How to Wire a Float Switch | Hunker Twist the end of one float switch lead around the end of the wire that connects to the power source. Then cover them with a wire nut. If the float switch protects an air-conditioning system, disconnect the furnace's transformer wire that connects to the red thermostat wire, and then connect the float switch lead to the transformer's wire. How to Troubleshoot an Air Handler Float Switch | Hunker Water dripping through the ceiling and an air handler sitting in a drain pan full of water directly above it is more common than you think, says Christian Smith, an HVAC contractor. The float switch, a safety device that shuts the system down when condensation overflows into the drain pan, can fail for a number of reasons, according to Smith.