A Well Water Holding Tank Is Not a Pressure Tank

	Holding (storage) tank	Pressure tank
Its job	Stores a large supply of water	Keeps water under pressure and keeps the pump from starting every time a faucet opens
Pressure inside	Atmospheric (the same as the air outside), plus a little pressure at the bottom from the weight of the water	Your house's water pressure, rising and falling as the pump turns on and off
Vented or sealed	Vented	Sealed, with compressed air inside
Usable water	Most of what it holds	About a fifth of the tank's size comes out before the pump turns back on
What it connects to	Filled by the well pump (or by rain or hauled water), with a second pump drawing water out	Sits between the pump and your house, next to a pressure switch that turns the pump on and off
Where you buy it	Tank suppliers like us	Well contractors and plumbing suppliers
Who installs it	You set it in place, and a well contractor or plumber connects the pumps	A well contractor or plumber

That pump pressure would push on every part of the tank, including the lid. This tank has a 16-inch threaded lid with a breather vent. At 50 psi, the water would push up on that lid with about 10,000 pounds of force. On a vented tank, the lid normally holds back no pressure at all. Neither the lid nor the tank is rated for pressure.

I took a call from a homeowner on a 50 to 60 psi well system who was about to order a 500-gallon cistern. He planned to plumb it into his line so his house would have a big supply of pressurized water. I told him the tank wasn't rated for pressure and would fail over time. I didn't want to sell him a tank that would eventually burst, so I talked him out of that order and walked him through a holding tank setup with its own pump instead.

If a storage tank is already tied into your pressurized line, have a well contractor or plumber take it off the line.

How Does a Well Pressure Tank Work?

A well pressure tank works by squeezing air. The tank holds a pocket of air along with the water. When the well pump runs, it pushes water into the tank. Air compresses and water doesn't, so the incoming water squeezes the air into a smaller space. When you open a faucet, that squeezed air pushes the water back out to your house.

A pressure switch decides when the pump runs. A switch set to 40/60, for example, turns the pump on when the pressure drops to 40 psi and turns it off when the pressure reaches 60 psi.

A pressure tank holds much less usable water than its size suggests. Only about 20 percent of a pressure tank's size comes out before the pump has to turn on again, so a 42-gallon tank gives you about 8 gallons. That's why a pressure tank can't do a holding tank's job of storing a large supply.

The pump makes the pressure, and the pressure tank holds it between pump starts. That also protects the pump. Without a pressure tank, the pump would short cycle, turning on and off again and again and running only a few seconds each time. All those starts may shorten the life of the pump motor.

Can You Get Water Pressure From Gravity Instead?

Gravity can give you some water pressure, but rarely enough to run a house. A faucet below a tank gets pressure the same way the bottom of a tank does: about 1 psi for every 2.31 feet of water above it. That's very little pressure for each foot of height, so getting house-level pressure from gravity takes a tank set very high above the faucet:

Real pressure at the tap is lower than these numbers, because water loses some pressure to friction as it moves through the pipe.

Almost every faucet and fixture in a house needs at least 10 psi to work properly, and water softeners, clothes washers and dishwashers need at least 30 psi. A full tank on the ground or on a stand usually gives you less than 10 psi. That's enough to let water run downhill into a stock trough, but not enough to run a shower or a washing machine.

A bigger tank doesn't give you more pressure either. Pressure depends only on how deep the water is, not on how many gallons are behind it. Two tanks filled to the same height give the same pressure at the bottom, even if one holds ten times as much water.

What Does a Well System With a Holding Tank Need?

A well system with a holding tank needs two pumps: one to fill the holding tank from the well, and a second one to send water from the holding tank to the house at pressure. Water moves through the system in this order:

1. The well pump pulls water from the well and sends it to the holding tank, at whatever pace the well can supply.
   
   
2. A float switch in the holding tank turns the well pump on when the water level drops and off when the tank is full.
   
   
3. The holding tank stores the water until the house needs it.
   
   
4. A booster pump, also called a transfer pump, draws water from an outlet near the bottom of the holding tank and pushes it toward the house.
   
   
5. A pressure switch and pressure tank sit after the booster pump, on the house side. The switch turns the booster pump on and off. The pressure tank holds the water under pressure between pump starts, just as it does on any well.
   
   
6. Water reaches your house at normal household pressure.
   
   

If you already have a pressure tank, ask your well contractor whether it can be reused on the house side of the new setup.

It's a good idea to add a second float switch, set low in the holding tank. The first float switch runs the well pump. This second one turns the booster pump off when the water level gets too low. That keeps the booster pump from running with no water, which can damage it.

A rainwater cistern is set up the same way from the tank onward. Rain fills the cistern instead of a well pump, and the booster pump, pressure switch and pressure tank do the same jobs they do with a well.

You'll get the pumps, float switches and pressure tank from a well contractor or plumbing supplier. Installing pumps and pressure tanks isn't a do-it-yourself job, so have a well contractor or another water system professional do it.

Do You Need a Holding Tank for Well Water?

A holding tank for well water is usually worth adding when your well can't keep up with your household's busiest hours.

A well that can't keep up with your busiest hours is called a low-yield well. A low-yield well may still produce plenty of water over a full day. It just can't produce it fast enough when everyone is using water at once. Penn State Extension gives the example of a well that pumps 1 gallon per minute. Over a full day, that adds up to 1,440 gallons. But in a home without water-saving fixtures, a family of four could use more than 300 gallons during a busy two hours of laundry, showers, dishes, toilets and sinks. In those same two hours, the well supplies only 120. A holding tank lets the well refill it slowly around the clock, so the water is already stored when everyone needs it at once.

A holding tank also helps when:

• You collect rainwater and need a cistern to hold it
• You haul water in and need a place to keep it
• You're building off-grid and need a stored water supply
• You want an emergency reserve of water on hand
• You water livestock and need more water at once than the well can pump

A holding tank probably isn't worth it when:

• Your well keeps up with your household's busiest hours. The pressure tank already does everything you need.
• Your well falls only a little short. A larger pressure tank may be enough.

A bigger pressure tank still isn't real storage, but it holds more usable water. An 82-gallon tank gives you about 16 gallons before the pump turns on, and a 120-gallon tank gives you about 24. That compares with about 8 from a 42-gallon tank. The extra water means the pump starts less often, and you have more water on hand for a short burst of use. Ask your well contractor whether a larger pressure tank would be enough for your busiest hours before you add a holding tank and a second pump.

What Should You Look For in a Holding Tank?

When you shop for a holding tank for your well, check for:

• A vented lid
• An outlet near the bottom of the tank, where the booster pump's line connects
• A drinking-water (potable) rating, if the tank will hold water you drink

For drinking water, start with our potable water tanks. Drinking-water ratings vary from one tank to the next, so check each tank's product page for its details. Rules for storing drinking water also vary by state and by local area, so check what applies where you live before you buy.

Our guide on how to choose a water storage tank walks through choosing the size, material and placement.

If you're trying to work out whether your well needs a holding tank, or which tank fits your setup, give us a call at 833-465-8265 or reach us through our contact page. We'll help you find the right tank, and if a holding tank isn't the answer for your well, we'll tell you that too.