Definition & explanation of well pump drawdown volume & typical drawdown cycle times:

Here we explain how to diagnose & correct water pump intermittent cycling "on-off" when no water is (known to be) running in a building. Well pump & water tank off-use cycling or water pump turning on-and-off: diagnosis & repair procedures.

SHORT CYCLE vs INTRMITTENT CYCLE: Comparison of Intermittent Well Pump Cycling with a Short-Cycling Water Pump Problem or with Lost Water Pressure. INTERMITTENT CYCLING REPAIRS: How To Fix Water Pump Intermittent Cycling

This article defines drawdown volume and time for well or water pump systems. Thanks to a suggestion from reader A.L. we include a table of typical water pressure tank or well tank sizes and we show those tank rated volumes, actual drawdown cycle water volume, and typical drawdown cycle "on" times. Shown at page top, a reader's water pressure tank and her well pump's controls.

This article series explains how to diagnose & repair water pump cycling problems like short cycling, intermittent cycling, continuous pump operation, or well pump chattering.

Water Pump On-Off Cycling Rates & Water Draw-Down Times

So is your well pump short-cycling? Intermittent cycling? Or is something else wrong? Or are there two things wrong?

Our photo shows one way to measure the actual delivery rate of water at a plumbing fixture: measure the time required to fill a 5-gallon bucket. Note that this is the flow-rate in GPM of the plumbing system including pump, pipes, controls etc.

It does not tell us the capacity of the well to deliver water.

[Click to enlarge any image]

Definition of draw-down pump cycle

The draw-down volume in a water pump system is the volume of water that can be run out of a pump and pressure tank system before the pump turns on, starting with the pressure tank fully charged and the pump having turned-off at the system CUT-OFF pressure, and ending with the water system reaching the CUT-ON pressure and thus turning the pump back on.

Typical water pressure tank draw-down volumes

A typical draw-down volume is the rated "equivalent gallons" of a water pressure tank is 20 gallons, 32 gallons, 40 gallons, or sometimes larger.

Just how much time is required to draw down the pressure tank's rated volume depends on the plumbing system's flow rate in gallons per minute but the draw down volume will always be less than the tank's specified "volume".

That difference is a feature of pressure tank design including the requirement that about half of the tank's volume might be used by its air space, and it's also a feature of the operating pressure range of the tank.

The time required for a well pump to re-charge the water pressure tank depends in turn on the pressure tank volume in gallons, the well pump's discharge rate in gallons per minute, and of course the well's flow rate, since a limited-flow-rate well may require limiting the well pump's actual discharge rate.

Keep in mind that a well pump's discharge rate in gpm will be reduced depending on the height to which the pump has to lift or push the water out of the well and into the water pressure tank.

See WATER PUMP CAPACITIES TYPES RATES GPM
and

See WELL FLOW RATE for details of measuring the capacity of a well to deliver water.

Examples: Amtrol's WellXTrol water pressure tanks are sold in tank volumes from 2.0 gallons to 119.0 gallons.

The actual draw-down volumes of these tanks when operating in a 40/60 psi pressure range will vary from 0.6 gallons (for the WX-101 2.0 gallon tank) to 28.1 gallons (for the WX-350, a 119 gallon tank). A 20-gallon WX202 has a draw-down volume of 6.2 gallons (at 20/40 psi) or 5.4 gallons (at 40/60 psi) or 4.7 gallons (at 50/70 psi).

Now if we're running water into a typical residential bath tub at 4 gpm, that means that if a 20-gallon pressure tank and a 40/60 pressure control switch is going to give us (5.4 drawdown / 4 gpm usage rate) 1.35 minutes of running water (from full at last pump shut-off) or 81 seconds before the pump has to turn on again.

Notes to the table above

1. Adapted & expanded (with drawdown time) from

• "WellXTrol, Next Generation Well Tank Quick Sizing Chart" [PDF], Amtrol Corporation, retrieved 2016/12/03, original source: http://www.amtrol.com/media/documents/wellxtrol/MC7025_04_14_WXTsizingCard.pdf
  Amtrol Technical Support (USA) Tel: 401-535-1216, Email: info@amtrol.com Website: http://www.amtrol.com/
• "Amtrol's Well-X-Trol Specifications Chart" [PDF] - Op. Cit.
• "Well-X-Trol Diaphragm Well Tanks: WX-100, 200, and 300 series" Submittal Data" [PDF] - Op. Cit.

2. Amtrol's Well-X-Trol well tanks are sold in at least 16 different sizes. The table above, adapted from Amtrol's tank sizing data, offers and expands on water tank specifications for typical residential and light-commercial sized well tanks.

3. Drawdown volume in gallons = the volume of water that can be drawn out of a fully-pressurized well tank with pump off until the time that the pump turns on.

4. Example drawdown time = time from start of running water out of a fully-pressurized well tank with pump off until the time that the pump turns on. For the purposes of this example we assume that the water flow rate out of the well tank is at 4 gallons per minute - a typical residential bathing tub spigot flow rate.

We also assume that the well pump pressure control switch is set to turn the pump ON when system pressure falls to 40 psi and that it turns the pump OFF when system pressure rises to 60 psi.

Really? Well no. Actual well tank water volume drawdown cycle times and volumes will vary considerably depending on the pump's delivery or discharge rate, well flow rate, the lift height of water from the well, piping sizes and run lengths, elbows, fittings, friction losses and other features.

Typical well pump discharge rates range from 5 to 40 gpm but may be further limited by well flow rate restrictions and restrictors and by pump lift height.

5. Typical design specifications for well systems using a 3/4 hp well pump expect the pump to be "on" for one to two minutes during water tank re-charging provided that no water is being consumed in the building served.

This well pump drawdown time table © 2016 InspectApedia.com

Pump clicks ON for 2 minutes then is OFF for 7 minutes regularly. No water is being run. Is this normal?

Our reader asked:

I'm not sure if my pressure tank is short cycling or not. I've read through much of your info. & can not find a definitive answer. Our tank is a bladder tank. It is at least 13-15 years old. I do apologize in advance, I do not know the specs. of the tank.

But I know by comparison to the tanks I've seen at Lowes, that our bladder tank is rather large, and appropriately so. ~ I've attached an image. (We just replaced the 50g water heater & had the plumbing cleaned up - the image is just for visual aid of the bladder tank size because I believe it may be relevant.)

So these are my questions:

1. The audible clicks our pressure tank makes are very clear. I've sat and listened and timed the span between the clicks (while no water is being used in the house or outside.)
   
   The tank clicks on, runs for about 2 mins, then turns off for approximately 7 minutes. Then it turns on again, and repeats at these regular intervals. I do not hear any water flow, or anything else. IS THIS NORMAL?
2. Or is my tank short cycling?
3. Is there a chart or something, somewhere that can be referred to that talks about presser tank sizes, how often said tank should click on, & when a tank of *blank* size is "short cycling" and how often *tank of blank size* should turn on & off when it is functioning NORMALLY?

I read somewhere that if a bladder tank clicks on/off more than 6x's an hour, that there is an issue, somewhere.

That more than 6x's an hour is considered "short cycling." But is this referral "6x's an hour" for ALL sized tanks? All I really need to know is, is my tank operating within normal parameters, for a tank in it's size range? Or should I be concerned? Is my tank in fact short cycling?

And lastly; WHEN a tank is operating normally, how often should it click on/off in an hour when not in active use?

Does the size of the tank matter when it comes to switching on/off so many times in an hour when it is idol?? I can not find this info, & I believe it would be helpful to me/others so we have a basis for comparison.

At least for me, if I can recognize that the clicks are getting closer together, I can get proactive before my well pump is damaged. - 2016/12/03 Anonymous (by private email)

Reply:

Our sketch shows the components of a typical drilled well with submersible pump, courtesy of Carson Dunlop Associates, a Toronto home inspection & education firm cited at References or Citations .

Am I right that your home is served by a private well system and that your well pump is a submersible unit that's in the well?

And also am I right that your pressure control switch (at the water tank) is turning on and off the pump directly?

That switch is described

at WATER PUMP PRESSURE CONTROL REPAIR

That is to say, there is no second heavy-duty relay control mounted nearby that is actually running the pump? (That second control is used on higher horsepower pumps and is described at WATER PUMP RELAY SWITCH ) If your system does use a second pump control relay you might hear two clicks when the pump turns on or off.

If so, then when the pressure drops in your water system, the pressure switch clicks closed and the well pump turns on. Standing anywhere near the pressure tank and control switch you won't hear anything except that click - that's the pressure control switch relay clicking to "close" to turn on the pump or clicking when it "opens" to turn off the pump.

Now to the question at hand. IF no water is being run out of your plumbing system, that is no faucets or fixtures running indoors, no running toilets, no water supply pipe leaks, no outdoor faucets running, no outdoor irrigation system running, THEN the pump should not be turning on and off as if some fairy godfather were mischievously flipping a switch somewhere.

SO if the pump is turning on and off by itself for no apparent reason, EITHER you're wrong and water is running somewhere in the building (or there's a leak) OR there is a leak outside of the building in the well piping.

For example, a bad check valve or failed foot valve in the well or a hole in well piping will permit water to leak backwards out of the pressure tank and back down the well piping when the pump has turned off. As that drainback leak occurs, pressure in the pressure tank falls, and eventually the pump pressure switch (not the fairy godfather) turns the pump on and you hear a CLICK!

This sort of mystery pump cycling on and off is explained further

at WATER PUMP INTERMITTENT CYCLING

An easy way to diagnose this condition is to turn off the main water shutoff valve between your pressure tank and the building. Now if the pump turns on again you can figure the leak is in the well piping.

The advice you read that a pump cycling on and off 6 times an hour as the normal maximum is malarkey.

When no water is being used the pump might finish an "on" cycle, bringing pressure in the system up to the cut-off pressure setting, but after that, as long as no water is being used the pump should never run. I cannot imagine what the heck your "6 times an hour" writer was thinking.

The rate of pump cycling on and off depends on the water usage rate, pressure or water storage tank size, type of water pressure tank, building water piping sizes, lengths, number of fixtures running and even the pump's horsepower or ability to deliver water from the well.

It's possible to run so much water so fast that the well pump simply runs continuously.

It's also possible that a slow leak such as a sneaky running toilet that's leaking so slowly you've not noticed it is wasting water and turning on the pump from time to time when no other water is being used.

Compare Normal Water Pump On-Off Cycling, Intermittent Cycling, Short Cycling & Chattering Water Pumps

1. Normal well pump operation 
   
   with a pressure tank will give a water volume draw-down cycle of 30 seconds to 1-2 minutes for a typical water pressure tank, or much longer if larger water pressure tanks or water storage tanks are installed.
2. Intermittent pump operation: 
   
   If a pump is running for no apparent reason and we're not running water in the building (or we think we're not running water there), then I call
   
   this WATER PUMP INTERMITTENT CYCLING.
   
   The time between pump-on cycles will depend on how fast water is leaving the system and might be anywhere from "pump won't stop running at all" (though other problems can cause that too) to "pump runs once in the middle of the night".
   • Fast plumbing leaks:
     
     If a running plumbing fixture or a well piping leak or some other leak is losing water fast, intermittent cycling might be happening every few minutes. In this case the time between pump-on cycles will depend on the pressure tank size.
     
     A large pressure tank might give you 30 seconds to many minutes of "draw-down" water volume before pressure drops enough to turn on the pump, but a typical pressure tank will give you about 45-seconds to 2 minutes.
   
   
   • Slow plumbing leaks:
     
     If a running plumbing fixture or well piping leak or some other leak is a slow one, intermittent cycling might happen at intervals of minutes, hours, or even longer.
3. Short Cycling pumps: 
   
   Watch out: Very short pump-on cycles followed by short pump-off cycles is caused by a water-logged pressure tank (lost air charge) combined with a rapid use of water in the building or by a significant water piping leak somewhere.
   
   See WATER PUMP SHORT CYCLING. These cycles are typically under 30 seconds., and in general a pump-on cycle for less than one minute is hard on the pump, motor, and controls.
4. Chattering pumps:
   
   Extremely-short pump on/off cycling or pump chattering is usually caused by a blockage at the pump or water system output, like a clogged debris filter. This on-off cycling rate will typically be a second or less.
   
   See CHATTERING RELAY CONTROL SWITCHES
5. Continuous pump operation:
   
   if the pump won't stop running the cause may be simply a high water usage rate that exceeds the pump or well's capacity to deliver water or there may be a leak or a bad control.
   
   See WATER PUMP WONT STOP RUNNING
   
   We note that some water pumps may be rated for continuous duty, such as irrigation pumps or pond aerators.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Is it OK for the pump to run for shorter time to increase life of pump?

I noted, lower tank head pressure increases tank water capacity and decreases pump run time. I lowered tank pressure to 11 psi and house water pressure is still OK . Pump now runs for 1.5 minutes, It runs for 2 minutes with tank pressure at 22 psi.

So is it OK for the pump to run for shorter time to increase life of pump. I was told the pump would have less wear if it runs for 2 minutes due to the water cooling the pump On 2020-12-29 by dave

by danjoefriedman (mod) - here's the proper pressure tank air charge

That's pretty interesting, Dave, thanks for the posting; I'll do some further research, and it'll help if you can give the source for your "I was told ..." remark; we might check that against the recommendations of the manufacturer of your pump brand and model.

In general we set the pressure tank air pre-charge to 2 psi below the pressure control switch CUT-IN pressure. That's the standard manufacturer recommendation found in just about any water pressure tank installation manual.

by dave - Manufacturer raises concern for over-stretching the water tank bladder; most of the pump wear is cycling on and off in less than 1 minute

I called a well pump install company and go this info . the most wear on the pump is the cycling on and off , run time of one minute is the minimum for house small pumps for 1/2 to 1 Hp . but tank pressure at 11 psi could over stretch the tank bladder. ,

called Amtrol tech - was told that may over stretch bladder in tank and no less than 5 psi below tank cut in pressure. . my tank is over 30 years old and was set at 11 psi for 10 + years till i adjusted it to 22 psi . AMTROL TANK WX 250

by danjoefriedman (mod) - The pump duty cycle designed by the manufacturer varies by pump brand and model

Thanks Dave, that's useful data.

The pump duty cycle designed by the manufacturer varies by pump brand and model; for example some pumps are rated for continuous on-duty - for hours at a time, with no damage while others might overheat and suffer damage.

In general I'd listen to the installing experts and to Amtrol about the tank pressure.

What's the definition of water pump and tank "drawdown cycle time" ?

"Normal well pump operation with a pressure tank will give a water volume draw-down cycle of 30 seconds to 1-2 minutes for a typical water pressure tank, or much longer if larger water pressure tanks or water storage tanks are installed."

What is "volume draw-down cycle"; the time it takes the pump to fill the tank at cut on up to cut off pressure? IOW the time a pump will be on (not how often)? On 2019-04-09 by Anonymous

by (mod) - how long before the pump should turn on

Anon

Thank you for asking. I should clarify as you request. By drawdown cycle I mean the amount of water that can be drawn before the pump has to turn on.

Typically that's 15 seconds to about a minute and a half - depending on a number of variables described above on this page

...

Continue reading at FOOT VALVES, WELL PIPING or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see WATER PUMP DRAWDOWN VOLUME & TIME FAQs - questions & answers posted originally at the end of this page.

Or see these

Well Pump Articles

• FOOT VALVES, WELL PIPING
• WATER PUMP CAPACITIES TYPES RATES GPM
• WATER PUMP DRAWDOWN VOLUME & TIME
• WATER PUMP WONT STOP RUNNING
• WELL FLOW RATE
• WELL FLOW TEST PROCEDURE - how much water can we get from the well?

Suggested citation for this web page

WATER PUMP DRAWDOWN VOLUME & TIME at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to WATER SUPPLY, PUMPS TANKS WELLS

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