Short definition
A float switch is an electrical switch actuated by a float that rises and falls with water level. It’s how a sump pump, sewage ejector, or well pump knows when to turn on and off. In WA, sump-pump float switches are critical — they’re what stand between a wet-season basement and a flood.
What it is
The float carries a magnet (vertical-style switches) or simply tilts a tethered cable (tethered-style switches) that closes an electrical contact when the float rises past a set point and opens it when the float falls back below. The switch is wired in series with the pump motor, so the pump runs only when the float commands it.
Common types:
- Tethered float. A cable-mounted float that swings freely; the cable’s length sets the on/off differential. Standard for sump pumps where the pit allows enough swing room.
- Vertical / rod-style. A float slides up and down a vertical rod; a magnet inside the float activates a reed switch. Tighter on/off differential; used where space is limited.
- Diaphragm switch. A pressure-sensing switch using a diaphragm; works in some sealed-tank applications.
- Electronic / probe-style. Conductive probes detect water level without moving parts; used where mechanical reliability is critical.
The mechanical equivalent is a float valve (mechanical fluid shutoff like a toilet fill valve). The float switch is the electrical version.
Why it matters to a homeowner
In Washington, the float switch is the single most common sump-pump failure point during the October-through-May wet season. When it fails, the pump either won’t start (basement floods) or won’t stop (pump motor burns out from continuous operation).
Three high-value behaviors:
- Annual sump-pump service. Clean the float, exercise the switch by lifting the float manually, verify the trip point. Five minutes of annual maintenance, usually in September before the wet season.
- Battery-backup pump install. A second pump with its own float switch above the primary’s trip point — runs on battery during a power outage and during primary-pump failure. WA’s Pacific windstorm season routinely takes power offline at the worst times.
- Replacement before failure. Float switches and the seals around them are wear items. After 7–10 years of use, replace proactively rather than wait for the failure during a November storm.
Common failure modes
- Float fouled by debris, soap film, or organics — sticks open or closed.
- Tethered float tangled around the pump cord — can’t rise to trip the switch.
- Internal switch contacts corroded — switch fails to make.
- Magnet on a vertical-style demagnetized after years — fails to actuate the reed switch.
Common variants and what a float switch isn’t
- Float switch (electrical) vs. float valve (mechanical fluid shutoff). Float switch energizes a pump or alarm; float valve closes water flow.
- Tethered vs. vertical. Tethered allows wider on/off differential; vertical has a tighter differential and works in tight spaces.
- Float switch vs. pressure switch. Float switch responds to water level. Pressure switch responds to pressure in a closed system. Both control pumps; different signals.
Washington note
Wet-season basement and crawl-space dewatering is one of WA’s signature plumbing duties. SCL and PSE windstorm-driven power outages routinely happen during the wet months when sump pumps are running hard, and the redundancy provided by a battery-backup pump with its own secondary float switch is the standard PNW protective layer. Annual fall service of every float switch in the house — sump, sewage ejector, ejector for any below-grade fixture — is small effort for substantial protection.