Pressure tanks are used in a variety of applications, but a common usage is system efficiency. For example, one reason someone might install a pressure tank in a plumbing system would be to keep the pump from constantly running. In doing so, the pressure-regulating tank increases the longevity of the pump/motor and reduces maintenance and downtime – ultimately resulting in lower operating costs. Let’s dive into a step-by-step how-to of sizing a pressure tank.
Info You NEED to KNOW Before Starting
Before beginning the process of sizing a tank, there are a few important input data points to know in order to properly size a pressure tank:
- Flow Rate
- Cut-in/Cut-out Pressure
- Target Run Time
A general rule of thumb, that most manufacturers suggest, is a run time of less than one minute if the horsepower is less than 1HP. If the motor is over 1HP, then a good guideline to follow is a run-time of 2 minutes or more. Always confirm this, with your tank manufacturer of choice, as guidelines can vary.
General Rule of Thumb for Sizing a Pressure Tank
Generally, as a rule of thumb, one can follow these guidelines when sizing a pressure tank:
- 0-10 GPM: 1 gallon of drawdown per 1 GPM of flow
- 10-20 GPM: 1.5 gallons of drawdown per 1 GPM of flow
- 20 GPM+: 2 gallons of drawdown per 1 GPM of flow
Drawdown can be defined as the amount of volume loss in the tank as the plumbing system “draws” off this pent-up pressure. After all, the purpose of a pressure tank is to maintain pressure in a given system and give the pump a break. This way, the pump doesn’t need to run constantly to remain at system pressure. While a pressure tank can appear costly upfront, it will save in the long run. Less run time for the pump means less maintenance and less money in energy costs.
There are various orientations of pressure tanks and the most common are horizontal, inline, and vertical. Be sure to determine which orientation works best for your plumbing setup.
Once we have identified our flow rate in gallons per minute (GPM), have identified our cut-in/cut-out pressure, and confirmed our target run time – we must determine what cut-in/cut-out pressure we want to set the system at.
Pressure Tank Sizing Explained
An important equation to remember when sizing a pressure tank is below:
Flow Rate X Run Time = Tank Draw Down Capacity
Let’s say we have a pump that produces 5 GPM and is ran by a ¾ HP motor. Since I’m operating a motor that is less than 1 HP, we are going to assume that “ABC Manufacturer” recommends a 1-minute runtime. We want to design this system to cut-in (turn on) at 40psi and cut-out (turn off) at 60psi.
5 (Flowrate) X 1 (Runtime) = 5 gallons of Draw Down (at 40/60PSI)
So, I will need to select a tank that allows for 5 gallons of drawdown at a pressure setting of 40PSI cut-in and 60PSI cut-out. If I need a vertical tank, I could select a WOMAX-220. If my plumbing layout would accommodate a horizontal tank better, I could select a WOMAXH-220. This would give me approximately 3.5 minutes of run time before the pump would cycle back on. Horizontal pressure tanks have a plastic pump stand so you can maximize space when designing a plumbing system. This is certainly a nice feature when working in confined spaces where space is at a premium.
Relationship Between Pressure & Tank Size
An important thing to remember, the higher the operating pressure – the larger the tank must be. Pressure and tank size have a direct correlation – as one increases, so does the other. The higher the pressure setting, the less the drawdown is and thus, the need for larger tank capacity.
After we have these three points determined, we can then proceed with sizing a pressure tank. Pressure settings are another important factor with any plumbing system. The most common pressure settings are 30/50; 40/60; 50/70. Most manufacturers will have a pressure tank sizing chart that will allow viewers to quickly size a tank’s drawdown based upon their system’s pressure settings.
We can supply you with this information on the Wilo MaxAir® product line if you want to get into the details. Just give us a ring or visit www.dultmeier.com 24/7. Here is a drawing of a Wilo MaxAir® horizontal tank that outlines some features which set this product line apart from the rest of the pack and really make it one of the top-line products in the marketplace.
You can view the full offering of Wilo MaxAir® Pressure Tanks Right Here on dultmeier.com. As always, should you have further questions about pressure tank sizing or other applications – don’t hesitate to contact us. That’s what we are here for. Your Experts in Delivering Fluid Handling Solutions – We Know Flow!
23 thoughts on “Sizing a Pressure Tank – Your Step-by-Step Guide”
I am replacing an existing system (pressure switch, piping, and tank) on our well. Pump has been in use for about 6 years. It is a 1 hp pump with a 12 gpm rating. Pressure switch (new) is 40/60. The old tank measures (eyeball) at 21 inch dia by 29 inches tall. Can you advise what size tank I need?
Bill, you will need to know your desired run time to use the formula. You’re probably in the neighborhood of a 35 gallon tank with those dimensions but best not to guess. At 35 gallon capacity with a 12gpm pump flow – you’re just under 3 minutes of run time. Remember, just because a pump is rated 12gpm flow – doesn’t mean it will produce that in your system. We always need to account for pressure loss through the plumbing system. Hope this helps.
Is just getting a bigger tank than you need the smarter option? Or is that bad? I have an EXTREMELY old galvanized pressure tank ( I dont think it even has a bladder) the pump switches on and off constantly so I have to blow compressed air into the Schroeder valve…. I need a new tank but dont know what size I should have but though bigger was better. This old tank is probably around 80 gallons but I think they put it in in the 50’s or 60’s to help the neighbors whose well ran dry at the same time (just a rumor though). I know smaller than what you need is bad but is too big a bad thing if you have space and the cost is nominal to get a bigger tank?
A bigger tank will cause your pump to run much longer to fill it up. Most pumps don’t want to run too long as they overheat and shorten the life of the pump. There’s a balance between cycling the pump on and off multiple times when using a lot of water (shower, washer, etc.) and also how long the pump needs to run to get to full pressure. At least that’s what I’ve been told.
Hi Bart, What did you decide. I have the exact same setup as yours and I am trying to decide the same. I didn’t see a straight forward response to your question. Just curious what you went with and why. Bryan
Hi Bryan, I have the same setup as you & Bart. Could you please let me know what size you went with?
I would like to replace my tank and like you both I’m trying to figure out the new tank size. My tank was put in when the house was built in 1987 and since I’ve replaced everything but the tank(and the well casing) over the last 3 months I figured now would be a good time to replace the old tank.
Any info would be greatly appreciated!
(who doing this solo since Hubby past 2 yrs ago)
Bart what area are you in. And are you using a submersible/ centrifugal / or jet pump In Florida, if I was getting 12 gpm off a 1 horse submersible pump I would inform you the liquid end is starting to fail very well could be because it has been running with a waterlogged pressure tank. I usually base sizing of tanks on how many people live in the house 1-2 people can get by with a 20-gallon equivalent if your frugal. Otherwise 2-4 people will need a 40-100 gallon equivalent
Can someone help me figure out what size pressue tank i need We have a 1 horse pump 5 gallond per minute to the cistern could be a bit more been drilled for 15 years our cistern is 1500 gallons
Susan, I would suggest getting with a well driller or installer to assess the water quality after that length of time. If you have a fair amount of sediment and buildup in the plumbing it could affect your flow. This is something you want to know prior to sizing your tank. The pump may be rated to 5GPM but you might not be getting that flow rate…
I’m supplying my house with tank water 4×5000 gallon tanks I have a Davey’s XP350P8C pump using an Onga 24L air tank and I would like to know what PSI I should be applying to the air tank.
Daniel, I would suggest contacting a well driller/installer or Onga for their cut-in/cut-out specifications for your specific tank.
Thank you for including the general rule of thumb guidelines for figuring out the right size of a pressure tank. My wife and I just moved into a home that has a well and we need to get a new pressure tank set up. This article will definitely help us figure out the right size to buy!
Will a bigger tan give me more water pressure
James, a bigger tank does not equate to more pressure. A larger tank equates to more run time off your tank supply. Thus, keeping your pump from cycling on and off – that is the purpose of a pressure tank, to help prevent the pump motor from cycling constantly (which shortens the life of the pump/motor).
I have a shared well. There are 4 households on the well, with horses and gardens at each. I understand the need to know the answer to Flow Rate X Run Time = Tank Draw Down Capacity. My question is does each house need it’s own tank or is there 1 large enough for all for households?
Christina, thanks for your question. That is a function of how much water will be needed in each household and how much flow the pump produces. In most instances, you would most likely need one tank per household. It would be rare for houses to share one large community pressure tank.
We have a well that’s 22 years old and it supplies 2 house’s. We have replaced the pump 6 months ago. We keep losing water pressure about every 2 weeks.What would be the next step to do so we can have pressure.
Hey, Larry. Thanks for posting. Sounds like you have a leak in the system somewhere. If there is a check valve inline, that might be a good place to look. Seal failures in new pumps are not unheard of but would be rare. I would get the contractor that replaced the pump back out to inspect. Best to get a professional on site to troubleshoot.
I have a 36 gallon pressure tank. Over the past 6 months I have had a decline in pressure. I am assuming I need a new tank (it’s 12-15 years old). Since the well was drilled and the pressure than was installed we put in a sprinkler system 5 years ago. Should I consider increasing the tank size as a result of the sprinkler system we use for about 3 months of the year. Flow rate is approximately 7 GPM, cut in/out is set at 40/60 and each zone of the sprinkler system runs for 15 minutes with a lag time of 10 minutes between zones (total of 3 zones)
I have no idea what kind of pump is in the well. Someone put it and then left me hanging with finishing it. My well/water is 165 ft. deep. I was told that it had a flow of I want to say 31 gallons per minute?? right now it has good pressure when I turn on the electricity to get water. It is actually extremely good pressure. I need to put on a pressure tank and have no idea what size I should use. I have tried to call the guy who put in the pump several times and cannot get an answer or a call back.
I have an unusual setup with (2) wells on an alternating controller and lead (50/70) / lag (40/60) pressure switch configuration. Well #1 is a .5 GPM and well #2 is 2.5 GPM. Both pumps are 1.5HP Grunfos hung at 600′. I have a 27 year old 86 gallon pressure tank and am wanting to replace proactively. Given the variations on output I am having difficulty determining a correct tank size. Currently the 86 gallon is impossible to find… could I drop to the more readily available 81 gallon tank without issue?
Jeffrey, you should be ok with a 5 gallon differential but would suggest that you err on the side of a larger tank than smaller.
I building new house one floor. I have to pump water from well. I will have 2/3 people in house.
What do I need size of tank and pump to suck water and pump for pressure tank to the house