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WATER PUMPS, TANKS, TESTS, WELLS, REPAIRS
WATER CONSERVATION MEASURES
WATER CONTAMINANT LEVELS & LIMITS
WATER FILTERS, HOME USE
WATER HAMMER NOISE DIAGNOSE & CURE
WATER ODORS, CAUSE CURE
WATER PUMP REPAIR GUIDE
WATER PRESSURE LOSS DIAGNOSIS & REPAIR
WATER PUMP SHORT CYCLING
WATER SOFTENERS & CONDITIONERS
WATER TANK REPAIR PROCEDURES
WATER TANK: USES, TROUBLESHOOTING
WATER TESTS for CONTAMINANTS
WATER TREATMENT EQUIPMENT CHOICES
WELLS CISTERNS & SPRINGS
WELL CHLORINATION & DISINFECTION
WELL FLOW RATE
WELL WATER PRESSURE DIAGNOSIS
WELL YIELD IMPROVEMENT
WINTERIZE A BUILDING
This article explains Water Tank Size - how much water is in the water pressure tank?
We address the question of how much water volume we need to avoid short cycling the water pump?
This article series answers just about any question you may have about pumps, wells, and drinking water.
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How to Find Out How Much Water is in the Water Tank, Considering the Space Taken up by Air in a Water Pressure Tank
Even before performing water quantity, quality, equipment function tests, there is an enormous amount we can determine about a building's water supply just by looking at the equipment. Articles here provide details on water pumps, tanks, controls, and wells and water supply inspection, diagnosis, and repair.
The page top photo shows our client holding the open top of his well casing along with some unusual well casing exit plumbing at a drilled well with a modern steel casing.
Finding the location of your well and inspecting the condition of the well piping and equipment are an important first step to assure a functional and potable drinking water supply - that is, having enough water supply and having water that is safe to drink. The articles listed below provide detailed advice on diagnosing and repairing problems with water pumps, water tanks, wells, and other water supply equipment.
It's not how much water the tank holds that is useful to know. It's how much water we can get out of the water tank before the pump has to turn on. This is the "draw down" volume, which we can measure or calculate, or we can focus instead on how long (in time) we can run the water before the pump has to turn on. (See our discussion of "short cycling" water pumps at Water Tank Repairs: Diagnose "Water Pump Short Cycling" & Restore Air in a Building Water Tank. If your pump is short cycling you want to fix it, as we explain in that article.
How to calculate the volume of water in a water tank
IF we just wanted to calculate the volume of water inside of a round water tank (photo at left), for simplicity ignoring a concave or convex (domed) water tank top and maybe bottom, we can use the formula to calculate the volume of a cylinder. Just measure the height and circumference of the water tank.
Reader Craig Revill send us this photograph of a large water storage tank in an 80 to 100 year old home, along with the tank dimensions: 36" in diameter and 99" in length. You can use the following formula to calculate the number of gallons of water (or any other liquid) in a storage tank:
pi * radius2 * height (pi is 3.1416)
Gallons of water in a tank: one gallon = 231 cu. in.
Reader Roger Davis reminded us to make clear that in the formula above, the radius (which is half of the tank diameter) should be squared. That is, divide the diameter in half to obtain the radius, and multiply r x r to obtain r2.
For the 36" diameter x 99" long water tank above, we calculated the water tank volume as follows:
[3.1416 x (18")2 x 99" = 100,782 cu .in.] / 231 = 436 gallons in this water tank.
That's a big tank storing a lot of water, probably indicating a well with a very low flow rate at the home where this water tank was installed. Or someone was planning to survive a long dry period. The small control connection shown at the 11 o'clock position on the end of this water tank was an air volume control.
Domed water tank volume: If we needed to be precise and if the bottom and top of a water tank are domed at the top and convex at the bottom (usually) we can measure these areas and calculate their volume using the formula for the volume of a sphere (or part of one). But we suggest skipping this detail. Probably the spherical volume lost from the convex tank bottom is about equal to the spherical volume of the tank top, so it's a wash and we can just use the tank's overall height and diameter.
A 30 gallon water tank does not give you 30 gallons before the pump turns on
The photograph shows a one-line jet pump, the water pressure tank, and a water softener. We know from the fact that this is a single line jet well pump that the well is a shallow one, probably less than 27' deep. Well depth may have implications for water quantity and quality and vulnerability to surface water contamination.
Remember that a "30 gallon water tank" used to control water pressure and pump cycling in a building does not hold 30 gallons of water, but something less than that (say 20 gallons of water max and 10 gallons of air at the point of pump cut-off). Then as you draw water out (and the in-tank pressure falls down to the pump cut-on point) the pump is going to come on before all of the water leaves the tank.
So the maximum actual water you get out of the tank is less than the tank size, maybe 15 to 20 gallons max. The bladder-type pressure tank manufacturers cite an "equivalent" draw-down water volume as that provided by the older bladderless tanks.
How to measure the draw-down water volume provided by a water tank
What are some typical water tank draw down volumes?
A 10 gallon water pressure tank that starts fully empty and is pumped up to about 50 psi will contain about 3 gallons of air and 7 gallons of water. The water tank in normal operation does not draw down to 0 gauge pressure before the pump comes on.
The water tank provides out flowing water down to 20 psi (on a 20-40 psi system or down to 30 at a 30-50 psi system) when the pump comes on.
A water pressure tank with a total volume of 10 gallons and operating between 20 psi and 50 psi of pressure will have a draw down water volume of just 4.35 gallons of water.
A typical kitchen water faucet runs between 3 gpm and 5 gpm (varying as the water pressure in the system varies as the water pump cycles on and off), so we can expect to run the water at the tap for about a minute before the pump will come on with this theoretical water tank.
Because the pressure drops as the water tank empties and then increases as the water pump comes back on, the water pressure at a faucet or other plumbing fixture will vary between the pump cut-in pressure (typically 20 psi or 30 psi) and the pump cut out cycle (typically 40 psi to 50 psi). quoting from Water Tank Pressure, Temperature, and Air Volume Calculations.
The above-cited article, which we admit is a bit unnecessarily complex (I was answering someone else's query) has the math you need to calculate the actual draw-down volume of water you get with a given sized tank, with a given in-tank water volume when the pump has reached its shutoff point.
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Frequently Asked Questions (FAQs)
Question: ran out of water after filling up the swimming pool
(June 21, 2012) Brody said:
Brody: WOW. Filling a pool with filtered softened water - expensive, and certainly exhausted your water softener's salt charge as well as, apparently, your well. Your well will probably recover if you wait - it may take several hours or longer. Many residential wells do not have the flow rate and capacity to fill swimming pools.
If you still have no water after 3-6 hours, I'd begin to worry that your long water run may have damaged a pump or pump control.
(Oct 23, 2012) Shawn said:
Regarding the filling of the pool with well water: The most common cause of well failure is overpumping.
Did you notice sand or silt in the water?
You may have "eroded" the aquifer pulling in fines and plugging the pores of the aquifer and slots or holes in the casing or liner. Other problems can be caused as well such as precipitation of chemicals (iron. etc. - can occur fairly quickly) and increased biofouling (this is longer term though).
The rate at which the water flows into the well may be permanently decreased now.
Sometimes rehabilitation (by a well driller) may help but may not be possible.
Question: how to calculate water tank volume in gallons
how much water does a 56 IN BY 26IN BY 17 IN TANK HOLD
Anon: calculate the water tank volume in cubic inches and multiply that by 0.004329 to get the volume in gallons
Question: where to put the pressure control switch
(Mar 20, 2014) Paul said:
The tank in the house would only be connected by the actual water pipe that exits the pressure tank at the well, no pressure switch connection. So basically it would be tank #1 being filled then water going from tank #1 to tank #2 about 100 feet away, and when tank #2 is full then the pressure switch at tank #1 would cut out the well pump. Other than the initial filling of tank #2 would their be any issues to the well pump or tank #1 pressure switch?
Paul, generally we want the pressure control switch to be close to the pressure tank. That pair of components can be located wherever it is convenient. I'm unclear of why we're talking about two pressure tanks unless one of them is actually to function as a water storage device.
(Mar 21, 2014) Paul said:
The original tank at the well head would retain the pressure switch and that tank in turn would feed the 2nd tank 100 feet away at the water pipes entry into the house. I am thinking, and this is just a thought which may be incorrect, is that since another tank, in the house, is also holding water under a pressurized bladder that it would provide a bit better pressure in the house due to the current pressure tank being over 100 feet away and also "pushing" water uphill.
If you float a second pressure tank on the supply piping at a 100 ft distance from the original pressure tank (and pressure control switch) it's about the same functionally as if you had simply put a larger pressure tank at the original location. There are flow losses due to friction losses and bends and restrictions through a water supply system, but pressure sensing is consistent throughout.
That is, with the pressure switch remote, at the well, it's not going to change when the pump turns on and off to add a second pressure tank in the building.
However, a water storage tank high in a building that feeds building plumbing fixtures may give some improved performance by benefiting from gravity as it feeds the rest of the building. In communities served by municipal water mains that operate at low pressures or intermittent pressures, many buildings take this approach: a rooftop or attic-located tank is filled slowly by pressure from the street (or using a booster pump in tall buildings), then that tank feeds the building by gravity (or by gravity and a second booster pump)
Search InspectApedia for ROOFTOP WATER TANKS to see examples.
(Mar 24, 2014) Paul said:
Appreciate your insights.
Paul I may not have the correct understanding but in general, a second tank, pressurized by the first one, is never going to be able to "push" water harder into the building than the first one did unless it includes a higher location or a booster pump. You are chiefly adding water storage volume, not water pressure.
Question: could the pressure tank be defective?
(May 2, 2014) Larry said:
It's like most of the pressure building is in the plumbing and not much going into the tank. In fact you can grab the top of the tank and it moves around like it feels very light as in no water in it. Also the gauge in line says 60 pounds at shut off as it should but the air pressure in the top of the tank is 44 pounds. My well pump guy said they should be equal which makes sense to me too. By the way the pressure in the tank at pump kick on is 38 pounds as it should be.
Oh, and the manufacturer tells me, "my tire gauge must be wrong". I don't think so.
Take a look at the condensation line on your water tank and you'll see the level to which it's actually usually filled when in service - at the point of pump shut-off. If the "sweat line" is near the top of the tank it may be close to water-logged.
Question: low flow well troubles
(June 29, 2014) walterwoj said:
We have capped the pumps output at 2.0 GPM with a dole valve and now I want to know if I can adjust the pump switch from 20-40 to something like 45-50 so the pump will kick on sooner when water is drawn to help it keep up with demand better. Considering my flow problem will changing the pressure settings work without damaging the equipment?
Walter you could go to 30/50 or even 45/60
Don't set the on-off too close or short cycling an damage the pump. Youd be better off with a large pressure tank and a longer drawdown cycle.m
(Aug 12, 2014) Teferi said:
Question: insufficient water draw down before the pump turns on
23 January 2015 Eric said:
The Pressure Guage was replaced in Jan 2007. The pressure Tank was installed in July 2009. Shouldn't I be getting close to the 9.7 gallons before cut in? What should I be looking into? Air Pressure on the Tank? Bad Gauge? or Something Else? When I tap on the Pressure Tank, it sounds very hollow, so I know it isn't water logged. Is the Pressure in the Tank too low perhaps? Thanks in advance!
One additional comment. The Gauge shows the Cut-In PSI @ 43 and Cut-Out PSI @ 61.
I made an error in my calculation. The drawdown should be 26% which would be 8.7 gallons. Is there possible sediment in the Pressure Tank? I drained the system and checked the pressure on the tank and it was 35 PSI. I suppose part of the Bladder could be stuck to the side of the tank from the very begining when it was installed? Trying to figure out if there a good way to flush the tank without taking it apart? Thanks.
Well, I tried flushing the pressure tank to see if I could get any sediment out and no change in the drawdown -- about 5 gallons. My main concern is that I have a 10 GPM pump and the time from Cut-In to Cut-Out is only about 35 seconds when there is no demand. How much good would it do (or harm) if I lowered the Cut-In pressure down to 39? It current seems to kick in about 42 (according to the gauge). I wonder if my gauge is 2 PSI off?? Is there any problem in checking the PSI on the Pressure Tank with a gauge when it is at rest after cutout with no demand?? I guess my other option is to simply get new tank or add a second tank so that I have volume plus the 5 or so I have today.
Well, it appears the mystery is solved. It's simple physics. I plugged in all of the figures (Tank Air PSI, Cut In PSI, Cut Out PSI) into the formula. I first verfied that my Tank had 35 PSI using 3 different tire gauges. Then after shutting down the system a couple of times and draining down to 0 PSI, I could see that the gauge read 35 right after I powered it on, so I think my Pressure Gauge is pretty accurate.
So, I wound up adjusting the Cut-In to 38 and the Cut-Out to 60, I now get a solid 7.5 gallons during drawdown, and it takes at least a minute to get back to cut-out PSI with a single faucet running. That should hopeful reduce the cycling. I am considering adding a second tank and am looking into Cycle Stop Valve, which I just learned about. Three cheers for Physics and the great information here. I hope someone can learn from my experience. Thanks.
Nice going Eric.
Typically we want the tank pre-charge to be just 2 psi below the pressure switch cut-in setting.
Your feedback will surely help other readers.
And yes I like the cycle stop valve; there are several short-cycle protection devices: a Cycle Stop valve, a well piping tailpiece, and some pressure switches have other built-in sensors that detect various pump problems, interpret them (usually correctly) and will shut off the pump if necessary.
See our description of the Cycle Stop valve at
27 January 2015 Eric said
I will take a look at the Cycle Stop Valve information. This is really interesting stuff. With my 40/60 Switch, are there any concerns with having a greater than 20 PSI between the Cut-In and Cut-Out? Because if I use different figures in my calculations, for example, 33 PSI on the Tank, 35 PSI Cut-In, and 58 PSI Cut-Out, my "theoretical" drawdown would increase by almost another gallon. Is it just a matter of making sure the pressure in the house is sufficient?
Do you think it is better to simply add another tank? or better to use something like a Cycle Stop Valve? The only big stressor on my system would be the Sprinkler System, and the number of heads in each zone varies from 3-5.
I am thinking I may need to swap out some more efficent heads with only a 10 GPM pump (and it seems to be doing that from my estimations). Thanks again for your input. I am passing some of this knowledge already to a work colleague who just bought a house with a well!
Eric these are two different approaches to low flow rate wells:
Adding more on-site water storage- a cascade of tanks or a very large tank - assures that you will always have enough water and lets the well and pump re-fill the tank(s) at a slow rate, catching up when not much water is being drawn = say when you're asleep.
Adding a cycle stop valve or other pump protector guards against pump burn-up or control burn-up should someone leave the water running and the pump running dry. So adding more onsite storage reduces but does not completely eliminate the risk of pump or control damage.
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