Most people start with a fairly small tank (or even a rain barrel) and a gravity hose bib. This is works really well when you have trees downhill that need watering, or you can fill a watering can to hand water your potted plants or veggie beds. But at some point, a pump becomes very useful. Using rainwater outdoors, there are three basic reasons you may find adding a pump to your system useful. First, you may have multiple tanks at different downspouts and you want to transfer water between tanks to the tank that is in the best location for water use. Second, you would like to use your rainwater for hand-watering, but you have to walk a long distance between locations so a pressurized hose bib that allows you to take your hose with you and turn on a hose nozzle at the plants you wish to water is very helpful. Finally, you may have existing irrigation valves that handle most of your water needs so it makes sense to use your rainwater as the water source for those valves.
The pump that will work well for these tasks will be different in each case. In general, the cost and complexity goes up as you move through the applications — transferring water between tanks is the least complex, it doesn’t have to happen quickly or at high pressure. Using a pump for a pressurized hose bib can work well with the same pump that you use to transfer water — it can be a simple utility booster pump and can be portable if being used in different applications. The above photo shows a simple transfer utility pump on the left (Flotec) and a booster pump with a pressure switch on the right. The second pump can be sized from 1/4 HP to 1 HP depending on the application, and is typically installed with a pressure tank for constant pressure.
The diagram above depicts a situation in which a pump is used for both irrigation valves and an indoor non-potable function such as toilet flushing or laundry. The next post in this series will go into more detail for the indoor applications. A more complex pump is needed to connect to irrigation valves so that the function is fully integrated, typically using a pump relay that connects to the irrigation controller.