Continuing our series of posts on Rainwater Harvesting 101, today we will look at pump selection. If you plan to use gravity to force water from your tank to your garden or lawn, this probably isn't the post for you. But, if you need to pump water at a higher flow rate and pressure than gravity will allow, let's get started!
Above are a few examples of pumps and we'll go into the specifics a little bit more below on what to consider when selecting a pump.
There are a few important things to consider when selecting a pump:
- Flow rate required
- Pressure or head required
- Installation Location/Pump Style
- Electrical Requirements
Flow rate comes into play as a pretty basic requirement for selecting a pump because an end application will require a certain amount of flow.
Here are a few examples of flow rate requirements
- Garden Hose: 2-4 gallons per minute
- Toilets: 1.6 gallons per flush
- Washing Machine: 2 gallons per minute
Make sure to account for all of your potential combined uses and choose a pump that can accommodate the maximum total flow rate.
Pressure or Total Dynamic Head
Pressure or total dynamic head required are arguably just as important as flow rate. If you have a below-ground installation and need to pump to the 7th floor of a building, you need enough Total Dynamic Head (TDH) to push the water to overcome that height difference, otherwise the water will never reach that portion of the building. Likewise, if you need your pump to provide enough pressure to mimic that of a normal residential system, you'll need one that is able to produce 45-60 PSI.
So, now that you know your flow rate and your pressure/TDH required, you'll need to make sure your selected pump can accommodate both. Let's take a look at the example pump curve below.
Say for example, you need 25 Gallons per minute at 100' TDH. The 1/2 HP and 3/4 HP options do not quite get there, but the 1 HP pump (1S51E-H) does and would be the pump you need to select.
Installation Location/Pump Style
This is typically a very project specific factor. The installation location typically refers to whether or not the pump is installed inside the storage tank (submersible) or outside the tank (booster/jet). Each style has it's own benefits. For example, a submersible pump is often quieter as it is submerged in the water tank, it is out of sight/not in the floor space, allows for constant pump access to water (as long as float switch is used for dry run protection). However, on the negative side, it must be disconnected and removed in freezing conditions, is more difficult to maintain, and typically requires stainless steel components for long-term submersion, which could affect cost. A booster/jet pump is easy to maintain, can be disconnected easily during freezing conditions, and can sometimes be more cost effective with lower cost materials since it is not submerged. However, they can sometimes be noisy, they take up floor space, and can lose prime if they are not in flooded suction state.
SUBMERSIBLE PUMP EXAMPLE
BOOSTER/JET PUMP EXAMPLE
This is probably the most straight forward of the bunch: what electrical characteristics do you have available? Some flow rates and pressures are only able to be accomplished with higher voltage and three phase power. For most residential applications, 115V/1 phase/60 Hz or 230V/1/60Hz power will suffice as these pumps are providing flow and pressure at rates similar to a city or well line.