Mold rarely starts where you see it. A dark speck on a supply register, a musty smell near an air handler closet, or a stain on an attic ceiling usually traces back to water that condensed in the wrong place and stayed there. The right fix depends on which water path failed, so walk that path before you buy a cleaner or book remediation.
A proposal in the mold and HVAC space makes the same point at a larger scale. ACHR News covered the proposed MOLD Act and its push to expand HVAC inspections in military housing, summarizing a plan that would require "independent inspections, environmental standards and stronger accountability measures." Military housing is a different setting from your house, but the failure points are the same: a condensate drain, a drain pan, a return leak, and a duct running through hot, humid space.
Start at the Coil
The evaporator coil pulls moisture out of the air. That water drops into a drain pan under the coil, flows through a trap, and leaves through a drain line to a floor drain, a condensate pump, or an exterior termination. Overflow happens when the drain line clogs with slime, the trap dries out, the pan rusts through, or a pump float sticks. Each failure leaves a different trail, which is why you check them in order.
The Five-Point Walk
Start with the pan. Use a flashlight to look for standing water, rust, or a slick biofilm along the edges. Check whether wet insulation or dust is sitting in the water. A pan that sits out of level lets water pool in one corner, so set a level on the pan lip and note any tilt.
Next, check the trap. It should hold water in its U-bend. A dry trap lets air pull through the drain and can allow the pan to overflow when the system runs under suction. Pour a measured cup of water into the pan or the drain access and watch whether it clears freely toward the termination.
Follow the drain line from the trap to its exit. Look for sags where water and slime collect, and for an outlet that drips long after the test water has passed. A line that drains into a sink or floor drain should show steady flow during the test.
If the system has a condensate pump, cut power at the air handler disconnect first. With power off, check the float switch for continuity with a multimeter while you lift and lower the float by hand. A stuck float that never opens is a common reason the pan fills while the pump stays silent. Restore power only after the panel is closed.
Finally, follow the ducts. In an attic, sweating supply ducts, wet insulation beneath them, and return leaks that pull hot attic air into the system all raise humidity around the air handler. A cheap hygrometer in the return grille and another in the attic will show whether the air being pulled through the system is contributing to the problem.
Read the Stain Pattern
Stain location is evidence. A ceiling stain directly below the air handler usually means overflow or a pan leak. A ring on the outside of a supply duct usually means condensation from cold air moving through warm, humid space, which calls for insulation rather than drain work. Staining near a supply register often points to moisture-laden air meeting a cold surface close to the grille. Map the stain before you pick a treatment, since a surface cleaner hides the evidence without fixing the path.
Keep the Record
Photograph each checkpoint, note the date, and write down what you saw in the pan, the trap, the drain exit, and the duct runs. Repeat the cup-of-water test after any fix. A dated log helps any technician you bring in and gives you a clear comparison if the problem returns.
If you are comparing coverage for home systems and appliances, read how each plan treats water damage and mold before you assume a drain failure is covered. Ask what documentation the insurer expects, and keep your own dated record either way.
The fix often costs less than the stain suggests: clear the line, level the pan, seal the duct, or replace a float switch. The goal is finding which water path failed before you treat what it left behind.