Well Pump Sizing

[Tall Pines]

Would like to get a second opinion for well pump sizing for a new well. I got two estimates; one called for a 1.5HP 5GPM pump and the other called for 1.5HP 7GPM pump. Both specified 1” drop pipe. My concern is that both are not enough GPMs for peak demand.


The well is 650’ deep, static level 176’, produces 3GPM and pump will be set at 620’. The house is about 120’ from the well head and about the same height as the well head.


The house is 5 full bathrooms (although 1 bathroom is a guest bath and will only have occasional use) and we are a family of 5 with two adults and 3 young kids that take regular baths. There will be no irrigation or sprinkler usage. Shower heads will be normal GPM rates but there will be times when 2-3 showers will be running at the same time.


5GPM seems way undersized for my application. I’m thinking a 10GPM or 16GPM pump with a 3HP motor, 1.25” drop pipe and a CSV is a better fit for my situation. I could go with a 2HP pump but both need 6-3 wire so I figured a 3HP would be better in the long run.


Am I overthinking this?

Thanks for the help.

 

[Valveman]

With 3 showers at at times you will need 8-9 GPM minimum. The 5 GPM pump can't do that. A 7 GPM, 1.5HP can only deliver 8 GPM at 50 PSI from a depth of about 350'. A 10 GPM, 2HP Goulds can deliver 8 GPM at 50 PSI from about 480' deep. I really don't see any reason to set the 7 GPM, 1.5HP or the 10 GPM, 2HP Goulds any deeper than about 520'. They won't deliver pressure if the water level gets deeper than 520' anyway. #8 wire is good for a 2HP up to 620' total, and the CSV1A will work with either the 7 GPM, 1.5HP or the 10 GPM, 2HP Goulds.

The 16 GPM, 3HP will only let you utilize the water down to 580' with pressure, and then only at 4 GPM. The 10 GPM, 2HP Grundfos won't really pump any more water than the 10 GPM, 2HP Goulds. But both the 10GPM, 2HP and the 16 GPM, 3HP Grundfos builds more pressure than the CSV1A can handle.

A lot of this is dependent on how fast your well will recover. If the well will recover pretty fast, I think I would go with the 10 GPM, 2HP Goulds on 1 1/4" steel pipe, a CSV1A, #8 wire, and about a 20 gallon size pressure tank.

If the well is a weak producer or slow to recover, you might consider a cistern system with a booster pump. You could set a 5 GPM, 1HP at 600' on 1" steel pipe, #10 wire, and have plenty of water to fill the cistern. Then a little 3/4 or 1HP jet pump or submersible in the cistern with a PK1A kit would give you more water than you could use with every faucet open.

 

[Sponsor]

 

[Reach4]

At 40/60, a 7 GPM 1.5 HP pump will deliver over 9 GPM until the water drops some amount. If you started to run out of water, the pressure would drop. That's not all bad IMO.

The well diameter determines how much storage you have per foot.

 

[Tall Pines]

Thanks for the replies.

Reach4s chart lit the light bulb for me. The entire time I had been thinking the pump had to be sized based on the depth of the pump instead of pumping water level PLUS pipe friction loss. Assuming the pumping water level is somewhat consistent, then the pump would be sized based on the 176' pumping water level PLUS 68 feet of head (600' 1" sch80, 1" pitless adapter, 125' 1" Pex) from pipe friction loss which means I would use 244 TDH when looking at pump curves.

Did I get that calculation correct?

Reach4 - Does this comment "That's not all bad IMO." mean that if the pumping water level gets low then the PSI would start dropping which would indicate to the users (myself, wife, kids) that something is wrong BEFORE the well and pump ran dry? If that's the case then I could definitely see how that is a good thing.

I'm now starting to understand why a 1.5HP/7GPM/1" Sch 80 drop pipe setup would work in my situation. Under 'normal' conditions with a 176' pumping water level, I could easily get 9-10GPM.

My gut tells me that I still want a bigger pump. Other than upsizing drop pipe to 1.25" sch 120, what are the downsides of going with a 1.5HP/10GPM setup?

 

[Boycedrilling]

One problem with your thinking. Your initial post states that your well yields 3 gallons per minute. If that is correct, anytime you are pumping more than the 3 gpm, you are drawing the water level in the well down by the amount of additional pumping above the 3 gpm.

I'm assuming that you have a 6" diameter well. A 6" well holds 1 1/2 gallons per foot of depth. If you are pumping at 10 gallons per minute and the well is yielding 3 gallons per minute, you are "mining" the well by 7 gallons per minute. That means that the water in the well is being drawn down an extra four and a half feet for every minute the pump runs. This is IN ADDITION to whatever amount the well draws down to produce the 3 gallon per minute yield. You will run out of water in about 1 1/2 hours of continuous pumping at 10 gpm.

 

[Reach4]

Assuming the pumping water level is somewhat consistent, then the pump would be sized based on the 176' pumping water level PLUS 68 feet of head (600' 1" sch80, 1" pitless adapter, 125' 1" Pex) from pipe friction loss which means I would use 244 TDH when looking at pump curves.

Did I get that calculation correct?

I did not check the friction numbers. 68 ft equiv is more than I would have guessed. In addition, add 40 PSI * 2.307 (ft/PSI) = 92.28 ft to your calculations. The table takes that into account with its separate rows for pressure. With the graph, you have to add in the pressure head yourself. The pressure tank should have enough drawdown to let the pump run at least 1 minute each time. So if you will pump 10 GPM, a diaphragm pressure tank would need a drawdown of at least 10 gallons. To get that would take about a 40 gallon tank, which means you would normally use a 44 gallon tank. Bigger is better. For pumps it is not the case that bigger is better.

If using a CSV, add some more to compensate for some drop. With the CSV, you can use a much smaller tank, because that slows the flow considerably as the pressure gets closer to cutoff. So that could use as little as a 1 gallon drawdown tank.

Reach4 - Does this comment "That's not all bad IMO." mean that if the pumping water level gets low then the PSI would start dropping which would indicate to the users (myself, wife, kids) that something is wrong BEFORE the well and pump ran dry?

That's what I was thinking. If the flow dropped to 2 GPM before water dropped to the pump intake level, you would probably never run dry. That would be the extreme.

If you decide that you are in danger of running out of water, there are devices to detect the current drop when the pump runs out of water, and will shut down the pump for an amount of time to let the well recover. If you went to a higher volume and power pump, that would be more important to have. Cycle Sensor is one such device. https://terrylove.com/forums/index.php?threads/pump-saver-plus-questions.63486/#post-470929

 

[Valveman]

Like Boyce says, if you are using 10 GPM and there is 3 GPM coming into the well, you are "mining" (good word) 7 GPM out of the well, which holds about 1.5 gallons per foot. And you will be able to sustain the 10 GPM flow for 90 minutes at a time. I doubt that you will use 10 GPM for 90 minutes at any one time. If you make a guess as to how long you might every use 10 GPM at one time, you can set the pump more shallow and use a smaller pump.

Lets say that you might use 10 GPM for 30 minutes max. That would only draw the water down by about 140'. So if you start with the static level at 176', and you draw it down another 140', the total depth to water would only be 315'. Then you could set the pump at about 350', which would allow a smaller pump and smaller wire. I would still use 1 1/4" pipe, but at 350', it could be SCH 80 or SCH 120 PVC instead of steel.

A 10 GPM, 1.5 HP set at 350' could use #10 wire. This size pump would work fine with the standard PK1A kit. At most you might upsize to the 10 gallon size tank instead of the 4.5 gallon size. This system would supply 10 GPM at 50 PSI for 30 minutes, and would supply lower flow rates for a lot longer than that.

 

[VAWellDriller]

I like everything valveman just said. I've never been a big fan of putting a pump at the bottom of the well just because you could, and wanted every available drop of storage the well offered. In his scenario of 10 gpm for 30 min, you would have more than a days worth of available water stored at the 350' mark (not to mention what the well produced during that 30 min), and not waste time, money, and effort to get to the bottom when you really don't need to....

 

[Reach4]

Here is a table for a 10 GPM 1.5 HP pump.

 

[Tall Pines]

Great stuff, thanks for all the replies. Never mentioned it before but the well is a 4" well which will store .66 gallons per foot. I believe this is the final version of plan, let me know what you think.

10 gpm Berkeley pump (L10P4GMGS) on Franklin 1.5 HP 3 wire 1 phase motor with a Franklin controller. Pump will be set at 440' with 1.25" sch 80 pvc pipe. I could set it at 350' but wanted access to additional storage. Found some inexpensive 8-3 submersible wire online so I'll use that to wire the pump. This will be a temporary setup (while the house is getting build) and connected to a yard hydrant and generator. The permanent setup will add a CSV, tank and cycle sensor.

On a related (kinda) note, the excavator is almost finished excavating the foundation hole for a walkout basement. The control box, tank, CSV and cycle sensor will eventually go into the mechanical room that's in the basement. Do you guys normally have the pipe go through the foundation wall or under the footer and up through the slab? I've seen it done both ways out here...

I'm learning a lot from everyone and am very thankful for this forum!

 

[Reach4]

Do you guys normally have the pipe go through the foundation wall or under the footer and up through the slab? I've seen it done both ways out here...

I would think that would depend on how deep the frost line is, and how deep the floor is. I would think wall would be most common.

On a related (kinda) note, the excavator is almost finished excavating the foundation hole for a walkout basement.

Nice.

Run your networking and camera lines while things are open, or run conduit. Take pictures of the framing. Screw some extra 4x4 wood into the framing by the tub or shower in case you want to put in a solid grab bar some day.

Maybe have a shower or floor sink near the outside basement entrance.

The Franklin AIM manual says you need a 5 KW generator if the generator is externally regulated, and 3 KW if internally regulated.It says

There are two types of generators available: externally and internally regulated. Most
are externally regulated. They use an external voltage regulator that senses the output
voltage. As the voltage dips at motor start-up, the regulator increases the output
voltage of the generator.
Internally regulated (self-excited) generators have an extra winding in the generator
stator. The extra winding senses the output current to automatically adjust the
output voltage.
Generators must be sized to deliver at least 65% of the rated voltage during starting to
ensure adequate starting torque. Besides sizing, generator frequency is important as the
motor speed varies with the frequency (Hz). Due to pump affinity laws, a pump running
at 1 to 2 Hz below motor nameplate frequency design will not meet its performance
curve. Conversely, a pump running at 1 to 2 Hz above may trip overloads.​

 

[Tall Pines]

Given my setup, do you think I need check valves in the drop pipe? Is the the Berkeley pump (L10P4GMGS) internal check valve something I can rely upon as the only check valve to protect the pump?

 

[Tall Pines]

I would think that would depend on how deep the frost line is, and how deep the floor is. I would think wall would be most common.



Nice.

Run your networking and camera lines while things are open, or run conduit. Take pictures of the framing. Screw some extra 4x4 wood into the framing by the tub or shower in case you want to put in a solid grab bar some day.

Maybe have a shower or floor sink near the outside basement entrance.

The Franklin AIM manual says you need a 5 KW generator if the generator is externally regulated, and 3 KW if internally regulated.It says

There are two types of generators available: externally and internally regulated. Most
are externally regulated. They use an external voltage regulator that senses the output
voltage. As the voltage dips at motor start-up, the regulator increases the output
voltage of the generator.
Internally regulated (self-excited) generators have an extra winding in the generator
stator. The extra winding senses the output current to automatically adjust the
output voltage.
Generators must be sized to deliver at least 65% of the rated voltage during starting to
ensure adequate starting torque. Besides sizing, generator frequency is important as the
motor speed varies with the frequency (Hz). Due to pump affinity laws, a pump running
at 1 to 2 Hz below motor nameplate frequency design will not meet its performance
curve. Conversely, a pump running at 1 to 2 Hz above may trip overloads.​

Frost depth is 3' but everyone out here buries water lines at 6' or 8' if it's under a road or driveway. I'll just go through the bottom part of the full height foundation wall.

Thanks for the detail about the generator. I've got a 5KW military generator (MEP-802a) and have heard of lots of people running well pumps from these gensets.

 

[Valveman]

Given my setup, do you think I need check valves in the drop pipe? Is the the Berkeley pump (L10P4GMGS) internal check valve something I can rely upon as the only check valve to protect the pump?

The check valve in the pump should be fine as long a you don't cycle the pump on/off too much, which is what wears out check valves. And the CSV will keep that from happening. If you want an extra check just in case, put it a few inches above the pump and built in check valve.