A well pressure tank is a water storage reservoir that helps keep water pressure at a constant level.
If you’re drilling a new well or replacing an old well pressure tank, you might have some questions about which size well pressure tank you need. That depends on several factors, such as flow rate, pressure switch setting, and minimum run time.
Still, the most relevant factor when choosing a well pressure tank size is the drawdown capacity, which is calculated by multiplying the flow rate of the pump by its minimum run time.
What is a Well Pressure Tank Drawdown?
A pressure tank drawdown is the amount of water that the tank stores and makes available when the pressure switch is off. In other words, it’s the amount of water the pressure tank can store and/or deliver between the time the pump shuts off and restarts.
Here’s an example of the action of a well pressure tank with a 30/50 pressure switch:
- The water pressure is at 50psi (pounds per square inch) when the tank is full, so the pressure switch turns off.
- The water pressure gradually decreases to 30psi as you use the stored water. There will be no water incoming to the tank until the pressure hits 30psi.
- Once the pressure reaches 30psi, the pump will reactivate until it climbs back up to 50psi.
In this scenario, the volume of water that can be drawn from the tank when its water pressure goes from 50psi down to 30psi is the pressure tank drawdown.
You shouldn’t confuse tank drawdown with tank volume. Only ⅓ of pressure tanks actually hold water; the rest is reserved for the air that’s responsible for producing pressure.
In addition, there should always be some water left inside the tank to maintain the minimum pressure point, which is 30psi in the case of tanks with 30/50psi pressure switch.
So, considering the air and unused water storage inside the tank, the volume of a tank should be at least four times its drawdown capacity.
The bigger the well pressure tank, the more water you can store. If you have a larger drawdown, your tank can run longer and will go through fewer pumping cycles.
What Size Pressure Tank Do I Need?
Pressure tanks come in 20-, 30-, 50-, 85-, and 120-gallon sizes. Here’s what you need to know before deciding the right tank size for you:
- Flow rate: What is your pump flow rate in gallons per minute (GPM)? The flow rate is usually initially tested when you drill a new well.
- Minimum run time of the well pump: How long will your pump run? More often than not, pumps are designed to run for a minute, and they are at their most efficient with a one-minute run time. However, you can consult the manufacturer of the pump to get a better idea about its specific run time.
- Pressure switch setting: The pressure switch setting is the pressure at which the pump turns on and shuts off. This setting can affect the tank's drawdown.
When you know the flow rate, minimum run time, and pressure switch setting, you can then determine the pressure tank drawdown. Knowing the drawdown will help you determine your tank size:
Drawdown capacity = Pump flow rate x Minimum run time
Here’s an example of a 10GPM flow rate and a 1-minute minimum run time:
10×1 = 10-gallon drawdown capacity.
Then, to determine the tank size you need, you should multiply the drawdown number by four because of the reason we mentioned in the previous section: only about ¼ of a pressure tank’s volume consists of usable water.
That being said, if you don’t know the flow rate, but want to find out the drawdown capacity you need from your pressure tank, you can run a quick test:
- Grab a couple of 5-gallon buckets.
- Fill them with maximum water pressure for exactly a minute after ensuring no other faucet is running at your household.
- Measure how many gallons you have at the end of that one minute.
- The number of gallons you have is your flow rate in gallons per minute. You should then multiply it by the minimum run time number.
- Then, multiply the final number by four and round it up to the closest available tank size.
With that calculation and with the available tank sizes we mentioned above, your best bet for a 10GPM flow rate and 1-minute run time would be a 50-gallon tank.
However, the pressure switch settings might also play a role in your decision on the proper tank size. Next, we’ll give you a general idea of what this role is, but we recommend you also consult the manufacturer.
Understanding Pressure Switch Settings
There are three pressure switch settings for pressure tanks: 20/40psi, 30/50psi, and 40/60psi.
The first number in these settings is the lowest pressure at which the pump turns on, and the last number is the highest pressure at which the pump turns off.
Most well pressure tanks come with a chart to tell you the drawdown capacity based on your pressure switch settings, and there’s one single point to heed in these calculations. As the turning on point of a pressure switch increases, its drawdown capacity will decrease.
This is because of the difference in water pressure ratio. If a tank can handle a 60psi maximum and 40psi minimum, then it means that it makes only ⅓ (20/60) of its water pressure available, while that number would be ½ for a 20/40psi tank.
This only has a small effect on the calculations you need to make, and all pressure tank manufacturers provide their own charts that explain the relationship between drawdown capacity, pressure switch settings, and of course, tank size relevant to their products. So, before making a purchase, it’s best to consult these charts before making a decision.
Can a Well Pressure Tank Be Too Big?
You can’t have a well pressure tank that’s too big. A larger tank would have longer and slower cycles, and that’s it.
In fact, bigger is better when it comes to well pressure tanks because they wear out more slowly with fewer startups and longer run times. Having fewer pump cycles increases tank longevity.
The only downsides to having a larger tank are that they cost more and take up more space. There’s also a chance a bigger tank may draw water faster than the well could supply (although unlikely).
So if space and cost aren’t an issue, it’s in your best interest to size up.
On the other hand, it’s possible to have a tank that’s too small. If your tank is too small, the pump will cycle too quickly. This will shorten the lifespan of your tank, which means you’ll have to spend a lot of money to replace it rather quickly.
A correctly-sized, good-quality well pressure tank will last an average of 15 years. Some cheaper tanks last approximately five years, while high-quality pressure tanks can last up to 30 years.
By opting for a bigger tank, you might add a couple of years, while choosing a smaller-than-needed size will soon lead to problems because the tank won’t be able to handle your water needs.
Is a 20-Gallon Pressure Tank Big Enough?
A 20-gallon pressure tank is only big enough if your pump flow rate is below 5 gallons. The reason for this is that a 20-gallon tank only has six gallons of drawdown.
Let’s revisit our drawdown equation again for a 20-gallon pressure tank with a 5-gallon drawdown capacity and 1-minute run time:
5GPM flow rate x 1-minute minimum run time = 5 gallons of drawdown capacity.
A pump with a flow rate above 5GPM will need more than 5 gallons of drawdown to function properly and maintain the ⅓ water-to-air ratio in the pressure tank. Thus, a 20-gallon pressure tank is only useful if you have a flow rate at or below 5 gallons.
However, keeping in mind that bigger tanks are better, even if you don’t need the capacity of a tank that’s bigger than 20 gallons, you may still want to invest in one.
The larger the pressure tank, the longer the tank’s lifespan. Therefore, even if your pump flow rate is only 0 to 5GPM, a larger tank will serve you better for longer.
Will Installing a Larger Pressure Tank Increase Water Pressure?
Getting a larger well pressure tank will not increase your water pressure beyond the pump’s pressure control setting. In other words, getting a larger pressure tank won’t improve your water pressure.
However, a larger pressure tank will cause the water pressure to decrease more slowly since it has a longer drawdown time. So, with a larger tank, water pressure will be higher for longer.
You can always add a booster pump to improve water pressure and flow, including water pressure in buildings high above the waterline.
Conclusion
Drawdown capacity is the most important factor to consider when choosing a pressure tank. However, since the pump flow rate and minimum run time determines the drawdown capacity, they should also be considered.
That being said, there’s no scenario where your pressure tank is too large for your needs. On the contrary, a large tank will have less wear and tear, and it’ll serve you a lot longer than a small tank.
Of course, bigger tanks are more expensive, but the investment may just be worth the price when you consider that you’ll likely need to replace smaller tanks more quickly.
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You point out how a larger tank will last longer but what about pump run time each cycle? If your pump has to run longer to fill the tank each time it comes on then will this wear your pump out sooner?
Hi Richard, your point is logical. We believe that anytime a machine with moving parts does more work, it will hasten its lifetime or servicing required at its service intervals, generally speaking. That said, water pumps in these types of applications are engineered for exactly the qualities required such as endurance and longevity. In the final analysis, we believe that properly engineered systems that appropriately match the right pumps to the size of the tank bring into consideration all that is necessary to have a properly working and economical system. As always, if there are service intervals and preventative maintenance specified for in the system, all of those actions should be taken to ensure a cost effective and highly performing system.
Would a larger tank give me the ability to run two sprinkler systems at same time ? Now if I turn on a 2nd prssure drops to much for them to be effective.
Hi Gary, there are a lot of factors to consider about your specific system to say definitely. But, I’m confident that generally speaking, the situation you’re presenting is more about system design and making sure there is enough flow in terms of gallons per minute (GPM) and other factors related to how much water can flow versus how much water you have in reserve. So, it’s not about tank size as it is about the overall system’s ability to flow water. When you turn on a 2nd sprinkler system (or another load on your water system), you are stacking demands on the same VOLUME of water that your system is capable of delivering in any given period of time (per second/minute/hour). That’s why the pressure drops. So, it’s about right sizing your overall system’s capability to deliver water in a period of time rather than just the size of your tank. They are related as a larger tank can deliver a greater amount of water as needed by multiple sources. But, hopefully I’ve made the point clear. I would suggest a reputable water system business in your area to help ensure you have the right overall design for your needs.
A larger tank is not always the answer. It may be the piping between the tank and the point of use. Smaller pipes cause more friction loss and may not be able to pass the amount of water being used. Case in point a 1/2″ pipe can not pass the same volume of water as a 3/4″ or 1″ pipe. The number of fittings also comes in to play as each fitting causes friction loss. Also it does little good to increase the size of the pipe at the point of use as you are limited to the volume and pressure of the water delivered to that point in the system.