How to size a pump for a borehole?

Struggling to select the right pump?
An incorrect choice leads to poor performance and high energy costs.
Proper sizing ensures efficiency and reliability for your water system.

To size a borehole pump, you must calculate the Total Dynamic Head (TDH) and the required flow rate. TDH is the sum of the static water level, drawdown, and friction losses. The flow rate is determined by your household or irrigation needs. Match these two values to a pump's performance curve.

Choosing the right pump might seem complex.
Many factors influence the final decision.
However, breaking down the process into clear steps makes it manageable.
This guide will walk you through each calculation.
We will ensure you have the data needed to select a pump that perfectly matches your borehole's and your property's requirements.
Let's begin with the first critical measurement.

How do you determine the required flow rate?

Need a consistent water supply but unsure how much you need?
Estimating wrongly can leave you with a weak shower.
We will help you accurately calculate your peak water demand.

To determine the required flow rate, sum the demand of all fixtures that could run simultaneously. For example, a home might need 25-35 gallons per minute (GPM). Always plan for peak usage to ensure adequate water pressure and volume for all applications.

Calculating your required flow rate is the foundation of pump sizing.
This value, often measured in gallons per minute (GPM) or cubic meters per hour (m³/h), represents the volume of water you need the pump to deliver.
An undersized pump will result in disappointing water pressure.
An oversized pump will waste electricity and may cause excessive wear on your system.
A precise calculation ensures optimal performance and efficiency.

Step 1: Auditing Your Water Fixtures

First, list every water outlet in your home or property.
This includes showers, faucets, toilets, washing machines, and outdoor taps.
Each fixture has a typical flow rate.

These are average values.
It's always better to check the specifications for your specific fixtures if possible.

Step 2: Calculating Peak Demand

You rarely use all fixtures at once.
The goal is to calculate the peak demand.
This is the maximum flow required when multiple high-use fixtures are running simultaneously.
A common method is the Fixture Unit Count.
However, a simpler approach for residential use is to identify a realistic "high-use" scenario.
For instance, two showers running while a toilet is flushed and the kitchen sink is on.

• Shower 1: 2.5 GPM
• Shower 2: 2.5 GPM
• Kitchen Faucet: 2.2 GPM
  In this scenario, your peak demand is 7.2 GPM (2.5 + 2.5 + 2.2).
  For a larger household, you might add a washing machine cycle, bringing the total closer to 11-12 GPM.
  Industry practice suggests an average family home requires a pump capable of delivering 25-35 GPM to ensure no pressure drops during peak use.
  This higher figure accounts for future needs and ensures a robust system.

Step 3: Considering Irrigation and Other Demands

Do you have a lawn or garden to water?
Irrigation systems can be major water consumers.
Sprinkler systems can require anywhere from 5 to 20+ GPM, depending on the number and type of sprinkler heads.
If you plan to run irrigation at the same time as household use, you must add this to your peak demand calculation.
Often, it's more practical to schedule irrigation for off-peak hours, like overnight.
If so, your pump must be sized for the larger of the two demands: household peak demand or irrigation demand.

How do you calculate the Total Dynamic Head (TDH)?

Confused by terms like "static head" and "friction loss"?
Ignoring these factors will lead to a pump that can't lift water to where you need it.
Let's simplify the formula for calculating total head.

Calculate Total Dynamic Head (TDH) by adding the static water level, drawdown, vertical lift from the wellhead, and all friction losses. This total represents the entire workload the pump must overcome to deliver water. An accurate TDH is critical for selecting an efficient pump.

Total Dynamic Head, or TDH, is the most critical calculation for sizing your borehole pump.
It quantifies the total pressure the pump must generate.
This pressure is needed to lift the water from its source deep in the ground and push it through the pipes to your final destination, a pressure tank or faucet.
TDH is measured in feet, meters, or pounds per square inch (PSI).
Failing to calculate it correctly is the most common reason for pump system failure.
It's a multi-step process, but each step is straightforward.

Component 1: Static Water Level

The static water level is the distance from the ground level down to the water in your borehole before the pump starts running.
This is the resting level of your water table.
You can measure this with a water level meter or a simple weighted line.
For example, if the ground is at 0 feet and the water is 100 feet down, your static water level is 100 feet.
This is your starting point for the total lift required.

Component 2: Pumping Water Level (Drawdown)

When the pump operates, the water level inside the borehole will drop.
This drop is called drawdown.
The new, lower water level is the pumping water level.
Drawdown occurs because the pump is removing water faster than the aquifer can replenish it in the immediate vicinity of the well.

• Pumping Water Level = Static Water Level + Drawdown
  A well driller's report will often provide an estimated drawdown for a given flow rate (e.g., 20 feet of drawdown at 30 GPM).
  If this isn't available, a safe practice is to add 20-30 feet to your static water level as an estimate for drawdown.
  Let's assume a 20-foot drawdown.
  Your total lift from within the well is now 100 feet (static level) + 20 feet (drawdown) = 120 feet.
  It's crucial to set the pump well below this pumping water level, typically by at least 15-20 feet, to ensure it remains submerged.

Component 3: Elevation and Pressure Requirements

Next, measure the vertical distance from the ground level at the wellhead to the highest point the water will be delivered.
This could be a second-story shower or a storage tank on a hill.
If this point is 40 feet above the wellhead, you add 40 feet to your head calculation.
You also need pressure at the destination.
Household water systems typically operate between 40-60 PSI.
To convert PSI to feet of head, you use a simple conversion: 1 PSI = 2.31 feet of head.
So, to achieve 50 PSI at the pressure tank, you need:
50 PSI * 2.31 = 115.5 feet of additional head.

Component 4: Friction Loss

As water travels through pipes, it rubs against the inner walls.
This creates friction, which the pump must overcome.
Friction loss depends on three main factors:

• Pipe Diameter: Smaller pipes cause significantly more friction.
• Pipe Length: Longer pipes result in more total friction.
• Flow Rate: Higher flow rates increase friction exponentially.

You must calculate friction loss for the total length of pipe from the pump to the pressure tank.
Friction loss charts are used for this.

Example Friction Loss (for 100 feet of Schedule 40 PVC Pipe)

If your total pipe length is 300 feet and you need 20 GPM through a 1.5" pipe, your friction loss would be 7.1 feet (per 100ft) * 3 = 21.3 feet.
You also add head loss for any fittings like elbows and valves, which typically add another 10-15% to the pipe friction loss.

Final TDH Calculation

Now, let's sum all the components.

• Static Water Level: 100 feet
• Drawdown: 20 feet
• Elevation to House: 40 feet
• Pressure Requirement (50 PSI): 115.5 feet
• Friction Loss: 21.3 feet
  Total Dynamic Head (TDH) = 100 + 20 + 40 + 115.5 + 21.3 = 296.8 feet.
  Your pump must be able to deliver your required flow rate (e.g., 30 GPM) at approximately 297 feet of head.

How do you read a pump performance curve?

Staring at a pump curve full of lines and numbers?
It can be intimidating, but it's the key to matching a pump to your needs.
Let's decode these charts together.

To read a pump performance curve, find your required Total Dynamic Head (TDH) on the vertical (Y) axis. Follow that line horizontally until it intersects the pump's curve. Then, drop vertically to the horizontal (X) axis to find the flow rate the pump will deliver.

The pump performance curve is a graph provided by the manufacturer.
It is the single most important tool for selecting the right pump.
This chart visually represents a pump's capabilities.
It shows the relationship between the flow rate (GPM or m³/h) and the pressure it can generate (TDH in feet or meters).
Understanding this chart ensures you choose a pump that operates efficiently and reliably for your specific application.
It prevents the costly mistake of buying an underperforming or oversized model.

Understanding the Axes

A pump curve has two primary axes.

• The Vertical Axis (Y-axis): This represents the Total Dynamic Head (TDH), measured in feet or meters. It shows how high the pump can push the water. The higher up on the axis, the greater the pressure the pump produces.
• The Horizontal Axis (X-axis): This represents the Flow Rate, measured in Gallons Per Minute (GPM) or cubic meters per hour (m³/h). It shows how much water the pump can move. The further to the right, the higher the volume of water.

The Main Performance Curve

The main, downward-sloping line is the head-flow curve.
This line shows the inverse relationship between head and flow.

• At maximum head (the point where the curve hits the Y-axis), the flow rate is zero. This is called the "shut-off head." The pump is working its hardest but moving no water.
• At maximum flow (the point where the curve hits the X-axis), the head is zero. The pump is moving the most water possible with no resistance.
  Your job is to find a pump where your calculated system requirements fall on a favorable point on this curve.

Finding Your Operating Point

Let's use the values from our previous example.

• Required Flow Rate: 30 GPM
• Calculated TDH: 297 feet
  You start by finding 297 feet on the Y-axis.
  Then, you move horizontally across the chart.
  Next, find 30 GPM on the X-axis and move vertically up.
  The point where these two lines intersect is your "design point" or "operating point."
  You must select a pump whose performance curve passes directly through, or very close to, this point.

The Best Efficiency Point (BEP)

Most pump curves also include efficiency curves, often shown as arcs or looped lines with percentages (e.g., 60%, 65%, 70%).
The goal is to have your operating point as close as possible to the center of these loops, known as the Best Efficiency Point (BEP).
Operating a pump at its BEP has significant advantages:

• Lower Energy Consumption: The pump converts the most electrical energy into water movement, saving you an average of 10-15% on electricity bills.
• Longer Pump Life: A pump operating at its BEP experiences the lowest levels of vibration and radial thrust on its shaft and bearings. This dramatically reduces wear and tear, extending the pump's lifespan by years.
• Quieter Operation: A balanced, efficient pump runs more smoothly and quietly.

Choosing a pump where your operating point is far from the BEP leads to wasted energy and can cause premature failure.
Some manufacturers also show the required power (in HP or kW) and Net Positive Suction Head required (NPSHr), which is critical for preventing cavitation in surface pumps but less so for submerged borehole pumps.

Why choose between a fixed-speed and a variable-speed pump?

Thinking a simple on/off pump is enough?
This can cause pressure fluctuations and high energy use.
A modern variable-speed drive (VSD) pump offers superior control and efficiency.

A fixed-speed pump runs at 100% capacity or is off, which can cause pressure swings. A variable-speed drive (VSD) pump adjusts its speed to maintain constant pressure, matching real-time demand. This results in superior comfort, system protection, and significant energy savings.

The final major decision in sizing your pump is choosing the type of motor control.
This choice impacts your system's daily performance, energy consumption, and long-term operating costs.
The two main options are traditional fixed-speed pumps and modern variable-speed drive (VSD) pumps.
While fixed-speed pumps have been the standard for decades, VSD technology offers compelling advantages that are becoming the new industry benchmark for performance and efficiency.

How a Fixed-Speed Pump Works

A fixed-speed pump operates in a simple on/off cycle.
It is controlled by a pressure switch connected to a pressure tank.

1. When you open a tap, water is drawn from the pressure tank, and the pressure in the system drops.
2. Once the pressure hits a pre-set low point (the "cut-in" pressure, e.g., 40 PSI), the switch turns the pump on.
3. The pump runs at full speed, refilling the tank and supplying your tap.
4. Once the pressure reaches the pre-set high point (the "cut-off" pressure, e.g., 60 PSI), the switch turns the pump off.
   This cycle repeats constantly.
   The main drawbacks are noticeable pressure fluctuations (from 60 PSI down to 40 PSI), high inrush current every time the motor starts, and wasted energy, as the pump always runs at 100% power regardless of whether you are using one small tap or five large ones.

The Advantages of a Variable-Speed Drive (VSD) Pump

A variable-speed drive (VSD) pump, also known as a variable frequency drive (VFD) pump, is a smarter solution.
It uses an intelligent controller to adjust the motor's speed in real-time.

1. You set a desired constant pressure (e.g., 55 PSI).
2. A pressure sensor monitors the system pressure continuously.
3. When you open a tap, the pressure begins to drop slightly. The VSD controller immediately detects this and speeds up the pump just enough to meet the new demand and maintain the target pressure.
4. If you open another tap, the pump speeds up more. If you close a tap, it slows down.
   The pump only works as hard as it needs to.

Key Benefits of VSD Technology

Choosing a pump with an integrated VSD provides a premium user experience and protects your investment.

• Constant Water Pressure: This is the most noticeable benefit. VSD pumps eliminate pressure swings, providing a consistent and comfortable 'city-like' water supply. Showers no longer fluctuate in temperature or pressure when a toilet is flushed.
• Energy Savings: According to the U.S. Department of Energy, VSDs can reduce a pump's energy consumption by 30% to 50% or more. The pump rarely runs at full speed, leading to substantial savings on electricity bills, especially in high-use applications.
• Soft Start/Stop: VSDs gently ramp the pump motor up to speed instead of slamming it on at full power. This "soft start" eliminates the high electrical inrush current and mechanical shock (water hammer) that stresses the motor, pipes, and check valves. This feature alone can double the lifespan of the motor and related components.
• System Protection: Many VSD controllers have built-in protections against dry-running, over-voltage, under-voltage, and overheating. This advanced monitoring safeguards your pump from the most common causes of failure, providing peace of mind.

While the initial investment for a VSD pump may be 15-25% higher than a fixed-speed model, the energy savings and extended equipment lifespan typically result in a lower total cost of ownership, with a return on investment often seen within 2-3 years.

Conclusion

Sizing a borehole pump is a process of matching your system's needs with a pump's capabilities.
Calculate flow rate and TDH, then use a pump curve to find an efficient model.

FAQs

What is the best size pump for a well?
The best size depends on your specific needs. Calculate your required flow rate (GPM) and Total Dynamic Head (TDH) to find a pump that matches those specifications.

How many GPM does a house well pump need?
An average 3-4 bedroom house typically requires a pump capable of delivering 25-35 GPM to ensure adequate pressure during peak usage periods without performance drops.

How far can a 1 hp well pump push water?
A 1 HP pump's capability is defined by its performance curve, not just horsepower. It might push water 150 feet at 20 GPM or 250 feet at 10 GPM.

Is a bigger well pump better?
No, a bigger pump is not always better. An oversized pump will run inefficiently, waste electricity, and can cause damage to your system through rapid cycling.

How deep should a well pump be in the water?
A well pump should be set at least 15-20 feet below the calculated pumping water level (static level + drawdown) to ensure it remains submerged and avoids damage from running dry.

What happens if a well pump is too small?
If a well pump is too small, it will fail to provide adequate water pressure and flow. This results in weak showers and poor performance from appliances.

Can you have too much water pressure in a house?
Yes, excessive water pressure (above 80 PSI) can damage plumbing fixtures, pipes, and appliances. It can also lead to leaks and wasted water.

How long should a well pump last?
A quality submersible well pump should last between 8 to 15 years. Proper sizing and the use of a VSD controller can help extend this lifespan significantly.