Condensate Float Switch

For homeowners

A safety device on the condensate drain that shuts the system down before water can overflow the pan and damage your ceiling. Common brand names: Diversitech SS-1, Safe-T-Switch SS2, EZ Trap series.

Inside a sealed plastic body sits a small float carrying a magnet. In normal operation, the chamber is dry and the float rests at the bottom. When the primary drain clogs (which happens slowly over years as algae builds up in the drain trap), condensate backs up through a secondary outlet, fills the chamber, and lifts the float. The magnet rises near a reed switch hidden in a dry compartment at the top of the device. The reed contacts open, breaking the 24V control circuit, and the entire system shuts off — compressor, blower, everything.

The system staying off is the alarm. Customers usually call and report “AC stopped working,” at which point the tech can check the float switch first thing and either clear the drain or replace a stuck switch.

---

For technicians

Construction. The device is typically a small plastic body, maybe two inches by three inches, with two threaded ports on the bottom for the drain line connection (inline style — primary drain enters one side, exits the other, with a tee for the secondary float port). The float chamber is on top of the inline section, and the reed switch compartment is on top of the float chamber, separated by a thin plastic divider that the magnetic field can penetrate but water cannot.

The reed switch is a glass-encased pair of ferromagnetic reeds. Most modern float switches use a normally-closed reed — contacts held closed by spring preload, opens when a magnetic field is applied. Normal operation: float is down, magnet is far from reed, spring keeps contacts closed. Tripped: float is up, magnet approaches reed from below, field overcomes spring, contacts open.

The choice of NC reed over NO reed is deliberate — it makes the switch fail-safe. Any failure mode that breaks the electrical path (broken wire, corroded terminal, dead reed) shuts the system off, which is the desired behavior. A NO-reed design would fail open and leave the system running without protection.

Wiring. The float switch is wired in series with the R leg of the 24V control transformer secondary. When the switch opens, every 24V load downstream loses power simultaneously — contactor coil, fan relay coil, heat strip sequencers, gas valve, the lot. Some installations wire the switch on the Y signal only, which kills cooling but leaves the blower running and the heat available. The R-leg wiring is more conservative and is what’s now considered best practice in coastal Florida installations.

The drain plumbing this is plumbed into. A typical setup:

• Primary drain port at the bottom of the evaporator coil’s drain pan
• 3/4” PVC trap (the P-trap that holds water and prevents air leaks past the drain)
• Cleanout tee for vacuum access
• Run downhill to outdoor termination or condensate pump

The secondary outlet on the pan (a separate fitting, usually capped at the factory unless used) is connected to the float switch port via a short length of PVC. The float switch sits inline OR off a tee, depending on style. When the primary backs up due to a clog, water rises in the pan, exceeds the height of the secondary outlet, and starts flowing through the secondary line into the float chamber. Within seconds the float rises and trips.

Newer style: in-line drain installation. Some float switches (the SS3 and similar) install directly in the primary drain line itself rather than on the secondary port. They have a small reservoir in the drain line that fills first when the drain restricts. Float rises, switch trips, before the pan even sees water. This catches restrictions earlier and works on systems that don’t have a secondary pan outlet.

Common failure modes:

Algae sticking the float. The chamber accumulates biofilm and algae over years, especially in humid climates. The float can stick in either position:

• Stuck UP — switch reads open, system never runs even when the pan is dry. Customer complaint: “AC stopped working.” Tech pulls the cap, finds the float hung up by algae, cleans the chamber, problem solved.
• Stuck DOWN — switch never trips, no protection. This is the bad failure because the homeowner doesn’t know it happened. Eventually the pan overflows. Damaged drywall, ruined flooring.

Annual maintenance protocol: pull the cap, flush the chamber with clean water and a small brush, verify the float moves freely up and down with finger pressure.

Broken reed. Glass reed switches can crack with mechanical shock — someone hit the cabinet, the device fell during service, a hurricane shook the air handler hard. Reads infinite resistance even when the float is down. System shuts off and stays off. Replace.

Corroded terminals. The terminals where the 24V wires connect can corrode in humid environments, particularly if the connection wasn’t made tight. Reads intermittent. System runs sometimes, doesn’t others. Strip back to clean wire and re-terminate.

Diagnostic test. With the unit powered off, pull the wires off the float switch terminals. Set multimeter to continuity or low-ohms. With the float in its resting (down) position, should read continuous (very low resistance). Lift the float manually with a fingertip through the access cap — should read open. Release the float, should return to continuous. Any deviation, replace the switch — they cost $15–25 and take five minutes to swap.

Florida-specific note. Coastal Florida humidity is brutal on these devices. The float bodies are typically polypropylene, which holds up well, but the reed switch electronics inside the sealed compartment do degrade over time from heat cycling in attic installations. Plan on float switch replacement every 8–12 years even if it tests good, just to maintain reliability. The cost is trivial and the alternative is a ceiling event.

A note on installation orientation. Most float switches are vertical-only — the float relies on gravity to return to the down position. Mounting at any angle other than vertical can cause sticking or false trips. Some newer models (SS3 again) are designed to mount in any orientation, using a different float geometry. Verify the manufacturer’s installation diagram before mounting.