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A 20-gallon pressure tank can be big enough for a house, but the tank size alone does not tell you whether it is a good fit.
A pressure tank does not provide its full labeled volume as usable water. A 20-gallon tank may supply only several gallons between pump starts, depending on the pressure switch settings and the tank's air precharge. For a small home with modest water use, that may work well. For a larger household, a high-flow well pump, or frequent irrigation, a larger tank may reduce pump cycling and usually be easier on the pump.
The main question is not how many people live in the house. It is whether the tank provides enough drawdown for the pump and water demand.
What a Pressure Tank Does
A pressure tank stores water under pressure so the well pump does not need to start every time someone opens a faucet.
Inside most modern pressure tanks is a bladder or diaphragm that separates water from compressed air. When the pump fills the tank, the air is compressed. When you use water, that compressed air pushes water out of the tank.
The pump starts again when system pressure falls to the pressure switch's cut-in pressure. It stops when pressure reaches the cut-out pressure.
For example, a system may operate between:
- 30 and 50 psi
- 40 and 60 psi
PSI means pounds per square inch. It is the amount of water pressure in the plumbing.
The pressure tank helps prevent the pump from starting and stopping too often.
A 20-Gallon Tank Does Not Give You 20 Gallons of Water
The rated tank size includes both water and compressed air.
The amount of water that can leave the tank before the pump starts is called drawdown.
A 20-gallon pressure tank commonly provides only a fraction of its total volume as drawdown. The exact amount depends on:
- Cut-in pressure
- Cut-out pressure
- Air precharge
- Tank design
Because of this, comparing pressure tanks only by their advertised gallon capacity can be misleading.
Always check the manufacturer's drawdown chart for the pressure settings you plan to use.
When a 20-Gallon Pressure Tank May Be Big Enough
A 20-gallon tank may work well when the house has relatively modest water demand and the pump is properly matched to it.
It can make sense for:
- A small house or cabin
- A low-flow well pump
- A household with moderate water use
- A system without large irrigation demands
- A booster-pump system designed for a smaller tank
- A pump with controls designed to limit cycling
For example, if only one faucet or fixture is normally being used at a time, a smaller tank may perform adequately.
The pump still needs enough run time after each start. That is one of the most important parts of sizing the tank.
When 20 Gallons May Be Too Small
A 20-gallon tank can become limiting when several fixtures or water-using systems operate frequently.
You may benefit from more pressure-tank capacity if you have:
- A high-capacity well pump
- Several bathrooms
- A large household
- Frequent toilet flushing and faucet use
- Irrigation connected to the well
- Large outdoor watering demands
- Appliances that use water in short cycles
- A pump that starts very frequently
The problem is usually not that the house will suddenly run out of water. The pump simply starts sooner because the tank has less drawdown.
Too many pump starts can increase wear on pumps, switches, relays, and other equipment.
Pump Cycling Matters More Than House Size
Pressure tanks are often described as being suitable for a certain number of bedrooms or people. That can be a rough starting point, but it is not the best way to size one.
The pump matters more.
Suppose a pump delivers 10 gallons per minute. If you want it to run for at least one minute after starting, the system generally needs enough usable drawdown to support that operating goal.
A small tank may not provide enough drawdown by itself.
However, pump-control systems can change the calculation. A variable-speed pump or constant-pressure system may use a much smaller pressure tank because the pump does not operate the same way as a traditional fixed-speed well pump.
Do not automatically install a large tank on a variable-speed system. Follow the pump and controller manufacturer's tank-sizing requirements.
Check Your Pump's Recommended Minimum Run Time
Traditional well pumps should normally avoid rapid starting and stopping.
Pump manufacturers may specify a recommended minimum run time or maximum number of starts per hour. Follow those requirements when sizing the tank.
A simplified way to think about the relationship is:
Required drawdown = pump flow rate × desired minimum run time
For example, if the pump delivers 8 gallons per minute and the system design calls for about one minute of pump run time, you would look for approximately 8 gallons of usable drawdown.
That does not mean you need an 8-gallon pressure tank. You need a tank large enough to provide that much actual drawdown at your pressure settings.
The manufacturer's drawdown chart tells you what nominal tank size is required.
Pressure Settings Change Tank Capacity
The pressure switch settings affect how much usable water the tank provides.
Understanding pumps designed for storage-tank water service helps assess allowable run time under normal household demand.
A common pressure switch has a 20 psi difference between cut-in and cut-out pressure. The tank's air precharge must also be set correctly for the system.
For many bladder-style tanks, the precharge is set slightly below the pump's cut-in pressure, but you should use the tank manufacturer's instructions rather than applying one setting to every system.
Precharge must normally be checked with the water pressure relieved from the tank. Working on a pressurized water system can cause injury or water damage, so shut off electrical power to the pump and depressurize the system before performing maintenance.
If you are unsure how the pressure switch, pump controls, and tank interact, a well or pump professional can check them together.
Signs Your Pressure Tank May Be Too Small
Tank size may deserve attention if the pump starts and stops repeatedly during normal water use.
Possible signs include:
- The pump starts every time a small amount of water is used
- The pump cycles repeatedly while someone showers
- Pressure changes happen very quickly
- Irrigation causes frequent pump starts
- The pressure switch clicks on and off unusually often
However, these symptoms do not always mean the tank is undersized.
They can also indicate:
- Incorrect air precharge
- A failed bladder
- A waterlogged tank
- A pressure-switch problem
- A plumbing leak
- A check-valve problem
- A pump or control problem
Do not replace the tank based on cycling alone without checking the system.
Is a Bigger Pressure Tank Better?
For a conventional fixed-speed well pump, more usable drawdown can reduce the number of pump starts.
That can be helpful, but bigger is not automatically better.
A larger tank:
- Takes up more space
- Contains more water
- Places more weight on the floor
- May cost more to install
- Must still match the pump and pressure controls
Water weighs about 8.3 pounds per gallon, so large tanks can become heavy when filled. Make sure the installation surface can safely support the complete system.
Using two compatible pressure tanks can also increase total drawdown in some systems. They need to be installed and precharged correctly.
What About a Rainwater Pump System?
The same basic principles apply if a pressure tank is being used with a rainwater harvesting system.
For example, a cistern pump supplying toilets, outdoor faucets, or irrigation may use a pressure tank to reduce cycling.
Tank sizing still depends on:
- Pump flow rate
- Pressure settings
- Expected water demand
- Pump-control type
- Required pump run time
The pressure tank does not increase the amount of rainwater available. It only changes how the pump and pressurized plumbing system operate.
If collected rainwater will supply indoor plumbing, keep it separate from potable plumbing where required and follow applicable local plumbing and cross-connection rules.
How to Decide Whether 20 Gallons Is Enough
Start with the pump rather than the house.
Find:
- Your pump's flow rate in gallons per minute.
- The pump manufacturer's recommended minimum run time.
- Your pressure switch's cut-in and cut-out settings.
- The pressure tank's drawdown at those settings.
- Whether you have a conventional pump or variable-speed control.
Then compare the available drawdown with what the pump system requires.
A 20-gallon pressure tank can be completely reasonable for one house and too small for another.
For a traditional well system, the safest approach is to size the tank around the pump's cycling requirements rather than simply choosing a tank according to household size.
Frequently Asked Questions
Is a 20-gallon pressure tank enough for a family of four?
It may be, but household size is not the main sizing factor. Pump flow rate, pressure settings, usable drawdown, and recommended pump run time matter more. A larger tank may be useful if the pump cycles frequently.
How much usable water is in a 20-gallon pressure tank?
Much less than 20 gallons. Part of the tank contains compressed air. The exact usable drawdown depends on the tank design, air precharge, and cut-in and cut-out pressures. Check the manufacturer's drawdown chart.
Will a bigger pressure tank increase water pressure?
Not normally. Pressure is mainly controlled by the pump, pressure switch, and other controls. A larger tank mainly provides more drawdown and can reduce how frequently a conventional pump starts.
Can a pressure tank be too small?
Yes. If there is not enough drawdown, a conventional pump may start and stop more frequently than recommended. Frequent cycling can shorten the life of pump-system components.
Can I replace a 20-gallon pressure tank with a larger one?
Often, but the new tank must be compatible with the pump, pressure controls, plumbing connections, available space, and structural support. Variable-speed pump systems may have specific tank requirements.
Why does my well pump keep turning on and off?
Frequent cycling can result from an undersized tank, but it can also come from incorrect air pressure, a damaged bladder, plumbing leaks, pressure-switch problems, or other system faults. Check the system before assuming tank size is the cause.
Does a variable-speed well pump need a large pressure tank?
Often not. Variable-speed and constant-pressure systems can use different tank-sizing methods than traditional fixed-speed pumps. Follow the pump and controller manufacturer's requirements.
