Float Switch Stuck? How to Protect Against Pump Failure.

A switching contact sits inside the float. When the water rises, it lifts the float, the contact closes and the pump runs. When the level drops, the float falls back and the pump switches off. Three designs are common:

• Tethered float: a float body hangs on a freely moving cable and tilts as it rises. You set the switch-on and switch-off heights via the free cable length, on the Zehnder ZPK for example by sliding the cable in its holder. Needs plenty of room.
• Compact or vertical float: the float slides up and down on a guide. Designed for narrow pits.
• Level sensing without a float: in lifting stations the level is often measured by a dip tube with a pressure sensor, conductivity probes or optical probes. Kessel, for example, offers its Comfort control units with a pressure sensor, conductivity probes or an optical probe.

The float has two jobs: it starts the pump in time, and it stops it before it runs dry. For the ZPK, Zehnder specifies that the lower switching point must be above the residual water level the pump can pump down to.

• Too little room: the float hits the pit wall or the pressure pipe, or gets caught in its own cable. For the ZPK with a tethered float, Zehnder requires a pit of at least 450 × 450 × 450 mm.
• Sludge and deposits: sand, sludge, limescale and grease settle on the float and the pit floor and slow it down.
• Loose cables: the pump’s power cable or another cable is in the way.
• Wear: plastic becomes brittle, cables break at the bend, the switching contact inside can fail.
• Shifted setting: if the cable length has changed, the pump switches on too late or no longer switches off.

The treacherous part is the long quiet spells. In our analysis of 8 Drainbee pump sumps in residential buildings (19 March to 6 October 2026, 1,616 device-days), the pump did not run once on 62% of days. For all that time the float does not move either. Whether it has seized up only shows with the next inflow. And on days when the pump ran, the float had to switch a median of 4 times, on individual days up to 49 times.

1. Visual check: open the lid and see whether the float hangs freely and does not touch anything.
2. Test with water: slowly pour a bucket of water into the pit. The pump must start as soon as the float rises. This tells you more than lifting it by hand, because it also shows that the pump actually moves water.
3. Watch it switch off: the pump must switch off before it draws air. If it keeps slurping for a long time, the lower switching point is too low.
4. Check the movement: unplug the pump and guide the float through its whole range by hand. It must not catch anywhere.
5. Clean: rinse the float and cable with clean water, remove sludge from the pit floor.
6. Tidy the cables: secure loose cables so they cannot get in the float’s way.

There is no rule on how often you must do this for simple basement pumps. Our recommendation: every few months, before the wet season, after every heavy rain and whenever someone has worked in the pit.

Work on hard-wired switches and on the pump’s electrics belongs in the hands of a qualified electrician.

• Float attached to the pump: on many submersible pumps the float is permanently connected. Some manufacturers sell it as a spare part, Zehnder for example for the ZPK. If the pump is old or noisy anyway, a new pump is often the better choice.
• Separate float switch with piggyback plug: many retrofit switches have a piggyback plug: the pump plugs into the float switch’s plug, which plugs into the socket. That way you can replace it without electrical work. You need a pump without its own float, which the ZPK, for example, is available as.
• Electronic level switches: sensors without moving parts that switch at a fixed height. They cannot get stuck, but they can get dirty.

Whatever you choose: test with a bucket of water afterwards. And the underlying problem remains: the new switch is also a single component that everything depends on.

A float switch that controls the pump cannot report its own failure. That is why a second layer that watches the level independently of the pump float is worthwhile:

• High-water float with alarm unit: a second float, mounted slightly higher, triggers an alarm unit. Mains-independent units such as the Wilo DrainAlarm also alert during a power cut. But the alarm stays inside the house unless you connect something for remote signalling.
• Water level sensor with app: a probe measures the level continuously and alerts your phone when it rises above a limit.

Drainbee is such a water level sensor: the hydrostatic probe hangs in the pit, measures up to 1 m to within ± 1 cm and alerts you by push notification or call-style alarm in the app when the level passes your threshold, as well as on power outage and Wi-Fi loss. If the float sticks, the app shows the level rising with no further pump cycles. Drainbee does not switch the pump; that remains the float’s job.