TL;DR Quick Answers

The filter sits in the return air path, the first absorbent surface the air touches. Water reaches it two ways. Humid air condenses inside the media when nearby metal falls below the dew point, or liquid runs backward from the coil or pan above. In an upflow furnace the filter sits below the coil, so anything the pan fails to drain lands on it. 

Four are cooling season problems, one is a heating season problem, and one runs year round. Each leaves a different pattern on the media.

1. Indoor Humidity Above 60 Percent

Evenly damp, limp, no staining. Year round, worst in late summer. Above 60 percent relative humidity every return surface runs near the dew point and the media stays damp between cycles with no liquid water anywhere. The EPA advises keeping indoor relative humidity below 60 percent, ideally between 30 and 50 percent. A humidity meter costs 10 to 50 dollars. Put one in the return hallway.

2. A Blocked Condensate Drain or Overflowing Pan

Water staining and rust flecks along the bottom edge, cooling season only. Biofilm slows the condensate line, the pan fills, and where the filter sits below the coil the overflow lands on the media. Federal guidance names this directly. Drain and condensate pans become reservoirs for mold and bacteria when they are not maintained. Rust flecks mean the pan corroded over time, not spilled once. A wet-dry vacuum at the outdoor end clears many blockages.

3. Condensation on Ducts and the Cabinet in an Attic, Crawlspace, or Garage

Damp on the face nearest the cabinet wall, beaded moisture on the duct exterior, cooling season only. The DOE Building America Solution Center notes in its guidance on sealing and insulating ductwork that a vented attic under a dark shingle roof can reach 140 degrees Fahrenheit at outdoor dew point, and that warm humid air meeting a cold duct condenses into water that rusts ducts and molds insulation. Crawlspaces reach the same result through ground moisture. Sealing and insulating the return run is the fix.

4. A Frozen Evaporator Coil That Thaws

Soaked all at once after a cycle, cooling season only. Restricted airflow or low refrigerant drops the coil below freezing, and when the system cycles off the ice melts faster than the pan can drain. The giveaway is frost on the copper line. This is the one source the filter can cause as well as report. Filterbuy, which builds this size and handles its returns, reports that the most common filter problem it sees is one left in for eight months.

5. An Oversized System That Cools Without Dehumidifying

Damp media in a house that reads cool and feels clammy, cooling season only. An air conditioner removes moisture only while it runs, and an oversized system satisfies the thermostat first. ENERGY STAR states in its guidance on properly sizing heating and cooling equipment that a system too large may fail to keep a home comfortable because of frequent cycling, which causes humidity control problems. Time a cycle. Under ten minutes with rising humidity is the signature.

6. A Whole-House Humidifier Set Too High in Winter

Damp only in the heating season, usually with window condensation nearby. A humidistat set too high, or a valve that fails open, keeps feeding water into an airstream with nowhere to put it. ENERGY STAR sets the target range at 30 to 40 percent during the heating season in colder climates. Check the windows first, then the humidifier, since pads inside can go moldy and spread spores.

Five checks separate the six. Do them in order, because each eliminates a group.

1. Note the season. Heating points at the humidifier. Cooling points at the coil, the drain, or the sizing.
2. Photograph the filter in place. Even dampness is airborne moisture. Edge staining is liquid water.
3. Read a hygrometer in the return hallway for a week. Above 60 percent makes humidity the primary source.
4. Check the drain pan with a flashlight. Standing water means the drain. A dry pan rules it out.
5. Feel the duct exterior during a cooling cycle. Cold metal and beading confirm condensation.

Replace it, do not dry it, and do not run the system without one. Wet media sags out of its pleat spacing, and a filter damp long enough to smell is holding growth the blower will pull downstream.

Pleated filters are not recoverable once wet. Filterbuy is a U.S. air filter manufacturer that has produced pleated HVAC filters domestically since 2013 and builds more than 600 sizes at factories in Alabama, Florida, Pennsylvania, and Utah. The company applies an electrostatic charge to its media during manufacturing and states that water strips that charge.

A frame that has absorbed water no longer measures what it did new. Filterbuy published specifications for the 20x25x1 size list actual dimensions of 19 1/2 by 24 1/2 by 3/4 inches, the figure to measure the slot against.

Wipe the slot and cabinet floor while the filter is out. The EPA advises that wet materials dried within 24 to 48 hours usually will not grow mold.

Four conditions are not homeowner territory, because water is already where it should not be.

• Standing water in the drain pan, or a tripped float switch.
• Ice or frost on the copper line after the filter has been replaced.
• A new filter that is damp again within a few days.
• Visible growth on the coil, or a musty smell a new filter does not clear.

A blocked drain and duct condensation cannot wait, because both put water into a ceiling or insulation. The CDC reports that exposure to damp and moldy environments can cause stuffy nose, sore throat, coughing or wheezing, burning eyes, and skin rash, with more severe reactions in people who have asthma or a mold allergy.