What it is

These two parts manage the water a cooling coil produces and prevent it from flooding the building:

  • Condensate float switch (safety switch) — a float-operated electrical switch that detects rising water and shuts the system down before the drain pan overflows. When the primary drain clogs and water backs up, the float rises and the switch opens the control circuit, stopping the equipment (and the condensate production) so it can't flood. Types include the in-line (in the drain line), the pan-mounted (in the secondary/auxiliary pan), and the primary-pan clip-in.
  • Condensate pump — a small reservoir with a pump and its own internal float switch, used when condensate can't drain by gravity (basement air handlers, high-efficiency furnaces, mini-split heads). The reservoir collects water; when it fills, the internal float starts the pump, which pushes water up and out through small tubing to a drain. A safety float in the pump shuts the system down if the pump fails and water rises too high.

This article covers identifying the switch types, how they're wired, pump operation, and their failures.

How it works

The float switch is a float on a pivot or a magnetic float on a stem, connected to a snap-action contact:

  • Most safety float switches are wired normally closed in series with the 24V control circuit (commonly in the Y/cooling call, or in the common/R path so it kills everything). Water rises → float rises → contact opens → the call is broken → equipment stops. As the water drains back down, the float drops and the contact closes again.
  • You wire it in series with the control wire you want to interrupt — typically the Y (cooling) signal so a clogged drain stops cooling (and thus stops making more water), or the R/24V feed to shut the whole system.

The condensate pump has two floats inside:

  • The pump float turns the pump motor on when water reaches the run level and off when it drops — normal automatic operation.
  • The safety/overflow float is a separate (often normally-closed) switch wired into the equipment's control circuit, like a standalone float switch. If the pump can't keep up or fails, water rises to the safety level, the safety float opens, and the equipment shuts down before the pump reservoir overflows.

In the field

Identify the switch type and where it's wired. In-line switches splice into the drain line at a tee; pan switches sit in the secondary pan or clip to the primary pan. Trace its two wires — they should be in series with a control conductor (often Y or R), not on line voltage.

Test a float switch. With it accessible, ohm across it: a normally-closed safety switch reads closed (continuity) when the float is DOWN (no water) and should read open when you lift the float to the trip position. Lift the float by hand — the contact should click open. If it stays closed when lifted, it's failed (and won't protect against overflow). If it reads open with the float down, it's tripped (water present) or failed open (which kills cooling for no reason).

Diagnose "AC won't run / shuts off" that traces to a float switch. A tripped condensate safety is a super-common cause of intermittent no-cooling. If the float switch is open, the system won't get the call. Check whether the drain is clogged and the pan/line has water — if so, the switch is doing its job; clear the drain. If the pan is dry and the switch is open, the switch failed open or is stuck — replace it.

Test a condensate pump. Pour water into the reservoir: the pump should kick on at the run level and push water out the discharge tubing, then shut off. If it hums but doesn't pump, the impeller's jammed or the pump's failed. If it doesn't run at all, check power to the pump and the internal float. Verify the safety float by raising it — it should open and stop the equipment.

Check the discharge line on a pump. A kinked, clogged, or too-high discharge tube makes the pump run constantly or overflow. The check valve (if fitted) should hold water from draining back into the reservoir between cycles.

Normal values & targets

  • Float switch contact state (normally-closed safety): closed with float down (dry), opens when the float rises to the trip level. This is the common configuration.
  • Wiring point: in series with a low-voltage control conductor — typically Y (interrupts cooling) or R/24V (interrupts everything). It's a 24V control device, NOT line voltage.
  • Some switches are SPDT (have both NO and NC plus common) so they can break one circuit and make another (e.g., kill cooling and trigger an alarm).
  • Condensate pump voltage: typically 120V line voltage for the pump motor; the safety float wires into the 24V control circuit.
  • Pump lift/capacity: small pumps commonly lift on the order of 10–20+ feet and move a modest gallons-per-hour rate — sized to the coil's condensate output; check the pump's rating against the required lift.
  • Discharge tubing: small-diameter (often 3/8") vinyl; needs a clear, downhill-after-the-peak run to a drain, with a check valve to prevent back-drain.

Common faults & what they mean

  • Cooling won't run, drain line/pan has water → condensate safety switch tripped on a clogged primary drain — it's working. Clear the clog; the switch resets as water drains.
  • Cooling won't run, pan is dry, float switch reads open → switch failed open or stuck up; replace it.
  • Drain overflows but equipment never shut down → safety float switch failed closed (didn't open on rising water), wired wrong, or not installed; water rose with no protection. Replace/repair and verify wiring.
  • Pump hums, doesn't move water → jammed/failed impeller or seized motor; replace the pump.
  • Pump runs constantly → stuck pump float, kinked/clogged/too-high discharge, or a failed check valve letting water drain back.
  • Pump overflows despite running → discharge restricted or pump undersized for the lift; water comes in faster than it goes out.

Tech tips & gotchas

A tripped condensate float switch is one of the most common "my AC just stopped" calls in cooling season — and it's usually not a switch problem at all, it's a clogged drain. Check for water in the pan/line first. If it's wet, clear the drain; the switch did exactly what it should.

Lifting the float by hand is the fast test. A good normally-closed safety opens (kills the call) when you raise the float and closes when you lower it. If it doesn't open when lifted, it's not protecting anything — replace it. That's the failure that lets a ceiling get ruined.

Wire it in the right spot. Putting the switch in the Y/cooling line stops the coil from making more water on a clog (good). Putting it in the R/common kills the whole system. Either is valid depending on the design, but make sure it actually interrupts the call you intend — and never wire a 24V float switch onto line voltage.

On condensate pumps, the discharge side causes most no-pump-but-running complaints. A kink, a clog, or a discharge run that's too tall for the pump's lift makes it run forever or overflow. Check the tubing and the lift height against the pump's rating.

Always verify the safety float on a pump after any pump work — pour water and confirm it shuts the equipment down at the high level. The pump's automatic float can work fine while the safety float is dead, and then a pump failure floods the space with no shutdown.

Keep a spare float switch and a small condensate pump on the truck in cooling season. Both are cheap, common failures, and clearing-a-drain-plus-new-float turns a callback into a one-visit fix.

Safety / code notes

A secondary drain or an auxiliary/overflow protection (such as a float switch) is required for condensate-producing equipment located where overflow would cause damage — see the mechanical code's condensate provisions (IMC §307). The float switch is the device that satisfies that overflow-protection requirement, so don't bypass it. Wire the float switch into the low-voltage control circuit only — never line voltage. The condensate pump's motor is line voltage; ensure it's on proper protected power, while its safety float ties into the 24V control. After service, verify the overflow shutdown actually works by floating the switch.