Less sediment in your pump pit: the flushing feature of the Pentair U-series
David MeijerQuick answer: the Pentair Jung Pumpen U3K, U5K and U6K have a built-in flushing feature. Drill out the 3 marked points (Ø 5 mm) under the strainer base, and the pump stirs up and cleans the bottom of the sump around the inlet. The result: far less sediment and odour in a permanent installation, while the capacity drops only slightly.
Video: drilling the flushing holes and the effect in the sump
Why does sediment build up in the pump pit?
In a permanent pump pit, for example for the washing machine, shower or groundwater, the pump always pumps down to the switch-off level. Anything heavier than water, such as sand, silt, lint and soap residue, sinks to the bottom between pumping cycles. That layer grows, starts to smell and can eventually obstruct the inlet or the float switch.
The flushing feature tackles this at the source. Through three small holes in the pump housing, part of the water is sprayed back along the bottom while the pump runs. This turbulence keeps the dirt moving, so the pump carries it away instead of letting it settle.
Which pumps does this tip work for?
| Pump | Flushing feature | Flat suction down to | Larger free passage |
|---|---|---|---|
| U3K(S) / U3K(S) SPECIAL | Yes | 5 mm | – |
| U5K(S) / FLUT-BOX | Yes, with markings | 6 mm | 10 → 20 mm |
| U6K(S) | Yes, with markings | 10 mm | 10 → 20 mm |
The U3K(S) SPECIAL is technically identical to the U3K(S), but made of a material that resists chemically contaminated water. Source: Pentair Jung Pumpen U3/U5/U6 operating instructions.
Step-by-step: drilling the flushing holes
- Unplug the pump. Always disconnect the pump from the power supply before working on it.
- Remove the strainer base. Carefully prise off the strainer base (the basket under the pump) with a flat screwdriver.
- Find the markings. On the pump housing you will see 3 markings labelled Ø 5. On the U5 and U6 series there are also markings on the strainer base, so you can see how it has to be refitted.
- Drill out carefully. Drill out the 3 markings with a 5 mm drill bit. Take your time and do not drill deeper than necessary.
- Deburr. Remove the burrs so that no sharp edges or plastic residue remain.
- Refit the strainer base. Click the strainer base back on and check that the new holes are not covered by the connecting pieces of the strainer base.
- Test. Put the pump back in the sump and let it switch on and off a few times.
Good to know: a small part of the water flows back through the flushing holes. As a result, the pump capacity drops slightly. In an ordinary basement or washing machine sump, you will hardly notice this in practice.
When to do it, and when not?
| Situation | Advice |
|---|---|
| Permanent pump pit with grey water (washing machine, shower, washbasin) | Recommended: less deposits and odour |
| Permanent sump with groundwater and some sand or silt | Recommended: the bottom stays cleaner |
| Mobile use during flooding | Not needed |
| Tightly sized installation where every litre counts | Check the pump curve first, as the capacity drops slightly |
3 more handy U-series tips
1. Flat suction without extra parts
Prise off the strainer base and the U-series pumps a flooded floor down to a few millimetres of residual water: U3K to 5 mm, U5K to 6 mm, U6K to 10 mm. On a version with a float switch, fix the float switch in the switched-on position. Automatic switching will then not work, so stay with the pump. Note: for safety reasons, only use the pump in mobile operation with the strainer base fitted. So only do flat suction without the strainer base under supervision.
2. Larger free passage on the U5 and U6
On the U5 and U6 series, the free passage increases from 10 to 20 mm. Prise off the strainer base and push the supplied extension pieces onto the pump feet. The strainer base will then no longer fit under the pump. Useful for water with coarser dirt.
3. Pump starts but does not pump? Think of air
If air remains trapped in the pump housing, for example because of water in a hose that was not empty beforehand, the pump will not move any water. Make sure the hose is empty before the pump goes under water. Also read Help! My Pentair submersible pump is not pumping.
Pentair Jung Pumpen U-series
COMPACT – BASEMENT AND SUMP
Pentair Jung U3K(S)
With flushing feature, flat suction down to 5 mm, with or without float switch.
View current price
ALL-ROUNDER
Pentair Jung U5K(S)
Flushing feature, flat suction down to 6 mm, free passage 10 or 20 mm.
View current price
HIGHEST CAPACITY
Pentair Jung U6K(S)
Flushing feature, free passage 10 or 20 mm, for heavier work.
View current priceAlso available as a complete emergency kit: the FLUT-BOX U5K(S).
Frequently asked questions
What is the flushing feature of the Pentair U-series?
Three small holes in the pump housing of the U3K, U5K and U6K. Once drilled out, some water is sprayed along the bottom while the pump runs, which largely prevents deposits in the sump.
How big should the flushing holes be?
According to the manual, you drill out the 3 markings labelled Ø 5, so with a 5 mm drill bit. Then deburr them.
Does the pump pump less because of the flushing holes?
The capacity drops slightly, because a small part of the water flows back. In an ordinary basement or washing machine sump, this is barely noticeable.
How many mm can the U-series flat-suction down to?
Without the strainer base, the U3K pumps down to 5 mm, the U5K to 6 mm and the U6K to 10 mm of residual water.
Not sure which pump fits?
Not sure whether this modification suits your sump or pump? Send us a photo of the installation and we will take a look for free.
Request free adviceView Pentair Jung Pumpen