Monitor Your Pump Sump: How to Prevent Water Damage.

A pump sump is a pit at the lowest point of the basement. It collects water that cannot drain away by itself: from the drainage around the house, from floor drains or from light wells. A submersible pump in the pit sends it through a pressure pipe outside or into the sewer. It is usually controlled by a float switch: when the level rises, the float lifts and the pump starts. When it drops, the pump switches off again.

Three things decide whether this works reliably:

• Pit size. The float needs room. For its ZPK drainage pump with a free-hanging float, Zehnder requires a pit of at least 450 × 450 × 450 mm. If it is narrower, the float gets caught on the wall.
• Pressure pipe. For lifting stations, DIN EN 12056-4 requires a backflow loop: the pipe is routed above the backflow level before it joins the sewer. The same principle applies to a basement pump that discharges into the sewer. The non-return valve in the pipe only stops water from running back into the pit after the pump switches off. It does not protect against backflow from the sewer.
• Type of water. A pump sump is meant for drainage, seepage and lightly soiled water. Toilet wastewater belongs in a sewage lifting station to DIN EN 12050-1, see lifting station.

We analysed the readings from 8 Drainbee devices in residential pump sumps: 19 March to 6 October 2026, 1,616 device-days in total. That is a small sample from our own fleet, not a representative cross-section. But it shows clearly how a pump sump works day to day:

• On 62% of days the pump did not run once.
• On days when it ran, the median was 4 pump cycles. On the busiest 5% of days it was 22 or more, with a maximum of 49 in one day.
• A median pump cycle lasted just over 3 minutes and lowered the level by about 10 cm.
• On its busiest day, a device’s pump ran about 7 times as often as on a typical pump day (median across the 5 devices with at least 20 pump days).

What this means for you: most of the time nothing happens in the pit. Whether the float and pump still work only shows when water arrives, and then at several times the usual volume. A fault that develops on a quiet day can therefore go unnoticed for weeks.

If the pump fails, the water in the pit rises until it spills over the edge into the basement. How expensive that gets depends on the volume of water, the floor construction and how quickly you notice. Two figures for context:

• Heavy-rain damage to residential buildings cost affected homeowners €7,600 on average, according to the German insurers’ association GDV (2002–2021).
• Around 41% of residential buildings in Germany had no natural hazard cover at the end of 2025 (GDV).

Whether damage from an overflowing pump sump is insured depends on your policy. Check what counts as flooding and backflow, and ask your insurer if in doubt. More under natural hazard insurance.

Regardless of cost: wet building materials and belongings should dry within 24 to 48 hours. In most cases mould will then not grow, according to the US Environmental Protection Agency (EPA). The sooner you learn about the failure, the better your chances of managing that.

• Stuck float. It catches on the pit wall, the pump cable or the pressure pipe, or sludge holds it down. The pump does not start.
• No power. Power cut, tripped RCD, unplugged pump. In thunderstorms this often coincides with heavy rain.
• Blocked or dirty impeller. Sand, grit and fibres lodge in the pump housing. The pump stops moving water, or the motor protection switches it off.
• Air in the pump housing. If the pump’s vent opening is clogged, it does not pump even though the water is high enough.
• Clogged or kinked pressure pipe. Especially with hose lines.
• Leaking non-return valve. Water from the pressure pipe runs back after every cycle, and the pump starts more often than necessary.

Most of these causes also appear in manufacturers’ troubleshooting tables, for example in the manual for the Zehnder ZPK. How to narrow them down in an emergency is covered under pump failure.

For lifting stations, DIN EN 12056-4 sets fixed maintenance intervals. There is no equivalent for a simple pump sump with a submersible pump. Zehnder, for example, specifies cleaning the intake strainer and pump base for its ZPK, with occasional visual checks in between. Our recommendation: every few months, and in any case before autumn and after every heavy rain.

1. Function test with water. Pour a bucket of water into the pit until the pump starts. It must lower the level and switch off cleanly. This tests float, pump and pressure pipe in one go.
2. Check the float. It must move freely and not touch anything. Secure cables so they cannot get in its way.
3. Check the cut-out point. The pump must switch off before it draws air. The lower switching point has to sit above the residual water level the pump can pump down to.
4. Clean the intake strainer and pit floor. Unplug first. Vacuum or scoop out sludge and sand.
5. Inspect the pressure pipe and non-return valve. Look for kinks, leaks and water running back after switch-off.
6. Power supply. Socket dry and above the highest possible water level, plug secure, RCD not tripped.
7. Lid on. It keeps leaves, small objects and children’s hands out of the pit.

If the pump hums without pumping, smells burnt or keeps tripping the fuse: stop using it and call a professional.

The checklist tells you whether everything works while you are standing in front of it. Between two checks you know nothing. Many failures happen in exactly that gap, because a fault only shows up with the next rain.

A water level sensor in the pit closes this gap. It measures the level continuously and alerts you when it rises above a limit, before the pit overflows. A water leak detector on the floor cannot do this: it only triggers when the water is already in the basement.

How Drainbee does it: a hydrostatic probe hangs in the pump sump and measures the water level up to 1 m to within ± 1 cm, adaptively every 10 seconds to 5 minutes. The wall box sends the data over Wi-Fi (2.4 GHz) to the cloud. In the app you see the level, pump cycles, inflow and the rain and inflow history. You get an alarm when the level passes your threshold, on power outage and on Wi-Fi loss, as a push notification or a call-style alarm in the app. €298 one-off, no subscription.

What Drainbee does not do: control the pump or detect water on the floor. So the probe replaces neither the float nor maintenance. It is the second, independent layer alongside them.