Continuing with our series of posts on Rainwater Harvesting 101, we will look at pump selection. There are a variety of different types of water pumps that each serve a different purpose in managing your rainwater. If you plan to use gravity to force water from your tank to your garden or lawn, this post probably isn’t for you. But, if you need to pump water at a higher flow rate and pressure than gravity will allow then it sounds like you need a rainwater pump. Let’s get started!
Above are a few examples of rainwater harvesting pumps and accessories. 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
Rainwater Harvesting Pump Flow Rate
Flow rate comes into play as a pretty basic requirement for evaluating which type of water pumps you might need because an end application will require a certain amount of flow.
Here are a few examples of flow rate requirements for rainwater collection pumps:
- 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 rainwater collection 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 rainwater harvesting pump to provide enough pressure to mimic that of a normal residential system, you'll need a rainwater pump 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 you selected one of the different types of water pumps 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 pump) or outside the tank (booster/jet). Each rainwater harvesting pump style has its own benefits.
For example, a submersible rainwater collection pump is often quieter as it is submerged in the water tank, it is out of sight/not in the floor space, and allows for constant pump access to water (as long as the 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 Rainwater Harvesting Pump Example
Booster/Jet Rainwater Pump Example
Rainwater Harvesting Pump Electrical Requirements
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.