What is a Pressure Line?

In a typical basement, several components sit in a row between the submersible pump and the sewer:

• Pump outlet: drainage pumps for basements commonly have a 1¼-inch connection (DN 32); the U5 by Jung Pumpen, for example, has exactly this outlet.
• Flexible connection: a short piece of hose that keeps the pump’s vibrations away from the rigid pipe.
• Check valve: lets water pass only in the pumping direction. For installation below the backflow level, EN 12056 requires a valve tested to EN 12050-4.
• Shut-off valve: so the pump or check valve can be replaced without the line draining.
• Backflow loop: the bend routed above the backflow level.
• Transition into a gravity pipe that leads to the sewer.

Up to the loop the water is under pressure; after it, the water flows by gravity. Two systems meet here, and that explains most of the rules for pressure lines.

In a basement, the pump almost always sits below the backflow level. If the pressure line joins the underground drain by the shortest route, sewage is pushed back through the line into the shaft during sewer backflow. That is why, under EN 12056, the discharge pipe has to be routed in a loop above the locally defined backflow level. Unless the municipality has set something else, the backflow level is the top of the road at the connection point.

The check valve does not replace the loop. Its main job is to hold the water column in the line when the pump switches off. Protection against backflow from the sewer comes from height: sewage can rise up to the backflow level, but not over the crest of the loop. The German federal flood protection guide (Hochwasserschutzfibel) names one exception: in flood zones, the water level in the sewer can rise to the flood level, so above the backflow level too.

In practice the loop often has to go up to the basement ceiling or into the ground floor. Where there is no room inside, heated backflow loops in an outdoor cabinet are available so the line doesn’t freeze in winter.

The pump determines the right diameter. Under EN 12056, the flow velocity in the pressure line must be at least 0.7 m/s; Jung Pumpen marks this limit directly in its pump curves. If the water flows more slowly, sand and suspended matter settle until the line clogs. A bigger pipe “to be safe” is therefore not a good idea.

Too small is just as bad. Every metre of pipe, every bend and the height of the loop cost delivery head. Whether the pump still moves enough water at the end, the installer reads off the pump curve: flow rate against head. If the output isn’t enough, the pump runs for a long time and empties the pump sump more slowly than water flows in during heavy rain.

If you swap the pump for a stronger or weaker model, have the line checked as well. The curve has to match the pipe.

• Check valve no longer closes: after the pump stops, you hear water running back into the shaft, the level rises again straight away, and the pump switches on conspicuously often.
• Valve stuck shut or shut-off valve half closed: the pump is audibly running, but the level barely drops or not at all.
• Deposits: pump cycles take longer and longer over weeks and months because the cross-section is shrinking.
• Frost: parts of the line in unheated areas or outdoors freeze. The pump then pushes against a plug of ice.
• Leaking joints: damp spots at fittings, clamps or the flexible piece, especially while the pump is running.
• Missing loop: in heavy rain the shaft fills up even though the pump works. The water is then coming backwards from the sewer.

A test run with a bucket of water tells you a lot. In dry weather, pour water into the shaft until the pump starts, and watch:

• Does the level drop quickly, and does the pump switch off cleanly?
• Do you hear water running back after it stops? Then the check valve is suspect.
• Do all joints stay dry while the pump is running?
• Is the shut-off valve fully open?

For lifting stations, operator rules based on DIN 1986-3 call for a monthly test run over at least two switching cycles and annual servicing in a single-family home. Servicing includes opening and cleaning the non-return valve, checking its seat and flap, and making the shut-off valves move freely. For a simple basement drainage pump, the monthly bucket test is a good habit.

You need a professional if the loop is missing or sits below the backflow level, if the valve has to be taken apart or replaced, if the line is being re-routed, or if you fit a different pump. Before any work on the pump: unplug it.

Drainbee doesn’t measure inside the line; it measures the water level in the pump sump or lifting station. Many line problems show up exactly there. If the check valve doesn’t close, the level drops after pumping and rises again right away, and pump cycles pile up in the app. If the line is blocked or frozen, the level stops falling, and the threshold alarm goes off before the shaft overflows.

Whether the cause is the valve, the line or the pump, Drainbee can’t tell. Someone has to look on site. And where there is no shaft with a pump, for example with a plain backflow valve, Drainbee has nothing to measure.