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Rainwater tanks do not all need the same kind of pump. The right pump depends on what you want the water to do after it leaves the tank.
For most home rainwater systems, the best choices are:
- A surface pressure pump for garden hoses, sprinklers, toilets, or other pressurized plumbing.
- A submersible pump when you want the pump inside the tank, quieter operation, or easier priming.
- A transfer pump when you mainly need to move water from one tank to another.
The tank size is usually less important than the flow rate, pressure, vertical lift, pipe length, and intended use.
Choose the Pump Based on How You Will Use the Water
Start with the job the pump needs to do.
| Rainwater use | Common pump type |
|---|---|
| Filling another tank | Transfer pump |
| Hand watering with a hose | Surface or submersible pump |
| Drip irrigation | Low-flow pressure pump, often with a pressure regulator |
| Sprinklers | Pressure pump sized for sprinkler flow and pressure |
| Toilet flushing | Automatic pressure pump |
| Washing machine supply | Automatic pressure pump, where permitted |
| Cabin water system | Pressure pump or submersible pump with suitable controls |
| Whole-house rainwater supply | Properly designed pressure system with treatment and backflow protection |
A small rain barrel used with a watering can may not need a pump at all. Gravity can be enough when the outlet is above the place where you are using the water.
Surface Pumps Work Well for Many Rainwater Tanks
A surface pump sits outside the tank and pulls water through a suction pipe.
This is one of the most common setups for above-ground rainwater tanks. It works well for garden hoses, irrigation, and other jobs that need useful pressure.
Many surface pumps designed for water supply are self-priming. This means they can remove some air from the suction line and begin drawing water after the pump has been filled correctly.
A surface pump is a good choice when:
- The tank outlet is easy to reach.
- The pump can sit close to the tank.
- You want easy access for maintenance.
- You need pressure for hoses or irrigation.
- You can protect the pump from rain, flooding, and freezing.
Surface pumps work best when the suction side is simple. Long suction pipes, small pipes, air leaks, clogged strainers, and excessive lift can make a pump perform poorly.
Whenever possible, position the pump close to the tank and follow the pump manufacturer's suction requirements.
Submersible Pumps Sit Inside the Tank
A submersible pump operates underwater inside the rainwater tank.
Instead of pulling water toward itself, it pushes water out of the tank. This removes many of the priming problems associated with above-ground pumps.
Submersible pumps can be useful when:
- You want quieter operation.
- The tank is some distance from the point of use.
- An outside pump would be difficult to protect.
- Suction lift would be a problem.
- You want a less visible installation.
Some submersible rainwater pumps include automatic pressure controls. Others require a separate pressure controller, pressure tank, or switching system.
Access matters. A pump inside a tank may be harder to inspect or replace than one installed outside. Never enter a cistern or other confined tank to retrieve or service equipment. Large tanks can present serious confined-space hazards.
Use a Transfer Pump for Moving Water Between Tanks
If your main goal is simply moving rainwater from one container to another, you usually do not need a household pressure pump.
A transfer pump is designed primarily to move water.
For example, you might use one to move water:
- From an IBC tote to a larger storage tank.
- Between two rainwater tanks.
- From temporary storage into a cistern.
- From a tank to a watering trailer.
For transfer jobs, flow rate often matters more than outlet pressure.
A transfer pump is not automatically suitable for supplying taps, sprinklers, or appliances. Those applications may require the pump to maintain a specific pressure and switch on and off automatically as water is used.
Pay Attention to Flow Rate
Flow rate tells you how much water the pump can deliver over a period of time. It is commonly listed in gallons per minute (GPM) or liters per minute (L/min).
Your required flow depends on what will operate at the same time.
A single garden hose needs much less flow than several irrigation zones running together. Likewise, a drip irrigation system may use relatively little water even though it operates for a long time.
Do not choose a pump simply because it advertises a very high maximum flow rate. Maximum flow is normally measured under favorable conditions. Actual flow drops as the pump has to produce more pressure or push water higher.
The pump's performance curve is more useful than its maximum-flow number. It shows how much water the pump can deliver at different amounts of head.
Understand Head Height
Head height describes the amount of resistance a pump has to overcome to move water through your system.
Vertical elevation is one part of it.
For example, moving water uphill from a tank to a garden 20 feet higher requires more pumping effort than moving it across level ground.
The complete requirement is often called total dynamic head. It includes:
- Vertical elevation.
- Pressure needed at the outlet.
- Friction inside pipes and fittings.
- Resistance from filters, valves, and other equipment.
This is why two systems using identical tanks may need very different pumps.
A pump supplying a nearby garden hose may have a relatively easy job. A pump supplying a building uphill through a long pipe has a much harder one.
Pressure Matters for Hoses, Sprinklers, and Plumbing
Pressure and flow are related, but they are not the same thing.
A pump might move a large amount of water at low pressure yet perform poorly with sprinklers that require higher pressure.
Check the pressure requirements of anything connected downstream, including:
- Sprinklers.
- Drip irrigation regulators.
- Hose nozzles.
- Filters.
- Toilets.
- Appliances.
- Treatment equipment.
For irrigation, excessive pressure can also be a problem. Drip systems often use a pressure regulator downstream of the pump.
Do not simply install the largest pump available. An oversized pump can cause rapid cycling, unnecessary power use, excessive pressure, and stress on pipes and fittings.
Automatic Pumps Are Convenient for Hose and Household Use
If you want water to appear when you open a tap, look for a system with automatic pressure control.
The controller detects water demand and starts the pump. When demand stops, the pump shuts down.
Some systems use a separate pressure tank. A pressure tank stores a small amount of pressurized water and can reduce how frequently the pump starts.
This can be especially useful where water use consists of many short draws.
The exact pump, controller, and pressure-tank arrangement should be compatible. Do not assume that any pressure controller can be attached to any pump.
Protect the Pump From Running Dry
A rainwater tank eventually gets low.
A pump that continues operating without enough water can overheat or damage internal parts. This is called dry running.
Dry-run protection can shut the pump off when the tank runs out of usable water.
Depending on the system, protection may come from:
- A built-in pump controller.
- A float switch.
- A tank level sensor.
- A separate dry-run protection device.
Check whether a pump for a rain barrel is necessary to verify switching behavior under realistic demand and pipe resistance.
This feature is especially useful for automatic systems because the pump may start when nobody is nearby to notice that the tank is empty.
Keep the pump intake above heavy sediment at the bottom of the tank when the system design allows it. Drawing directly from accumulated sludge can clog filters and increase pump wear.
Check the Pipe and Tank Connections
A good pump cannot overcome badly designed plumbing.
Before choosing one, identify the tank outlet size and the pipe sizes on both sides of the pump.
A bulkhead fitting is a sealed fitting installed through the wall of a tank. It provides a connection for a valve or pipe.
Avoid reducing the suction pipe unnecessarily. A narrow or restricted suction line can starve a surface pump even when the tank contains plenty of water.
Also check:
- Pump inlet and outlet sizes.
- Tank outlet size.
- Valve size.
- Pipe diameter.
- Pipe length.
- Number of bends and fittings.
- Filter connections.
A shutoff valve near the tank can make maintenance easier, provided it is sized appropriately and cannot accidentally remain closed while the pump is running.
Add Filtration Where It Helps the System
Leaves, grit, insects, roof debris, and tank sediment should not be allowed to pass freely through a pump.
Most rainwater systems benefit from keeping large debris out before the water reaches storage. A suitable inlet screen and other prefiltration can reduce the amount of material entering the tank.
A pump intake may also need a strainer.
Fine filters should be chosen carefully. A filter creates resistance as water moves through it, and that resistance increases as the filter becomes dirty.
If a filter is installed before a surface pump, make sure the pump manufacturer permits that arrangement. Excessive restriction on the suction side can cause poor performance or pump damage.
A micron rating describes the approximate size of particles a filter is intended to capture. A smaller micron number means finer filtration. It does not tell you whether water is microbiologically safe to drink.
Consider Your Power Source
Most permanent household rainwater pumps use mains electricity. Smaller systems may use low-voltage DC or solar-powered pumps.
Solar pumping can make sense for irrigation or slow tank transfer where water does not need to be delivered on demand at high pressure.
Check both starting and running power requirements. Electric motors can require considerably more power while starting than while operating normally.
Outdoor electrical equipment also needs suitable weather protection and safe wiring. Pumps near water should be installed according to applicable electrical requirements. Have a qualified electrician handle wiring when the installation requires permanent circuits, outdoor outlets, controls, or other work beyond a simple plug-in connection.
Protect the Pump in Freezing Weather
A surface pump, filter housing, valve, or exposed pipe can hold enough water to be damaged when it freezes.
If you live in a freezing climate, your system needs a winter plan.
That may mean draining above-ground equipment, locating components in a protected space, or designing the system specifically for year-round freezing conditions.
A submersible pump may be less exposed than an outside pump, but its discharge piping, valves, filters, and other components can still freeze.
Do not assume insulation alone will prevent freeze damage under every condition.
What About a Pump for Household Rainwater?
Supplying a house is more complicated than supplying a garden hose.
A household system may require:
- Automatic pressure controls.
- A pressure tank.
- Suitable filtration.
- Water treatment based on the intended use.
- Backflow protection.
- Separation from potable plumbing.
- Proper pump sizing.
- Low-water protection.
- Electrical protection.
If rainwater and municipal water are both present, the plumbing must prevent untreated rainwater from contaminating the drinking-water system. CDC specifically advises keeping collected rainwater from entering pipes carrying treated drinking water.
Local plumbing, health, and water-reuse requirements can also affect how rainwater may be connected and used.
A Pump Does Not Make Rainwater Safe to Drink
Potable means suitable for drinking. Non-potable means water that is not intended for drinking.
A pump only moves water. It does not make roof runoff potable.
Rainwater can pick up germs and chemicals from the air, roof, gutters, pipes, and storage system. CDC advises that rainwater used for drinking, cooking, or bathing should be regularly tested for harmful germs and chemicals, with treatment selected for the contaminants that are actually present.
A drinking-water rainwater system therefore needs to be treated as a complete system involving suitable collection surfaces, prefiltration, treatment, current laboratory testing, maintenance, and applicable local requirements.
How to Size a Rainwater Tank Pump
You can narrow down your pump choice with five questions:
- What will the water supply? Tank transfer, a hose, irrigation, household fixtures, and treatment systems have different requirements.
- How much flow do you need? Add up the likely simultaneous water demand.
- How high must the water travel? Measure the vertical difference between the tank water level and the highest or most demanding outlet.
- How much pressure is required at the destination? Check the requirements of sprinklers, irrigation equipment, appliances, filters, or other downstream equipment.
- How long and restrictive is the piping? Long pipes, narrow pipes, valves, bends, and filters increase resistance.
Then compare those requirements with the manufacturer's pump performance curve. Choose a pump that can deliver your required flow at your actual head, rather than choosing one based only on its maximum advertised flow.
For a simple backyard tank, this may be straightforward. For a house, large irrigation system, long uphill run, or complex pressure system, having a pump or plumbing professional calculate the requirements can prevent expensive sizing mistakes.
Frequently Asked Questions
Do I need a special pump for a rainwater tank?
Not necessarily. Many clean-water surface and submersible pumps can move rainwater. The important part is matching the pump to the required flow, pressure, head height, installation arrangement, and water quality. Check that the manufacturer permits the intended application.
Is a submersible or external pump better for a rainwater tank?
Neither is always better. An external surface pump is easier to access and maintain. A submersible pump is quieter and avoids many suction and priming problems. Choose based on tank access, distance, pressure requirements, maintenance needs, and installation conditions.
What size pump do I need for a garden hose?
Choose the pump based on the hose or nozzle's required flow and pressure plus the elevation and pipe losses between the tank and hose. Tank capacity alone does not determine pump size.
Can I use a sump pump in a rainwater tank?
A sump pump may work for basic water transfer if its manufacturer allows the application, but many sump pumps are designed for high flow at relatively low pressure. They may not provide the pressure needed for hoses, sprinklers, or household plumbing.
Can a rainwater pump run when the tank is empty?
It should not. Many pumps can be damaged by dry running. An automatic system should have suitable dry-run protection, a float switch, or another way to stop the pump when the water level becomes too low.
Do I need a pressure tank with my rainwater pump?
Not every system needs one. Some automatic pumps operate without a separate pressure tank. A pressure tank can reduce frequent pump starts and provide smoother operation in systems with small or intermittent water demands.
Can I pump rainwater directly into my house plumbing?
It may be possible where local rules allow it, but household rainwater systems require more than a pump. Plumbing separation or approved backflow protection, appropriate treatment, pressure controls, and compliance with local requirements may be necessary. Untreated rainwater must not be allowed to contaminate potable water piping.
