A customer recently asked us for a pump that could do three things at once: pump water down to 1mm, pass solids over 5mm, and switch itself on and off automatically. It sounds like a reasonable shopping list. In practice, no single pump on the market does all three.
This is not a gap in our range. It is a result of how pumps physically work. The features pull against each other, and understanding why makes it much easier to choose the right setup.
This guide shows how close you can get with real pumps, and covers the two-pump approach we recommend when you need everything at once. It is relevant to homeowners with flood-prone cellars, facilities managers, and anyone specifying pumps for unattended use.
What the three specs actually mean
• Low suction (residual water level): how much water the pump leaves behind. A puddle pump can clear water down to 1mm to 3mm. A standard submersible may leave 30mm to 70mm or more.
• Free passage (solids handling): the largest solid particle that can pass through the pump without blocking it. Puddle pumps typically manage 2mm to 5mm. Dirty water and sewage pumps handle 10mm to 50mm.
• Automatic operation: the pump starts and stops itself using a float switch or an electronic sensor, so it can be left unattended.
Why one pump cannot do all three
A puddle pump achieves its 1mm performance by drawing water through a very narrow gap at its base. That narrow intake is exactly what allows it to take water off a flat floor. It is also what stops larger solids getting in. If the gap were big enough to pass 10mm gravel, the pump could not create suction at 1mm depth.
High free passage pumps work the other way round. They use a vortex or channel impeller with large internal clearances, which is great for stones, leaves and debris, but they need a reasonable depth of water around the intake to operate. That is why a sewage pump like the EVAK Hippo 75 passes 50mm solids but needs around 280mm of water.
Automatic operation adds a third constraint. A traditional tethered float switch needs several centimetres of water to swing freely, so it cannot trigger at puddle depths. Electronic floor sensors solve this, but they are fitted to puddle pumps, which brings us back to small free passage.
Low suction means a small intake, high free passage means a big intake, and one pump cannot have both.
How close can you get? Real examples
Closest single pump: Tsurumi LSC1.4S Automatic with extension probes
The Tsurumi LSC1.4S has the highest free passage of any true puddle pump we supply, at 5mm. The automatic sensor version normally switches on at 258mm and off at 85mm, but optional sensor extension probes allow it to pump down to around 1mm automatically. That is 1mm suction, 5mm free passage and automatic operation in one unit. Over 5mm free passage, however, is simply not available in this class.
Best automatic puddle pump for cleaner water: EVAK Residox 400 Automatic
The Evak Residox 400 Automatic uses an integrated floor-level crab sensor that detects water at just 3mm, with adjustable start and stop delays to prevent rapid cycling. It moves 250 litres per minute with an 11m head. The trade-off is a 2mm free passage, so it suits silty or lightly dirty water rather than debris-laden water.
Budget automatic option: APP BPS100EA
If you want automatic operation at low levels without the price tag of a sensor pump, the APP BPS100EA is a cost-effective middle ground. It uses a compact float switch rather than a pendant float, so it works in narrow sumps and shallow pooling where a standard float cannot swing. It switches on at around 30mm, off at around 3mm, and pumps down to as low as 3mm, with a 3mm free passage that copes with grit and small debris. At 70 litres per minute with a 6m head, it will not shift flood water as fast as the Residox 400, but for keeping a cellar, sump or plant room dry unattended, it does the job for a fraction of the cost.
Automatic with higher free passage: accept a higher residual level
If solids matter more than the last few millimetres of water, an automatic drainage pump such as the KSB AMA Drainer 301 passes 10mm solids and runs unattended, but leaves roughly 70mm of water behind. For serious solids, sewage pumps like the EVAK Hippo 75 pass 50mm, at the cost of needing much deeper water.
The two-pump solution we usually recommend
When a customer genuinely needs all three capabilities, the answer is usually two pumps working as a team rather than one compromised unit:
• Stage 1: an automatic dirty water or drainage pump with high free passage handles the bulk of the water and the solids, switching itself on as levels rise.
• Stage 2: a puddle pump clears the remaining water down to 1-3mm once the debris has gone.
This setup often costs less than expected, and each pump is doing the job it was designed for, which means fewer blockages and longer service life.
Common mistakes when specifying this kind of pump
• Overestimating the solids. Much "dirty" flood water only carries silt and grit under 2mm. If that is your situation, an automatic puddle pump like the Residox 400 Automatic already meets the spec.
• Assuming automatic must mean a float switch. Floor-level electronic sensors trigger at 3mm, far below any tethered float.
• Insisting the automatic stage reaches 1mm. In most flood scenarios, the automatic pump handles the event unattended and a quick manual pass with a puddle pump finishes the job.
Conclusion
There is no automatic pump with 1mm suction and free passage over 5mm, because the physics of a narrow puddle intake and a wide solids intake cannot exist in the same casing. The Tsurumi LSC1.4S Automatic with extension probes gets closest at 5mm free passage. For everything beyond that, pair an automatic high-passage pump with a puddle pump.
