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Water on the floor at the furnace: find the source (condensate, AC coil, humidifier)

A furnace does not use water, so the leak comes from something attached to it: the condensate system of a high-efficiency (90%+) furnace, the AC evaporator coil and its drain pan sitting on top, a whole-house humidifier, a condensate pump, or a nearby water heater or washer. When it leaks tells you a lot: cooling season points to the AC coil; heating season points to furnace condensate or the humidifier.

Cross-checked ~40 min easyBrands: All

Before you start

If water has reached wiring or the control board, turn the furnace off at the switch or breaker before touching anything
Do not open or inspect the heat exchanger yourself; a suspected cracked or corroded heat exchanger can leak carbon monoxide and goes to a licensed technician

Diagnostic steps

Unlock all 6 steps with expected values

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