Choosing a pump is not really about horsepower. Two pumps with identical motors can behave completely differently — one will push water 50 metres up a hill at a trickle, the other will move four times the volume but barely lift it over a fence. The motor tells you how much energy is available. The pump design tells you what it does with that energy.
Working through four questions in order will get you to the right family of pump, and usually to the right model.
Question 1: Where is the water coming from?
This is the single biggest fork in the road, because it decides whether you need a surface pump that sits beside the water and sucks it up, or a submersible pump that sits down in the water and pushes it up.
| Water source | Pump family | Why |
|---|---|---|
| Rainwater or poly tank at ground level | Surface pressure pump | Water is at or near pump level, easy to prime |
| Dam, creek or river | Surface centrifugal / transfer pump | High volume, short lift, pump sits on the bank |
| Shallow well or sump (under 8 m to water) | Surface jet pump | Jet design handles suction lift |
| Bore or deep well (over 8 m to water) | Submersible bore pump | Suction physics rules out a surface pump |
| Pit, flooded area, sump | Submersible drainage pump | Pump goes into the water it is removing |
| Sewage, effluent, water with solids | Submersible macerator or sewage pump | Needs to pass solids without blocking |
The eight-metre rule
A surface pump does not actually suck water up. It lowers the pressure in the suction line so atmospheric pressure pushes water up into it. At sea level, atmospheric pressure can only support a column of water about 10.3 metres high — and that is a perfect vacuum with no losses, which no real pump achieves.
In practice, 7 to 8 metres of vertical suction lift is the working limit, and less if the suction line is long, the pipe is narrow, or you are above sea level. Rural Max jet pumps are rated to 9 m maximum suction under ideal test conditions; design for less.
If the water level is more than about 8 metres below where the pump would sit, no surface pump will work reliably. You need a submersible.
Question 2: How high and how far do you need to move it?
Pump performance is quoted as head — the vertical height a pump can push water, measured in metres. But the figure you need to match is not just the vertical rise. It is total dynamic head: vertical lift, plus the pressure you want at the outlet, plus the friction losses in the pipe along the way.
A rough working method:
- Static lift — vertical metres from water level to the highest outlet.
- Pressure required — add 20 m for reasonable household pressure, 15 m for garden sprinklers, 0 m if you are just filling a tank.
- Friction — add roughly 5% of your pipe run length in metres for correctly sized pipe, more if the pipe is undersized or has many bends.
Add those together and you have the head your pump must produce while still delivering the flow you need. This is the part people miss, and it is covered properly in Understanding Flow Rate, Head and Pump Curves.
The number on the box is not the number you get. A pump rated at 43 m maximum head produces that head at essentially zero flow. A pump rated at 55 L/min maximum flow produces that flow at essentially zero head. Your real operating point sits somewhere between the two.
Question 3: How much water do you actually need?
Flow is measured in litres per minute (L/min). Estimating it honestly prevents both undersizing and the more common mistake of buying far more pump than the job needs.
| Application | Typical demand |
|---|---|
| Single garden tap or hose | 15–25 L/min |
| Shower | 8–12 L/min |
| Household, 1–2 outlets at once | 30–50 L/min |
| Household, 3+ outlets at once | 50–80 L/min |
| Garden sprinkler system | 30–60 L/min per zone |
| Stock trough filling | 40–100 L/min |
| Irrigation transfer, dam to tank | 150–400 L/min |
| Emptying a flooded area | As much as you can get |
Add up what will realistically run at the same time, not the total of every outlet on the property. A house with six taps rarely has more than two open at once.
Do not oversize
An oversized pump costs more to buy, more to run, and wears faster. It also causes problems an undersized pump does not: rapid cycling on and off, water hammer, and cavitation when the suction line cannot keep up with the pump’s appetite.
Question 4: What is in the water?
Clean water pumps have tight clearances and will block or wear out quickly on anything else.
- Clean water — tank, mains, bore water. Any pump in the range.
- Slightly dirty water — dam or creek water with silt and some suspended matter. Suits submersible drainage pumps such as the RM-QDX370 and RM-QDX1100, rated for a pH range of 6–8.5 and fluid temperatures below 40 °C.
- Water with solids — construction runoff, slurry, farm washdown. Needs a macerator pump like the RM-WQD7, which cuts solids as it pumps.
- Sewage and effluent — needs a dedicated sewage pump such as the RM-WQD15.
Corrosive or aggressive water also affects materials. Cast iron bodies with brass impellers suit most tank and garden duty. Where water is mildly corrosive or the pump is exposed to weather, the stainless steel RM-SJP800 is the more durable choice.
Putting it together
Three worked examples using the Rural Max range.
A house on tank water
Rainwater tank at ground level, house 20 m away, bathroom 4 m above the tank outlet, two outlets running at once.
- Static lift: 4 m
- Pressure for household use: 20 m
- Friction on 20 m of correctly sized pipe: ~1 m
- Total dynamic head: ~25 m. Flow needed: ~40 L/min.
The RM-JP100 delivers 55 L/min maximum and 43 m maximum head, so at 25 m of head it comfortably supplies 40 L/min. Add an automatic controller so it starts and stops on demand — or choose the RM-JP100-E, which has one fitted. See Pressure Tank or Pump Controller?.
Filling a tank from a dam
Dam 60 m from the tank, tank inlet 8 m above the dam surface, want it filled quickly. Water is a little silty. Pump can sit on the bank, 3 m above the water.
- Static lift: 8 m
- Pressure required: 0 m (just filling)
- Friction on 60 m: ~3 m
- Total dynamic head: ~11 m. Flow: as much as possible.
The RM-TF1 centrifugal pump moves 267 L/min at maximum and produces 18 m of head — a good match for high flow at low head. At 267 L/min you will need 2” discharge hose rather than matching the pump’s 1.5” fittings; see Choosing and Using Lay Flat Discharge Hose.
Bore water for stock and irrigation
Bore with the water sitting 45 m down, drawing down to 60 m while pumping. Tank on a rise 15 m above the bore head.
- Pumping water level: 60 m
- Rise to tank: 15 m
- Friction: ~4 m
- Total dynamic head: ~79 m.
Only a submersible bore pump reaches this. The RM-BORE2 produces up to 132 m of head and 67 L/min, fits a standard 4” bore casing, and comes with 40 m of SAA-certified cable and a control box. Sizing a bore pump has extra steps — Sizing a Bore Pump covers them.
Quick reference
| If you need to… | Look at |
|---|---|
| Pressurise a house from a tank | RM-JP100, RM-SJP800, RM-JP200 |
| Same, with automatic start/stop built in | RM-JP100-E, RM-SJP800-E |
| Hold steady pressure across many outlets | RM-DJSM1025 multistage |
| Move large volumes at low head | RM-TF1, RM-CP-3 centrifugal |
| Push irrigation water uphill or a long way | RM-CP-4 — 200 L/min at 45 m |
| Pump from a bore | RM-BORE2 |
| Drain a pit, sump or flooded area | RM-QDX370, RM-QDX1100 |
| Move water containing solids | RM-WQD7, RM-WQD15 |
Still deciding between designs? Pump Types Explained walks through how each family works and where each one falls down.
