# Drawdown Calculation....

Discussion in 'Pumps and Tanks Well Forum & Blog. Water is life.' started by watrax, Sep 1, 2009.

1. ### watraxNew Member

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Jun 20, 2009
Location:
Philippines
i just learned that calculating a drawdown for pressure tanks is the poduct of pump delivery rate in GPM/LPM and pump running time? is there an ideal or minimum pump delivery rate for residential uses and what is also the ideal pump running time? im clueless..pls help. thanks

2. ### valvemanCary AustinStaff Member

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Pump Controls Technician
Location:
Lubbock, Texas
Size your pump for the delivery rate that you need. Size the tank to give you at least 1 minute of run time, and 2 minutes is better. Here is a draw down calculator that will help you understand.

http://www.cyclestopvalves.com/runtime_app.php

4. ### watraxNew Member

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Jun 20, 2009
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ok. is the pump delivery rate in GPM mentioned the same as the demand in GPM at the house? let say the demand at the house requires 8GPM. should i also size my pump at 8GPM?

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

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Eight gpm is not sufficient for more than a 1 bathroom house. I.E. a shower = 2.5 gpm, a toliet 1/2-1.4 gpm. a washing machine 3-4 gppm, a sink 2-2.4 gpm, a tub... large tubs with separate handles can flow over 9-10 gpm alone.

Two person showers and 2-6 body sprays 20+ gpm.

Pumps come in two parts, the wet end rated in gpm and the motor rated in hp. Once you know what peak demand gpm you need, then with a pump flow chart/pump curve chart, you find the horses needed to do the job.

6. ### upperDIY Senior Member

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May 30, 2009
Location:
Fresno, CA
First thing to know Weltrax above others,is what is your well capable of how much is avaliable.Your well does not care how many bathrooms you have,in fact it is better it does not know Upper

7. ### watraxNew Member

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Jun 20, 2009
Location:
Philippines
thanks for the link. DRAWDOWN (in Gals.) is the available water volums in the pressure tank regardless of its source (from well, cistern or storage tank). these drawdown will be divided by the drawdown factor to get the size of the pressure tank (in Gals). apparently drawdown depends on the selected pump delivery rate and the desired pump running time. i want know the basis of the drawdown you want to have for the pressure tank. like for instance for Storage Tanks, its size willl depend on the demand at the house. (1.) GPD per capita or (2.) Fixtures multipled by their corresponding fixture demands in gpm/gpd. THANKS

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

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You size a pressure tank so the pump is off between starts for a minimum of 60 seconds (for proper cooling of the motor) for up to and including 1.5 hp motors. Above 1.5 hp 120 seconds.

So, run the water until the pump comes on, shut off the water and at the same time the pump comes on (switch closes) time how long it takes 'til the pump shuts off. If less than a minute, the tank is too small for the pump.

The amount of the draw down gallons is dictated by the pressure range the pump is operated at; 30/50, 40/60 etc.. The lower the pressure, the higher number of gallons between pump off and on; the higher the pressure the fewer gallons. Of course that varies based on the size of the tank.

Anyone selling tanks can tell you the gallons per various pressure ranges.

This has nothing to do with your peak demand gpm in the house. That is a sum total of all the various fixtures using water in gpm at any given time but especially when the most water is being used.

Another way to go and to be able to use a very small tank (as small as 2+ gals), is to use a CSV (Cycle Stop Valve). It allows the pump to run continuously while you are using water, which gives you constant pressure and greatly reduces the number of start/stop cycling of the pump that kills motors and, you don't need a large tank taking up a large space and their high prices.

9. ### nhmasterMaster Plumber

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Household fixtures are measured in SFU's (supply fixture units) the general figures can be found in any plumbing code book and most likely online also I'm sure. remember that the figures generally listed are for standard fixtures and do not include things like roman tub fillers and such. Also loads for sprinklers must be figured directly. Also remember that all of the fixtures in a home will almost never be open at the same time so you need to take that into consideration also. There is no need to figure the load as though every faucet in the house will be running at the same time.

10. ### watraxNew Member

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Jun 20, 2009
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so you're saying Gary Slusser that if i choose 20/40 pressures it will give me a bigger tank and if i choose 40/60 its will give me smaller tank

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

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No I'm not. I am saying that the tank, regardless of its size, will have more or less draw down gallons depending on the pressure range the pump is operated at. I.E. a nominal 20 gal bladder type tank with 20/40 psi pressure switch settings delivers roughly 6+ gals from pump off at 40 psi down to pump on at 20 psi. Same tank, switch settings at 30/50, less than 5 gals. Same tank, 40/60 less than 4 gals.

It doesn't matter how many gallons, it is the length of time the pump is off between starts.

The fewer gallons means the quicker the pump starts. You get fewer gallons because higher pressure empties the tank faster (the water moves faster; the velocity is higher) than at lower pressures.

And you need 60 seconds off for proper motor cooling between starts.

Pump starts kill pump motors so the fewer starts the better. All well water pumps are rated continuous duty (running constantly). And an inexpensive CSV extends pump motor life better than any other choice; especially if the choice is buying a larger tank. Large tanks are very expensive, take up a lot of space and are heavy.

A CSV takes up no space because they are usually installed in the water line just before the pressure tank.

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Gary, This doesn't seem to be correct to me. I don't think that the rate that a bladder tank empties has anything to do with how much the bladder tank will hold.

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

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Yes you're right but I meant fewer gallons in the house before the pump comes on. We are talking about how to size a tank which is based on how long the pump is off while the tank is delivering the water between pump starts.

At higher switch settings you also are supposed to raise the captive air pre charge pressure, so there will be less space in the tank for water (fewer gallons) and then the higher pressure pushes the reduced volume of water out of the tank faster than a lower pressure range does which shortens the length of time for cooling of the pump motor.

14. ### nhmasterMaster Plumber

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Bob, take a look at this chart. It will explain things a little better for you.

http://www.goulds.com/pdf/BCPTP.pdf

15. ### magdiel1975New Member

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Nov 28, 2015
Location:
Florida
Hi..
I know this is a very old thread, but I was reading and this is a little confusing to me... so, higher water pressure gives fewer gallons?
I don't understand that...to me, higher pressure means more volume of water coming out... meaning more gallons... I would think I would be able to fill a gallon of water faster using 40/60 than 30/50, am I wrong?

16. ### Reach4Well-Known Member

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Sep 25, 2013
Location:
IL
The bulk of the discussion above got into the pump filling the pressure tank rather than how quickly the pressure tank delivering water to the user. As a user, the higher pressure will fill your glass quicker for a given opening of a faucet that does not have a flow regulator. It would also fill a bath tub quicker at higher pressure. As a user, the pump action is normally not your concern. I don't hear my submersible pump, and I don't notice a difference in flow when the pressure tank is at max vs min pressure. I am set to 37/57.

Many newer faucets and shower heads have a flow regulator that keeps the flow rate limited so that higher pressure does not make much difference.

17. ### magdiel1975New Member

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Nov 28, 2015
Location:
Florida
I have a 50 gal tank set a 40/60 and it takes 45 seconds from when the pump starts to stop. A user said if it's less than a minute then my tank is too small for my pump...so, does that mean I have to get an 80 gal tank then?

18. ### Reach4Well-Known Member

Joined:
Sep 25, 2013
Location:
IL
45 seconds is not terrible. You might check and maybe adjust your air precharge with zero water pressure (typically set to 38 PSI with your 40 cut on).
If you do decide to increase pressure tank capacity, you could add a new tank in addition to the old if that works better for you.

If you replace your pump at some point, you should consider a smaller pump, unless you are a very big water user.

19. ### magdiel1975New Member

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Nov 28, 2015
Location:
Florida
when I first moved in to this house, it had a 30 gal tank set a 30/50.. so I bought this new 50 gal and increased the pressure to 40/60 and 38 at tank. - I have noticed something very strange.. whenever the pump stops, it doesn't matter if it stops at 50 psi or 60 psi, it always drops 5 psi quickly from the cut off... I thought there was a leak, but once it drops the 5 psi, it will not continue to drop anymore at all, even after more than 1 hour.. so i know there is no leak.

I have replaced the pressure swtich, the gauge and the tank is new. This has been driving me crazy as I have never seen this before. Any idea why it would do that? - is it a bad sign?

Last edited: Nov 29, 2015
20. ### valvemanCary AustinStaff Member

Joined:
Mar 15, 2006
Occupation:
Pump Controls Technician
Location:
Lubbock, Texas
The old 30 gallon tank held about 8 gallons of water. The new 50 gallon size tank only holds about 15 gallons. That is just the way pressure tanks work. When working properly, about 75% of the tank is air and 25% water at best. So if you have the right air charge and the tank is good, you probably have about a 20 GPM pump.

When the pump fills a bladder tank that fast, many times the pressure switch and gauge will see 60 PSI and shut off the pump before the bladder has fully inflated. After the pump shuts off, the bladder finishes expanding and you see a slight drop on the pressure gauge.

An 80 gallon tank only holds about 25 gallons of water, which would make your pump run a little over a minute. But a minute of run time is the bare minimum for the pumps survival. 2 minutes of run time is better, but would take 2 of those 80 gallon tanks to do that. Running continuously while using water is the best thing for the pump. A Cycle Stop Valve would keep the pump running continuously as long as you are using water, then it would only allow the tank to refill at 1 GPM. So a 4.5 gallon size tank would really be all you need with a CSV. The 30 gallon or 50 gallon tank would also work fine with a CSV, and would be much more tank than is really needed.

So you could purchase an 80 gallon size pressure tank or two and slow the cycling down, or you can eliminate the cycling with a CSV1A and use whatever size tank you have. When the CSV is filling the tank at 1 GPM, the bladder has time to expand as needed and you won't see the pressure drop after the pump shuts off.

21. ### magdiel1975New Member

Joined:
Nov 28, 2015
Location:
Florida
That makes sense about the bladder expanding and the loss of 5psi..thanks for that explanation. Now, if the main thing about a pump is to not have it not run so often..how is a CSV which makes the pump run more frequently better?

Also.. with my 50 gal, should I keep it at 40/60 or bring it down to 30/50...which would be better for the pump?