# Well Pump Pressure Tank Size?

Discussion in 'Pumps and Tanks Well Forum & Blog' started by qcprc, Apr 19, 2005.

1. ### qcprcNew Member

Joined:
Feb 15, 2005
I've added several more fixtures to my house and I've seen where they want you to add up the amount of fixtures in order to determine what size tank you need. My question is: is there a relationship between pump size vs. tank size? I have 1/2 horse 3450rpm pump. So what could be the biggest size tank I could install. I want to reduce the amount the times the pump cycles etc. too. Please advise.

2. ### hjModerator & Master PlumberStaff Member

Joined:
Aug 31, 2004
Occupation:
Plumber
Location:
Cave Creek, Arizona
tank

The tank is a resevoir for the water from the pump. The smaller it is the faster it will "run out of water" and then the pump will start. THere is no direct relationship between the tank and anything else. A small tank will cause frequent short operations of the pump. A large tank will turn the pump on less frequently, and once it starts it will run for a longer period of time. The ideal solution is to install a larger tank if you have room for it.

4. ### qcprcNew Member

Joined:
Feb 15, 2005
re

Thanks for the information. I guess I'm confused with the pressure vs. volume issue. So what is the difference between, let's say a 1/2 horse pump vs. a 3/4 horse? More volume at a faster rate?

5. ### hjModerator & Master PlumberStaff Member

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Plumber
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Cave Creek, Arizona
pump

It depends on the pump. A pump for a deep well might have more stages to give additional force, but the same volume. You would have to compare the flow curves for the two pumps to see which has the necessary volume for the specifications of your well.

6. ### Gary SlusserThat's all folks!

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Retired
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Wherever I park the motorhome.
In general terms you could say that pressure is the resistance to flow and volume is how many gallons is delivered by the motor in overcoming the resistance to that flow. The bigger the horse, the more work can be done, but if the wagon is the same size and the same volume of material is in the wagon and the weight is the same..... the bigger horse eats more and takes up more room for no gain. Unless you drive the horse faster, the result is the same.

As to tank size.... The size of the tank is to be sufficient, at the pressure range the pump is operated, to allow the pump motor to be off for at least 60 seconds for motors of up to 1 hp. Over 1 hp up to 2 hp the minimum is 120 seconds; above that the time needed is increased. That is to allow for proper cooling of the motor betwen pump runs. The pressure range used dictates the drawdown gallons provided between pump runs. The higher the pressure the fewer gallons no matter what size tank. I.E. a 20 gal nominal will provide about 6.5 gals at 20/40 psi, about 5.2 at 30/50 and about 4 at 40/60 etc.. The larger the tank the more gallons but... you can go too large and cause water quality problems and/or pump problems depending on the pumping level and recovery rate of the given well and pump or cause a 'dry' well condition at times of heavy water use.

In more or less layman's terms... To size a pump you have to know the peak demand gpm the building requires (your fixture unit count) and how much 'work' is required to provide them, then you apply the horses to get the gpm to where you need them or where they ned to go, or in other words, to get the job done. The hp has little to do with the gpm output of the pump. That's because all pumps have two parts; the wet end or stages, and the motor. The wet end is rated in gpm and the motor in hp. Between the two, they deliver the gpm to where it needs to go at whatever 'head' (elevation/height/pressure) the system has to be able to overcome, including friction pressure losses of the plumbing material used.

Gary
Quaity Water Associates

7. ### PumpmanPump Sales

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Apr 12, 2005
Occupation:
Owner - R.L. Peeks Pump Sales
Location:
So. Cal
According to Sta-rite, the rule of thumb for sizing pressure tanks is this:
Size the tank so that you have one gallon of drawdown for each gallon of pump capacity. This ensures that the pump will get the proper run time.
Ron

8. ### qcprcNew Member

Joined:
Feb 15, 2005
Pump

Gary and others,
So if I have a 1/2 horse well pump (and I'm not interested in replacing that at this time) and I have 3 toilets, 6 sinks, 2 showers, and two sets of outside spickets, spread over a 3100 square foot house, whats the biggest size tank you would recommend? I have 20 gal right now...

9. ### jadnashuaRetired Defense Industry Engineer xxx

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Sep 2, 2004
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Retired Systems engineer for defense industry.
Location:
New England
If I'm reading the comments properly, you need to determine how many gallons/minute your pump supplies, then, possibly, find out how many gallons your well can suppy/minute. You need enough time for the well to recover inbetween cycles, too. Then, make a wag at how many fixtures are likely to run at one time to determine your demand. You could size for everything to be on full, but that is unrealistic. My unprofessional opinion.

10. ### Gary SlusserThat's all folks!

Joined:
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Retired
Location:
Wherever I park the motorhome.
Forget the hp, it has next to nothing to do with the situation. Run water somewhere until the pump starts. When the pump shuts off, drain the tank into a bucket until the pump starts. Measure the water in the bucket, that's your drawdown gallons. Then run water until the pump starts and time the time it takes it to shut off. Then run water and time the time until the pump starts, that time should be at least 60 seconds, and probably equal to the first time. If you catch one minute's flow into a bucket or any portion of a minute and multipy for one minute's flow, it won't be exact but close to the gpm of the system. If the pump doesn't stay off for 60 seconds, then you should get a larger tank which will provide a larger drawdown (equal to the gpm of the system) .

That gpm is what you have available for the house, and the square feet has nothing to do with your plumbing or water system's peak flow rate; it is the number of fixtures wanting water at the same time. But then no one truns on all the fixtures in a house at the same time so... count up the gpm of the fixtures you think will be used at the same time and see if the pressure tank gpm is equal to or above that number. If not, then you don't use all those fixtures or you replace the pump with another 1/2 hp but a higher gpm wet end. You can actually buy just a wet end for your motor. Like a 5-13 gpm as long as the hp is sufficient to do the job. Now you don't find wet ends at the local big box stores, you go to the local pump or plumbing supply houses or an internet dealer like me and many others. All you need to know is the brand of pump or the motor you have on the pump.

Gary
Quality Water Associates

Last edited: Apr 20, 2005
11. ### qcprcNew Member

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Feb 15, 2005
Re:

Thanks Gary and all the others for all your help on this matter. I appreciate the advice.

Jason

12. ### captwallyNew Member

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Apr 30, 2005
Occupation:
Apartment Complex Maintenance, Professional Chef
Location:
Florida
I can't imagine living in a house of 1500 square ft, let alone 3100 square ft with anything less than a 3/4 or 1 HP pump. In my experience 1/2 HP pumps whether Goulds, StaRite, F&W, RedJacket or what ever do not meet the needs of a family home

13. ### HubeNew Member

Joined:
Sep 1, 2004
Location:
Ontario
I agree with "pumpman"; size the tank according to the gpm capability of the pump."
For example; a 8 gpm pump should have no less than a tank that will hold at least 8 gallons. Also this will insure that the pump's running time will be at least 1 minute.
The confusion in buying a bladder type tank is the tank size may be listed as an 20 gallon but it will actually ony hold 5 or 6 gallons.
Some tanks at Home Depot are 60 gallons but they only will hold 12 gallons. In most cases the tank's size is listed in overall area but the true fact is that the actual "draw" of water is only a quarter of that area. A tank sized at 30 gallons will give off approx 8 gallons.

These sizes marked on the tanks can be very confusing to the diyer.

14. ### Gary SlusserThat's all folks!

Joined:
Aug 31, 2004
Occupation:
Retired
Location:
Wherever I park the motorhome.
Here's a tutorial on pressure tank sizing. Although I don't agree with the of time for the various hp. I was always told 60 seconds off for up to 1 hp and 120 for up to 2 hp.

http://www.aquascience.net/tanksizing.htm

Gary
Quality Water Associates

15. ### PAtrick CampbellNew Member

Joined:
Jan 20, 2015
Location:
Vermont
I'm building a new home and the well driller has suggested a 1/2HP 7gpm pump with a FL-12 pressure tank which has a 10 gallon 40/60 draw down.

If I put a 119 gallon pressure tank with 35 gallon draw down at 40/60 - will this be OK? Will it save pump life and will it help to mitigate issues like when we have guests over, taking multiple showers, laundry and dishes?

Last edited: May 14, 2015
16. ### craigpumpWell-Known Member

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Apr 12, 2012
Occupation:
Self employed water system tech
Location:
ct
Well......depends on the integrity of the guy selling you the equipment. If he wants to sell more pumps and tanks, he sells smaller tanks mated to larger volume pumps. If he's honest, he sells properly sized systems that allow for at least a minute run time. I like to see pumps run for a few minutes, but economics kind of dictate what gets installed.

17. ### PAtrick CampbellNew Member

Joined:
Jan 20, 2015
Location:
Vermont
From what I understand this is sized correctly. It's a 1/2hp 8gpm tank, and the draw down is 10 gallons, so I think that would run for over a minute? However, is there any harm in having a 30 gallon drawdown on that same pump?

18. ### valvemanModeratorStaff Member

Joined:
Mar 15, 2006
Occupation:
Pump Controls Technician
Location:
Lubbock, Texas
No matter how large a tank you use, you only have 7 GPM because it is only a 7 GPM pump. A larger tank will just act like an additional load for the pump. So while you are taking two 3 GPM showers, the tank will also be filling or draining and will make the pressure seem low as it will stay close to 40 PSI a lot of the time.

If you want good pressure use a CSV set at 55 PSI with the FL7 tank. Then the CSV will hold a strong 55 PSI for as long as the shower is running. The FL7 is actually much larger than you need when using a CSV. You still won't be able to use three 3 GPM showers at the same time, but the pressure will be better until you use enough water to run out of pump.

19. ### SHendersonNew Member

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Sunday
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I have a 44 gal. well/tech WX250 pressure tank that has failed. I used the equation on the Aquascience's Web site to calculate what tank size I need. The result was to get a 111 volume tank. That's quite a difference!

I have only lived in this house for four years, so don't know how long this tank has been here or why this size was installed.

I am scratching my head about the big difference between the suggested size and the existing size. What information do I need to pick the best replacement?

20. ### Reach4Well-Known Member

Joined:
Sep 25, 2013
Location:
IL
You have a 25 to 30 GPM from your pump? That is a big pump or a shallow well.

Rule of thumb is that the pump should take at least a minute to pump the "drawdown" amount.

21. ### valvemanModeratorStaff Member

Joined:
Mar 15, 2006
Occupation:
Pump Controls Technician
Location:
Lubbock, Texas
Larger tanks just slow down the cycling, they don't stop it. A Cycle Stop Valve will stop the cycling, will work with a very small tank, and delivers stronger pressure to the house.