What Is in Your Air Ducts Dust, Drywall and the Odd Mouse

The ordinary layer

Every forced-air system pulls room air through its returns, past the filter, across the blower and coil, and back out the supply runs. Whatever is floating in a room gets a chance to land inside. EPA's guidance says most of that dirt clings to duct surfaces and does not necessarily come back out into the living space, and that a light coating of household dust in the ducts has not been shown to carry any health risk. A gray film inside a register boot is normal and, on its own, no reason to spend money.

Returns and supplies are not the same

The return side sits upstream of the filter, so it collects the most. A return grille fuzzed with lint is what EPA calls expected. The supply side is downstream of the filter and should stay comparatively clean. Heavy dust on a supply register usually means air is getting around the filter, through a loose filter door, a gap at the slot, or a leaky return joint in an unfinished basement pulling in dirt from the laundry area. Cleaning the supplies without fixing that leak only resets the clock.

Drywall and construction leftovers

Wheaton's basement remodeling guideline runs inspections through the rough-ins, insulation and a final, but none of them look inside the ductwork, and taping and sanding happen after the rough inspections are signed off. If the furnace ran while joint compound was being sanded, the returns pulled the powder in and it packed onto the blower wheel and into the return trunk. Remodels also leave solid debris: drywall screws, wire nuts, wood shims and chunks of compound that fell into floor boots before the registers went back on. This is the case post-renovation duct cleaning is built for.

Mice and their leftovers

Mice look for warm places once Illinois nights turn cold. Illinois Extension says they get through openings of about half an inch, and that their urine and droppings carry viruses, bacteria and allergens; a dropping the size of a grain of rice is the usual sign. In a duct system they favor basement returns, panned joist cavities and abandoned branches left from an old furnace swap. The CDC says not to sweep or vacuum droppings or nesting material, and to soak them first with a disinfectant such as 1.5 cups of bleach in a gallon of water. Inside ductwork that means sealing entry points, then a full air duct cleaning with the system held under negative pressure so nothing is blown back into the rooms.

Moisture and mold

EPA says moisture should not be present in ducts at all, and that controlling it is the most effective way to prevent mold. In a Wheaton house the summer air conditioner is the usual source. O'Hare's normals list about a thousand cooling degree days a year, enough for the coil to sweat for months, and a clogged condensate pan or an uninsulated supply run through a damp crawlspace can keep a section of duct wet. Visible growth on sheet metal or the coil is one of EPA's three reasons to clean, after the water problem is fixed.

Old materials in an old house

About 62 percent of Wheaton's homes were built before 1980, and some of the oldest still hold parts of their first heating systems. EPA lists oil and coal furnaces with asbestos insulation and pipes covered with asbestos blankets or tape among the places asbestos turns up in homes. Its advice is to leave undamaged material alone and not touch anything suspect. A competent crew that finds old wrapped piping or furnace insulation near the work stops and says so rather than brushing through it.

Checking for yourself

With a screwdriver and a phone, remove a supply register in a bedroom and photograph down the boot with the flash on. Do the same at a return grille near the floor. Pull the filter and look at the slot for gaps. Then compare:

• Gray, even dust on the return side and a cleaner supply: normal, and a filter question.
• Dust caked on supplies too: look for air bypassing the filter.
• Dark specks the size of rice, shredded paper or insulation: mice.
• Fuzzy or spotted growth on bare metal, or a wet drain pan: moisture first, then cleaning.
• White powder or hardware in the boots: remodel debris.

What to do with what you find

Ordinary dust calls for a good filter, not a crew. Droppings, growth or construction debris call for a cleaning that reaches every return, supply and trunk, and a dusty blower wheel is handled through furnace blower cleaning. The pricing page lists the posted Chicago-area ranges and the scope each range assumes.