Understanding Pump Specifications for Domestic Rainwater Reuse
When purchasing a pump for your rainwater tank, you'll likely encounter a range of technical terms and specifications that may seem overwhelming at first. Whether you're planning to use rainwater for your garden, washing machine, toilet flushing, or even household supply, understanding these specifications will help you choose the right pump for your needs. This guide breaks down everything in simple language to help you make an informed decision.
Why Do You Need a Pump?
Unlike mains water, which is pressurised and readily available at the turn of a tap, rainwater stored in a tank needs a pump to move it to where it's required. A pump provides the necessary pressure to ensure a steady and reliable flow of water, whether it's to your irrigation system, plumbing, or even a high-pressure appliance like a washing machine.
Main Specifications to Consider
1. Flow Rate (Litres Per Minute - LPM or Litres Per Hour - LPH)
The flow rate tells you how much water the pump can move in a given time. Most specifications list this in litres per minute (LPM), while some might use litres per hour or litres per second. Always double-check the unit of measurement before comparing different models or brands.
As a rough guide:
- Small garden irrigation systems: 10–20 LPM
- Household supply (toilets, laundry, garden): 40–70 LPM
- Showers: 10–15 LPM per showerhead for modern, water-efficient showerheads
- High-demand use (multiple outlets simultaneously): 70+ LPM
For comparison, standard mains pressure will generally supply around 15-20 LPM per tap.
A higher flow rate means more water can be delivered at once. Still, it's important to match it to your needs to avoid excessive power use or an insufficient water supply with pressure or flow rate drops.
2. Head (Metres - m) – Think of it as Total Distance
Head refers to the maximum height or distance the pump can push your water. It's effectively thought of as the total distance the water needs to travel, accounting for both vertical height climbed and the routing of your pipework.
- If your tank is at ground level and the outlets are on a single-storey home, a head height of around 20–30 metres is usually a good goal.
- For two-storey homes, look for pumps with a head height of 30–50 metres to maintain good pressure upstairs.
- Always factor in the pipework layout, as the distance the water needs to travel may be further than a straight-line distance.
Example of a Pump Performance Graph:
Flow rate and head values are intertwined in pumps. The further you need to pump the water, the more the flow rate will decrease. This is because the pump has to work harder to push water to greater distances, reducing the amount of water that can be delivered. Understanding this relationship helps you choose a pump that can still meet your needs even at higher head distances. This is what is shown on a pump performance curve or performance graph.
In the image above, the RPS57E pump supplies approximately 30 litres per minute of water at 50 metres of head.
Secondary Characteristics – Understanding the Numbers on a Spec Sheet
Beyond flow rate, head height, and pressure, a pump's spec sheet may include several other important figures:
- Pressure (Kilopascals—kPa or Bar): Pressure refers to the force at which water is delivered. While often assumed to determine how "strong" the water feels coming out of a tap, pressure alone isn’t the full picture—it works in combination with flow rate and head height. Pressure is a secondary characteristic because it naturally results from selecting the right pump for your required flow rate and head height. Pumps shouldn't be chosen based on pressure alone, as too much or too little pressure can be a symptom of an undersized or oversized pump.
- Power (Horsepower - HP or Kilowatts - kW): This tells you how much energy the pump uses. A higher-wattage pump generally moves more water but also consumes more electricity.
- Current Draw (Amps—A): This helps determine whether your electrical circuit can handle the pump without overloading or requiring an electrical upgrade.
- Outlet Size: The diameter of the pipe fittings the pump is compatible with (commonly 25mm, 32mm, or 40mm). Having the right size prevents pressure loss.
- Voltage (V): Most household pumps run on 240V single-phase power, but some larger pumps require three-phase power, which is uncommon in residential setups.
Understanding Different Pump Types and Features
Common Pump Types
- Jet Pressure Pumps: These use a jet mechanism to create suction, which is commonly used for household water supply.
- Multi-Stage Pumps: These pumps feature multiple impellers to generate higher pressure and efficiency, making them ideal for homes with high-pressure demands.
- Variable Speed Pumps (VSD): Adjust speed based on demand, reducing energy consumption and wear on components. It can be ideal for solar setups as they ramp up gradually.
- Submersible Pumps: Installed inside your water tank, these pumps are quieter than external options because the water muffles the sound of the pump running.
Pump Features
- Automatic Pumps & Pressure Controllers: These pumps turn on and off based on demand, ensuring water is available when needed without manual intervention.
- Run-Dry Protection: Prevents the pump from operating when no water is available, protecting it from damage.
- Thermal Overload Protection: Shuts down the pump if it overheats.
- Pressure Sensors: Help maintain stable water pressure without constant cycling.
Considering Power Draw and Efficiency
Now that you understand how flow rate, head height, and power ratings work, it's important to also think about how efficiently the pump operates:
- Power Draw and Running Costs: A pump that is too powerful for your setup will consume more energy and cost more to run unnecessarily.
- Variable Speed drives (VSDs): These pumps will have smarter controllers that only operate the pumps' impellors at the speed needed to provide an adequate flow rate. This can lead to quieter operation and greater electrical efficiency, which can be especially helpful for solar setups since it will gradually ramp up the power draw.
- Energy Supply Considerations: If you run off-grid, choosing an energy-efficient pump or solar-compatible system can help reduce your system's demand.
Selecting the right pump for your rainwater tank doesn't have to be complicated. By understanding flow rate, head height, pressure, and different pump types, you can choose a pump that meets your needs without overspending on the pump or electricity costs.
By choosing wisely, you'll ensure reliable access to clean rainwater for years to come.