How to select the well pump size

Discussion in 'Pumps and Tanks Well Forum & Blog' started by BrianK, Apr 23, 2013.

  1. BrianK

    BrianK New Member

    Messages:
    26
    Location:
    Calgary, Alberta, Canada
    Hi All
    Does anyone have good rules of thumb (or real guidelines) for selecting the proper size well pump, pressure tank and how deep to put the pump? Also - comments on VFD versus a simple pressure switch with tank would be appreciated and any other suggestions or recommendations.

    Our new 2 story house with a basement (all 10 foot ceilings) has 19 fixtures with 3 1/2 baths. The well/utility room is in the basement. The well is 300 feet deep and water trickles over the top of the well normally and will be discharged outside the house. It has a 6" casing down about 15 feet with the rest a 4" well pipe. Well capacity is 4.5 GPM.

    Fixture count:
    Kitchen sink = 1
    Dishwasher = 1
    Fridge = 1
    Bathroom Master - upstairs - 2 sinks,1 toilet,1 shower,1 bath = 5
    Bathroom - Guest - upstairs - 1 sink, 1 toilet,1 shower,1 bath = 4
    Half Bath - Main Floor - 1 sink, 1 toilet = 2
    Basement bathroom - 1 sink,1 Toilet,1 Shower = 3
    2 outside fawcetts = 2
    Total 19 fixtures.

    I're read that you should have 1 gpm for each fawcett but 19 GPM seems excessive as they obviously won't all be used at the same time.
    Thanks for the guidelines.
    Brian
  2. valveman

    valveman Moderator Staff Member

    Messages:
    4,457
    Location:
    Lubbock, Texas
    OK, you brought up the VFD thing, so I am sure everybody is popping popcorn, waiting for the show to start. :) The reason is that Cycle Stop Valves or CSV’s were designed to replace Variable Frequency Drives or VFD’s. The CSV and VFD both achieve the same goal.

    The VFD is a computerized, expensive, problematic device that varies the speed of the pump/motor to reduce the amount of water from the pump, to match the amount of water being used. Many pump companies are pushing these devices because they are expensive, short-lived, hard on the pump/motor, and therefore very profitable. Which of course profit for them means extra cost to you.

    A standard pressure tank system requires a large pressure tank and still causes the pump to cycle on and off a lot. Cycling causes pressure fluctuations in the house and shortens the life of the pump, motor, control box, pressure switch, bladder in the tank, check valve, and anything else in the system.

    The CSV is a simple valve that varies the output of the pump to match the amount of water being used. Very few pump companies will even mention a CSV, as it is inexpensive, and makes pumps last several times longer than normal. The CSV holds a constant pressure in the house and works with a very small pressure tank.

    My rule of thumb is 5 GPM per bathroom. So your 19 GPM estimate is pretty close. Another benefit of the CSV is that it will allow you to install as large a pump as you think you may need, and the CSV will make it work all the way down to 1 GPM when needed.

    Even though your well only produces 4.5 GPM, the 300’ of water in the well is like having a 450 gallon tank that will let you draw as much GPM as you want. I would probably use about a 18 or 20 GPM, 2 HP pump, set at about 250’, controlled by a CSV1A Cycle Stop Valve, and a 20 gallon size pressure tank. The only reason I would use a tank that large is because I would set the CSV to run at 70 PSI with a 60/80 pressure switch because of the three story height, instead of the normal CSV at 50 with a 40/60 switch. 40/60 could use the 4.5 gallon size tank, but higher pressures like 60/80 needs a larger tank.

    See this link and click on the 1 to 6 different uses in the house to see how it works.
    http://www.cyclestopvalves.com/simple/home.php
  3. BrianK

    BrianK New Member

    Messages:
    26
    Location:
    Calgary, Alberta, Canada
    Thanks Valveman for the great info. I wondered about the VFD - high upfront cost and expensive to repair WHEN it fails and electronics will only last so long. I'll look into the CSV - I haven't heard of those before. My cabin has the old pressure switch and tank combo and has been working for 35 years - no problems. I should probably clean the contacts though LOL!.
    Brian
  4. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    3,965
    Location:
    NW Ontario, Canada
    I'm not sure how well I can read the pump curves, but a well that goes from 0 (overflowing) to 250 feet down would have a difference in head of over 100 PSI. What would the holdback pressure be against the CSV when the well is overflowing and what does being that far off the curve do to the pump? While it is tempting to use all that casing for storage capacity, would it not be pushing outside the design characteristics?
  5. valveman

    valveman Moderator Staff Member

    Messages:
    4,457
    Location:
    Lubbock, Texas
    Didn’t think I would have to show the math, but these are good questions so I will explain my thinking.

    With a flowing well the 18 GPM, 2 HP pump will have a back pressure on the CSV and drop pipe of 190 PSI. With the CSV set at 70, this would be a max of 120 PSI differential. I say set the pump at 250’, because that pump can still produce 10 GPM at 70 PSI from a depth of 240’. That is not utilizing all the water in the 300’ well, but it will still be able to access 360 gallons of stored water before the well yield drops to 4.5 GPM. With a recharge rate of 4.5 GPM, that would allow the use of 15 GPM for as long as 35 minutes at a time, or 10 GPM for as long as 65 minutes at a time.

    The 190 PSI backpressure doesn’t hurt anything. That just makes the pump think it is pumping 70 PSI from a depth of 250’, which is what that pump likes and is designed to do. Actually with a flowing well it is upthrust caused by too little backpressure that causes the impellers to rise too high and grind off the tops. The 70 PSI at the house is not enough to keep this pump from upthrusting if this were a pressure tank only type system. But adding 15 PSI friction loss from a CSV1A will prevent upthrust problems. So as far as the pump curve goes, the friction loss of the CSV is keeping the pump from going too far to the right of the curve, and the 1 GPM minimum of the CSV is keeping the pump from going too far to the left of the curve.

    I backed off to a 2 HP because I thought it more than sufficient. If I were pushing the limits of the well, I would have suggested a 3 HP, set at 300’. That would require using two CSV1A valves in series to handle the 260 PSI back pressure and 190 PSI differential. Because it is a 3 HP motor, it would require a minimum cooling flow of 2 GPM instead of 1 GPM. Now you need a 44 gallon size tank instead of the 20 gallon size tank. All of this would have only allowed the access of the last 50’ or 75 gallons of water stored in the well, and I didn’t think that would be necessary.
  6. LLigetfa

    LLigetfa DIYer, not in the trades

    Messages:
    3,965
    Location:
    NW Ontario, Canada
    I probably should not have combined two opposing questions in one sentence as it could be misconstrued.

    "What would the holdback pressure be against the CSV when the well is overflowing" was meant to discern the differential across the CSV and getting away with using one versus two.

    "what does being that far off the curve do to the pump" was the concern about upthrust, and the data I was missing was the friction loss in the CSV.

    Still, I think it may be shaved a little close and wonder if the curves of one pump may be different than the one you based this design on.

    Thanks for the lesson.
  7. valveman

    valveman Moderator Staff Member

    Messages:
    4,457
    Location:
    Lubbock, Texas
    I looked at two different brands of pumps. One was a 20 GPM and the other was an 18 GPM, both 2 HP, both curves look basically the same. With a minimum of 85 PSI pressure on the pump there will be no upthrust. With the 1 GPM minimum built into the CSV, there is no deadheading or overheating on the left side of the curve. I don’t understand what you mean by “shaved a little close”?

    Of course you could just use a ½ HP, 10 GPM pump set at about 100’. This would give 10 GPM at 60 PSI, but only utilize about 75 gallons of water stored in the well. Might be enough, might not.

    With the 2 HP pump I was just trying to maximize what you could get from this well, while still not going all the way to the edge of the wells production.
  8. BrianK

    BrianK New Member

    Messages:
    26
    Location:
    Calgary, Alberta, Canada
    I've been doing a little investigating and appreciate your comments on my assumptions below:

    1) 1 foot of head of water equates to 0.4335 PSI so the pump at 250 feet requires 108 PSI to the top of the well. Then adding say 25 feet of head to get to the shower on the 3rd floor adds another 10.8 PSI (~ 11PSI) plus the 70 PSI back pressure of the water system itself requires 108 + 11 + 70 = 189 PSI this your value of 190 PSI. So a rule of thumb is 1 foot of water - .5 PSI?

    2) I assume you suggest 70 PSI as a CSV pressure setpoint with a 60/80 pressure switch rather than the lower differential systems to provide the 25 feet of head required because it is a 3 story house with the basement and we want a decent pressure at the shower nozzle?

    3) #1 above assumes that we will be able to use all the water in the well down to the pump. If we only want to use 100 feet of well capacity, we could get away with a smaller HP pump. Say first thing in the morning, the well will have recovered fully so the pump will only have to over come the (70+11=) 81 PSI (to the 3rd floor shower) since the static suction head with the well being full will provide the 108 psi. As the demand draws down the well water, the required head will increase from 81 PSI to whatever is required by the drawdown of the well water height to a max of 190 as the water level approaches the pump level.

    4) I looked at the CSV placement in a number of examples on your web site and am confused with its placement. Were should it go? On the pump side of the pressure tank/check valve or on the house side (downstream side) of the pressure tank after the check valve and T. The main picture with the smiling faces show the CSV right on the T to the pressure tank but other examples show it upstream of the pressure tank. It would seem to me that mounting it downstream after the pressure tank is the right location but one diagram suggests to put it in the well - ie upstream of the pressure tank and its pressure switch. Please clarify.

    5) I don't understand the bleeding of 1 gpm through the CSV - where does it go? Is there a bleed line out of the CSV which should go to a sump or does it leak a bit and that is the 1 GPM? I have seem some references to a drain pan or a sump being nearby would be a good idea. In a finished basement , a 1 GPM 'leak' might not probably be desireable. I understand the requirement to provide some cooling flow for the pump do it doesn't deadhead and heat up the water/motor/pump. I assume the CSV is basically a downstream pressure control valve which tries to maintain 70 PSI or a constant discharge pressure (so we have enough pressure at the 3rd floor shower) to the house. So lets assume the system pressure to the house is 60.1 PSI (60 start/80 stop), the well is full and we start the shower which takes say 10 GPM. The pressure tank will provide all flow through the CSV (assuming it is on the downstream side of the pressure system) until the pressure drops below 60 PSI which turns the pump on. With the shower taking 10 GPM, the pump runs up its curve providing lots of pressure (say 190PSI I don't have a curve) and the CSV provides 70 PSI at 10 GPM plus what ever flow goes into the pressure tank which the pump supplies. As the pressure tank approaches 80 psi, the flow reduces. Then at 80.1 PSI the pump shuts off. So really what does the CSV do. I normally see that the air pressure in the bladder supplying the motive force to supply water when the pump is off.

    6) When buying a pump, can/should I specify the GPM AND head requirements or are they basically sold in HP increments and you get what you get from a pressure perspective which is dependent on the flow - as flow goes down/head goes up but follows the pump curve. Are there different impellors (thus different curves) available?

    Thanks for your help.
    Brian
  9. VAWellDriller

    VAWellDriller Member

    Messages:
    171
    Location:
    Richmond, VA
    I install the system Valveman mentioned all the time, in high end homes with rain showers and irrigation...Goulds 18GS20, 20 gallon tank and CSV....it's a good system and will do more than enough for your home....The reality is, about 90% of the single family homes around here have 10gpm series water pumps that deliver anywhere from 6-12 gpm in service...Actually I know one driller that installs a Meyers 3/4hp 8 gpm pump in every well, and they do just fine...If you aren't planning on irrigating, and you have standard flow fixtures,nothing elaborate, that's really all you need. You can save a lot of money in the pumping system, by installing a 1 hp 10 gpm pump at 200'. I agree with the 70psi csv setup and 60/80 switch. A couple questions above...the CSV goes between the well pump and the pressure switch...before any point of use. This should clear up the rest of your questions about what it does. With the CSV, the pump output is adjusted to match demand over 1 gpm, so once the pump is running, the pump is creating and mainting pressure in the system...not the tank. The 1 gpm minimum bleed as you mentioned is the rate that the pressure tank would fill once you stop using water...no water is wasted or leaves the system. You do need to specify HP and head when shopping for a pump....there are more combinations than you can imagine. There are devices available to stop the artesian flow and you should install one.
  10. BrianK

    BrianK New Member

    Messages:
    26
    Location:
    Calgary, Alberta, Canada
    Thanks VA. I appreciate the time you took reading my questions and providing your suggestions/answers. I guess it amounts to how much do I want to spend on the system. The more I spend (on a pump), the more capacity I will have. It seems that smaller HP pumps will work but just not as much capacity to pump the well down. Probably won't irrigate much and I am aware that this is a well and that all water usage will end up in the septic tank - a whole other concern.....and just because the pump will allow me to use lots of water, it doesn't mean I should (from the septic tank perspective).
  11. VAWellDriller

    VAWellDriller Member

    Messages:
    171
    Location:
    Richmond, VA
    You'd be surprised how little water you probably use....The 10 gpm pump will be more than enough....if the septic is designed right, you shouldn't need to worry.
  12. valveman

    valveman Moderator Staff Member

    Messages:
    4,457
    Location:
    Lubbock, Texas
    Of course I agree with VA. A lot of people would love to have 10 GPM to work with, and that will probably be all you need. The trick is to be able to supply peak demand. Anytime you have a house full of people, three showers going at the same time, washing machine running, toilets flushing, maybe a sprinkler or two in the yard, the total flow you need adds up. A 10 GPM pump probably won’t supply enough for a peak demand like I described. But 95% of the time, one shower at a time, maybe the washing machine, occasional toilet flush, 10 GPM will be plenty. If you have company stagger showers and other water use, you can get by with much less flow.

    Even though the pump will produce 20 GPM, you probably won’t be using that much for more than a few minutes at a time. So it won’t fill your septic tank any faster to have everybody showering at the same time, as compared to everybody waiting a few minutes between showers. You will still have used the same number of showers.

    Running irrigation, you shouldn’t use more than 4.5 GPM at a time, as that is all your well will produce on a long-term basis. Even then it matters WHERE the water is entering the well. If the water enters the well from up high like at 50’, then you don’t need a pump that can produce from very deep. But if that water enters the well down below, you may still need the higher HP pump just to produce 4.5 GPM, because the pumping level could be at 250’.

    The 25’ extra elevation is already figured in the 70 PSI CSV setting and the 60/80 pressure switch. You don’t need to add it in again. Just to clarify, the CSV will hold 70 PSI steady for as long as water is running, and won’t let the pressure build up to 80 and shut off the pump until you stop using water all together. Nothing drips on the floor or bypasses back down the well. The CSV just can’t close to less than 1 GPM. So when you are no longer using any water, the 1 GPM still going through the CSV has no place left to go except the tank. At that point the pressure increases from 70 to 80 and the pump shuts off. And as VA said, the CSV goes between the pump and pressure tank/switch, and before any waterlines tee off.
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