Poor indoor air quality usually comes down to four things your heating and cooling system has a hand in: a filter that’s too dirty, too coarse or a bad fit; ductwork that’s dirty or drawing in dust through leaks; moisture that lets mold grow inside the equipment; and not enough fresh air. The signs are dust that returns fast, allergy symptoms that are worse at home, musty or stale odors and visible mold. Some fixes cost next to nothing; others need a technician.
Two warnings before anything else. If the CO alarm goes off, or more than one person at home feels headachy or sick at the same time, get everyone outside and call 911. Gas appliances, the furnace and water heater included, can release carbon monoxide when venting or combustion goes wrong, and you can’t smell it. And if you smell gas, leave first and make the call from outdoors.
From complaint to cause
| Symptom | Likely cause | What it takes |
|---|---|---|
| Dust settles again days after cleaning | Dirty or loose-fitting filter; leaky return ducts | A filter that seals in its slot; duct inspection and sealing |
| Allergies flare indoors | Pollen, dander and dust circulating | Finer filtration; air purification |
| Musty smell when the system starts | Mold on the coil, in the drain pan or ducts | Coil and pan cleaning; drain repair; treatment |
| Mold spots around registers | Condensation from damp air or a cold duct | Moisture source fixed; cleaning |
| Air feels stale, cooking smells hang around | Too little ventilation | Exhaust fans checked; ventilation improvements |
| Debris puffing out of the vents | Dirty ducts, a torn filter or a gap around it | Duct cleaning; filter rack repair |
The filter: cheapest fix, most often wrong
No single part does more for your air. Three mistakes are common: the filter stays in too long, it’s the wrong size so air sneaks around the edges, or it’s so dense that the blower can’t push enough air through, which hurts heating and cooling too. A pleated filter that fits snugly and gets changed on schedule does more than an expensive one that’s bent or overdue. If you want finer filtration than your filter slot allows, have the system looked at first; a deeper filter cabinet with more surface area is often the answer. The guide on how often to replace an HVAC filter covers timing.
Ducts: when cleaning pays off
Air duct cleaning runs $299 to $599. It makes sense when the ducts hold real dust or debris, after a remodel that sent drywall dust through the system, or after rodents or insects have been inside. It does little for ducts that only look dusty at the grille. Leaks often matter more: a return duct with gaps in the attic or crawlspace pulls in dust and insulation fibers and sends them into the rooms. Sealing those gaps is often what finally stops the dust.
Moisture and mold
Mold needs water. In a heating and cooling system the wet parts are the indoor coil and its drain pan, which collect condensation every time the AC runs. A clogged drain, a rusted pan or a coil that never dries can grow mold and send a musty smell through the house. Professional cleaning and treatment of the coil and pan, plus a drain that flows, fixes most of it. When the house itself stays damp, a dehumidifier or a whole-home solution may be needed; a dry house in winter is the opposite problem. Our humidifier installation page explains which one a home needs.
Fresh air
Homes need some outside air. Bathroom and kitchen exhaust fans that really vent outdoors carry moisture and odors away. When stale air lingers even with working fans, ventilation improvements may be needed, and a technician can assess the options.
Your own checklist
- Put in a new filter and confirm the size printed on its frame matches the slot.
- Look at the pan and drain line under the indoor unit for standing water or slime.
- Run bathroom fans during showers and for a while after.
- Keep return grilles clear of furniture, dust and pet hair.
- Test your carbon monoxide alarms.
What the fixes cost
An AC maintenance visit, $89 to $199, is the natural time to have the coil, drain pan and filter looked at. Duct cleaning runs $299 to $599. Whole-house air purification ranges from $500 to $3,000 and more, depending on the system. You see the price before any work, and equipment is recommended only once the cause is clear.
Local note
San Jose summers are hot and dry, and many homes keep the windows shut and the AC running for months, so whatever the filter misses keeps circulating. In older homes with original ducts, such as many in Cambrian, leaky returns in the attic are a common source of dust. Larger homes in Almaden Valley often do better with whole-house air cleaning, since a portable purifier covers only one room.