Rainwater Catchment System Pump Sizing

by Doug Pushard

Pumps are an integral part of almost all rainwater catchment systems; however, sizing a pump correctly is not straightforward and installers often fail to make the appropriate calculations. Much has been written on pumps for irrigation systems and for wells, but rainwater harvesting pumps can be markedly different. This series of articles is aimed at shedding light on the differences and assisting in properly sizing rainwater pumps. This first article will explain pumps and general pumping concepts . The second article will discuss efficiency and pump curves and the last one will explain the math involved in properly sizing a pump and provide examples of sizing different systems.

Pumping Overview:

Though gravity flow can be used for some rainwater harvesting systems, most systems require a pump and pumps are often one of the most expensive components of a rainwater catchment system, excluding the tank. Unfortunately, most installers and designers just install a standard ¾ HP or 1 HP pump because they “feel” it will do the job. In some cases it will; however, in a majority of cases it will either be oversized or undersized. This is like installing a high performance car engine in a go-kart; it is possible to do but it will use far more fuel than a properly-sized engine and will not last as long due to the engine not operating at peak efficiency. Likewise, installing the wrong size pump wastes energy, decreases the life of the pump, and increases maintenance costs.

Why care if the pump is incorrectly sized? Money is the simple answer. A high quality submersible ½ HP costs nearly $500, a 1HP about $600 and a 1 1/2HP $800. All should have a life expectancy of between 10-15 years. If an installer uses a 1 HP pump in a situation where a ½ HP would have worked, he is initially costing his client $100 more than necessary. In addition, the over-sized pump will probably need to be replaced every 3-4 years rather than every 10 years. Therefore, a small $100 error can become a $1,000 error over the normal life expectancy of the pump and this does not take into account the increased electrical use due to improper sizing or labor costs for replacing the pumps. With the $200 price difference between a 1HP and a 1 ½ HP pump, this same error would add up to more than $2,000 over a decade. It just makes good economic sense to spend the time to make sure the pump is sized properly for a given system.

Sometimes not only is the wrong size pump installed but the wrong type as well. Rainwater catchment pumps are a little different from a standard irrigation or well pump. Irrigation pumps are designed to push the water out to sprinklers or drip heads and typically require less gallons per minute (GPM) and pressure per square inch (PSI) than most rainwater systems. Well pumps are designed to pull or push water up from far greater depths when compared to the requirements for a rainwater system. Depending upon the system, a rainwater pump may need to both pull water out of a cistern (e.g., pull water up from a buried cistern) and create the pressure necessary for its intended use (e.g., faucet, hose, sprinkler, pressure tank, etc.). Therefore, in a lot of cases rainwater harvesting pumps need to be sized to both pull water up and push water out. Properly sizing a rainwater pump requires detailed knowledge of where the water is being stored, where the pump is located and the intended use of the water. Every installation is just a little different; consequently, a one-size-fits-all design methodology simply doesn’t work.

Lastly, to make things even more complicated, neither industry nor the government has mandated a standard way to depict pump performance. Some pump manufacturers will include a fair amount of performance information, while others do not provide enough data to determine if a given pump is the optimal pump. Additionally, sometimes the information is published on the outside of the box, while other manufacturers bury it in a manual inside the sealed pump carton.

With all the above, it is no wonder pump sizing is a little like black magic and seldom done. But pump sizing is critical to the implementation of an efficient and successful rainwater system. Wasting electricity operating a system by installing a pump that is the wrong size is just not “green.” Make sure your designer or installer provides you with the pump manual and the sizing calculations as part of your installation documentation. If you are building a system yourself, become familiar with the math involved (as covered in the other articles in this series) and then get assistance from a trusted source on selecting the correct pump for your specific needs.

Pump Sizes:

Pumps are sized in units of horsepower (HP), sometimes referred to as brake horsepower; however, pumps also are rated in flow, i.e., gallons per minute (GPM) and for a given pressure, i.e., either Head or Pressure per Square Inch (PSI). For example, a typical pump might have a table like the following, which demonstrates an inverse relationship between PSI and GPM:

It is these two specifications that really assist in determining whether a specific pump is sufficient and efficient for any given application. Two pumps can have the exact same horsepower rating and produce very different flows and pressures.

Flow is the amount of water that can be delivered in a given unit of time (e.g., 12 GPM) at a given pressure (e.g., 30 PSI). For example, in the table above, a pump with a PSI of 25 with deliver a GPM of 14 gallons per minute. When thinking about flow, imagine a meandering river near you and how much water flows by as you watch it for a minute. This flow will increase or decrease based on the contour of the slope and the intensity of the inflows upstream. Pressure is the intensity of the flow.

Fixtures and sprinklers require a minimum amount of pressure to function and in some cases will not function properly over a given pressure level (e.g., drip irrigation heads start popping off at over 30 PSI). Beside horsepower and GPM, all pumps are also measured in pressure per square inch (or pressure per square millimeter, if not a US pump). In the river example above, the water pressure in the river changes based on the contour of the land. The water pressure increases with steep contours, and decreases with flat contours.

As a general rule, with centrifugal pumps (the typical pump used in rainwater systems) the primary relationship to understand is that as the flow INCREASES, the pressure DECREASES and visa versa.

This critical aspect greatly affects the efficiency and effectiveness of a given pump. To assist with understanding this concept, think about the water pipes in the walls of your home. With no faucet or water flowing (i.e., no flow), the pressure in those pipes is constant, generally between 30-40 PSI; however, when a faucet is turned and water flows out of the tap (i.e., flow increases), the pressure drops in the line (i.e., pressure decreased and flow increased). Turn the faucet off and pressure returns to the line (i.e., increases) while flow has stopped (i.e., decreased).

When calculating pump size, both pressure and flow in static state (faucet closed) and dynamic state (faucet opened) need to be included in the calculation.

Pressure is sometimes referred to as Head. Every pump is capable of developing a specific pressure (measured in PSI, which translates into feet or meters of Head) at a specific flow. In the US, pressure for pumps is typically measured in Feet of Head. One Foot of Head equals 0.433 pounds per square inch (PSI). Because pumps are measured in either Head or PSI, it is important to be able to convert between the two. These formulas are as follows:

Feet of Head = PSI * 2.31
PSI = Feet of Head / 2.31, or Feet of Head * 0.433

For example, to convert 55 PSI to Feet of Head, multiply 55 by 2.31, which equals 127 Feet of Head. To convert 127 Feet of Head to PSI, multiply 127 * 0.433, which equals 55 PSI.

In simple systems, the installer should first determine the number of faucets, outlets and/or sprinkler heads in the system, then calculate the required PSI and GPM, then match these specifications to the appropriate pump. In more complex systems, the installer should take into account not only the requirement to push water out to the faucets and sprinkler heads, but also to pull the water up, for example, from a buried cistern (Ignoring friction and a few other variables for the time being. These will be covered in another article.).

For example, drip irrigation systems require no more than 30 PSI. If this is the only intended use for the pump, and the cistern and pump are above ground at the same elevation, and there is no upward slope to the landscape, then the pump should be sized to the requirement of the drip irrigation system (i.e., 30 PSI). Otherwise, the pump will be oversized and will work too hard due to the constant back pressure, thus shortening the life of the pump and wasting electricity.

The pump’s Efficiency Rating is also important. This number provides the GPM and PSI at which the pump operates at optimal energy efficiency (i.e., using the least amount of electricity to deliver a given PSI and GPM). Efficiency Ratings are oftentimes published as a curve depicting the optimal operating range for that specific pump. It is critical for installers to consult this graph when properly sizing a pump for a given system. A pump that does not operate within its Efficiency Rating will not last as long or be as energy efficient as one that does.

To conclude, rainwater system pumps can be very different from irrigation or well pumps. Rainwater pumps need to be sized to both pull the water out of the cistern and push it to its intended end use (i.e., water fixtures, outside sprinklers or both). All pumps come sized in horsepower, PSI (or Head) and GPM. It is these latter two specifications that should be used to determine if a specific pump is the right pump for a given application. Most pumps also include an Efficiency Rating. To minimize life-time costs for the pump, it is critical to maximize the life of the pump and lower its electricity use. Consequently, it is critical to choose a pump that operates within its published Efficiency Rating given the demands of your rainwater harvesting system. Properly sizing a pump can be complex, but in the end it will save money and energy and provide years of carefree operation.

Related Links

Rainwater Harvesting - Pumps or Pressure Tanks
Pump Systems for Rainwater Catchment
Free Rain, Free Watering and Exercise All in One (Bicycle Pumps)
State and City Programs and Vendors

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