 years is on the same clock the tank was, and a $1,295 tank hanging off a corroded tee is a bad trade. That is why every package we ship includes the tee kit. When you compare prices, compare like for like: a bare tank at a supply house does not include any of it.

The four sizes: WX-202 (20 gal, $1,095) for a cabin or a one-bath house, WX-203 (32 gal, $1,295) for most homes, WX-205 (34 gal, $1,695) when the space is under 3 feet tall, and WX-250 (44 gal, $2,195) for bigger pumps and heavy irrigation. If you were quoted a 119 gallon tank at a box store, call me first; I would send you to a plumbing supply house for a WX-302 or WX-350 rather than a box-store tank, or talk to you about a constant pressure system. Which size fits your pump is on the sizing page, and what the job costs installed four different ways is on the cost page; the four packages are in the well tank collection.

Four minutes on camera: how the tank works, the 30/50 and 40/60 switch, how to tell one has failed, and what I have seen tanks last.

Frequently Asked Questions