How Do I Figure Out Which Pump I Need?

We will assume that you are not an expert and are just interested in selecting a pump to either replace a damaged pump you have now, or to install a new pump set-up in a lake or river.

To select a replacement pump from our inventory, your first concerns will be what Gallons Per Minute (GPM) and what horsepower was the original pump.  If you are unsure about your pump's gallons per minute, you can ball park GPM if you remember how many rotor heads you had on your largest circuit. 5 or 6 rotor heads would usually translate into an 18 gpm pump with an average (20-40 ft) water table (level of underground water), and 7-9 heads would usually mean a 25 GPM pump with the same 20-40 ft water table.

When you get to the pump lists, just go to the pump column that provides the GPM you need. If you are unsure of the water table, your neighbor may know or you can call whomever issues well permits in your area. The well report submitted with every well lists the water table and the agency will have no problem providing that information. Without any knowledge of the water table (how far the pump is lifting the water), you are really just guessing when you specify a pump. To have us assist you, just give us a call at 352-573-7531... weekends and evenings are okay too.

As a rule, you want your pump to run at about 50-65 lbs when operating your sprinklers. After friction loss through anti-siphon, valves, piping and fittings, you'll lose around 15 lbs to friction loss, so your rotor heads will be getting around 35-50 lbs psi at the base of the heads. While 25 lbs is minimum required for many rotors, they break up and give optimum performance with less instance of rotation failure if you run them at higher pressure. Higher pressure operation also builds in a safety margin beneficial during falling water tables caused by drought.

Once you determine your lift (measured from underground water level in well or pond vertically to a point level with your highest point of water usage) and a Gallons Per Minute need, the rest is easy. You'll find we stock numerous pumps that will put out the GPM and pressure you will need. We recommend selecting a pump that will run at 60 lbs psi while giving you the gallons per minute you need from your water table depth. A planned operating pressure of 60 lbs psi will provide excellent pressure for the sprinkler heads or the old water pic up in the second floor bathroom (if you're using the water in the house), and it will leave a safety margin in case water levels fall in future years.

When a well is drilled, the well driller decides what horsepower submersible and output pump is appropriate for your use requirements and for the limitations that he knows exist with the water table and recharge rate of your well. He isn't out to save himself money, he's out to create a well that will work and not create call-backs.

For instance, if the driller installed a one-half or three-quarter horsepower pump (for which he paid almost the same as he would have for a one horse) in your well, he had a good reason.

If you decide to replace that half horse or three-quarter horse pump with a one horse or two horse pump, you may find out the reason you had a three-quarter horsepower pump. You may discover you have created a real mess because your new BIG pump draws water out of your well faster than the well recharges (water entering the bottom of the well casing pipe through hydraulic pressure). 

Pumping faster than recharge is called overpumping and will cause the pump to suck down the water level in the well pipe and then run dry (ahhh, not a good thing), eventually burning the pump out.

Bigger is definitely NOT always better when replacing a pump. A bigger pump installed during the rainy season may work fine for months... until the dry season comes along and effects (reduces) the well recharge rate.

Bottom line is to be safe, you should replace a well pump with one of equal horsepower unless your are very very sure (it's a gamble) the water table and recharge rate of your well will support accelerated withdrawal.

Two wire or three wire pump?

A three wire pump has a control box, usually with a capacitor or two in it. The wire going from the control box down the well or out to the pump is four strand, because a 3 wire pump requires 2 hot wires, one start capacitor wire and one ground wire. Advantage is that if the start capacitor gets knocked out, you can replace the capacitor or control box easily and relatively inexpensively without pulling the pump up out of the well, and be back in business. We usually recommend going with a three wire pump on wells if possible, because of this factor when replacing a pump.  However, installing a lake pump is another matter. We usually install a two wire (internal starting capacitor) pump to avoid the hazards and intrusion of control box placement on shore or dock. A two wire pump requires no control box.

So, two wire pumps do fit better in some situations. For instance, we sell a lot of two wire pumps intended to pump out of lakes, ponds, canals or rivers. Installing a 2 wire pump in that scenario eliminates trying to find a water-safe and visitor-safe spot to mount the control box and allows the use of standard 12-2 UF insulated wire all the way from breaker or relay to pump on most installations and eliminates the problem of trying to securely run four strand wire out to the pump.

A two wire pump has only three wires going down the well or down into the pond to the pump, because a 2 wire pump needs only 2 hot wires and a ground. Advantage?  Easy to install, less electrical exposure, simpler wiring, less installation planning and time needed.  Disadvantage?  If a capacitor does get knocked out, it's pump-pulling time.

In an existing well, because a two wire pump set-up has only three wires going down to the pump, it isn't possible to properly change out an existing two wire pump set-up to a three wire pump unless you upgrade the three strand wire going down the well to four strand wire. Many times when we upgrade from a 2 wire pump to a 3 wire (plus ground) set-up, we will locate the control box right at the wellhead in order to allow us to use the existing 3 strand wire to power the control box, then go down the hole with 4 strand well wire from the control box to the pump.

When planning to pump out of a lake or river with intention to use a sub pump because of lift or pressure requirement factors (over 20 foot lift), you will usually do just fine if you choose a one  horse submersible pump and plan your system for around 20 gpm or less (6 rotor heads) per zone, 10-15 spray heads per spray zones, assuming a lift (from water surface to highest head) of between 20 and 40 feet. If you have less than a 20 foot lift, think about using an above ground jet pump. Of course the numbers here will change dramatically if higher lifts are involved and are also dependent upon pressure line size and length. You can contact us at 352-573-7531 to discuss your particular lake pump situation. We will ask you what your GPM and pressure needs are, the size of and length of your pressure line, and what your total lift is from surface of water source to highest point of water usage.

Selecting a Sub Pump can get a little confusing, because the amount of water and pressure is determined by not just horsepower, but also by the number of stages or impellers in the pump. The size and length of the pressure line also presents a large factor in sizing your pump. And of course for existing systems, the water volume and pressure demand is another factor. We are available to assist you at 352-573-7531

It is very important that your sub pump be sized correctly for your application, and a sub pump should not be an off-the-shelf item purchased from your local discount store. They know about as much about sizing pumps and impeller stages, as I do about purchasing products from China.

If you are planning an irrigation system for a larger piece of property and need to put 12-15 rotor heads per zone, then you may choose a two horse 50 gpm pump and plan your system for 35-45 GPM. Yes, I realize you can put a lot of heads on a smaller pump as long as you use itty-bitty nozzles that take an hour of watering to penetrate an eighth of an inch. But why waste time installing a system if it doesn't provide adequate irrigation when needed?

When pumping out of a lake or river, we recommend using a pump start relay or direct 230 switch for your pump. Do NOT use a pressure switch and tank set-up. Lake and river water levels tend to go up and down and will drive you nuts keeping the switch adjusted to operate the system correctly.

  • 1. Find out the depth in feet to your underground water table and add above-ground elevation to highest point of water use (figure height only).

  • 2. Find out the GPM water use you will need.

  • 3. Select a pump that will run at 60 lbs pressure while conforming to the above two factors. If you get stuck, give us a call at 352-573-7531

  • 4. Leave discussion of "head" to engineers and well drillers trying to impress customers. 

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