Does Your Spokane Home Smell Musty? It Might Be the Ducts

When a home takes on that damp, “old basement” smell, the HVAC system is usually involved. At Air Duct Cleaning Spokane in Spokane, WA, we routinely trace musty odors to microbes and moisture inside return paths, coils, drain pans, and duct liners—and to leaky returns that pull crawlspace air into the system. With disciplined inspection, moisture control, and standards-based cleaning, the odor can be eliminated at its source and kept from coming back.

Musty Odor 101: Why the HVAC Path Is a Prime Suspect

Microorganisms release microbial volatile organic compounds (mVOCs) that create the classic “moldy” odor. Environmental and public-health agencies note that a persistent musty smell is a warning sign that active growth may be present and should be investigated. Industry guidance recommends duct cleaning as needed—when there’s visible growth on hard surfaces, recurring odors tied to blower operation, vermin, heavy debris, or after moisture events—not on an arbitrary schedule.

Fast Self-Checks You Can Do in 15 Minutes

• Run the blower: If the smell spikes within seconds of the fan starting, suspect the return path, evaporator coil, drain pan, or duct liner—not carpets or closets.

• Check the filter: A collapsed, saturated, or bypassing filter lets debris and spores load the coil and ducts.

• Pop a register cover: Use a flashlight to look for matting, slime, or fuzzy growth near supply openings.

• Open the air handler panel (power off): Look for standing water or slime in the condensate pan; a clogged pan is an odor source.

• Measure indoor humidity: Keep 30–50% RH (and below 60% at all times) to starve mold of moisture.

What’s Usually Causing the Smell (and How We Confirm It)

1) Wet or Moldy Porous Duct Liner/Insulation

If fiberglass liner or ductboard gets wet and colonized, cleaning alone won’t stop regrowth. Industry bodies (EPA, NADCA, NAIMA) advise replacement of wet or mold-contaminated porous duct materials rather than “treating” them.

2) Slime in the Condensate Drain Pan and on Coils

Drain pans must self-drain and be inspected and cleaned at least annually to prevent microbial buildup. Operations and maintenance standards (ASHRAE, NIOSH) emphasize clearing blockages, correcting slope, and removing biofilms on pans and coils.

3) Leaky Return Ducts Pulling in Crawlspace/Basement Air

Leaky returns in unconditioned spaces (attics, crawlspaces) can draw in odors, moisture, and soil gases and distribute them through the house. Building-efficiency programs (DOE EnergySaver, ENERGY STAR) recommend sealing with mastic or UL-listed foil tape, not cloth duct tape.

4) Inadequate Ventilation and Pressure Imbalances

Insufficient outdoor air and unbalanced duct pressures concentrate pollutants and drive infiltration from “smelly” zones. Residential ventilation standards (ASHRAE 62.2) and indoor-air guidance highlight the need for balanced supply/exhaust and dedicated spot ventilation.

Health Context: Why Musty Air Matters

Spending time in damp buildings is associated with respiratory symptoms, asthma development or worsening, hypersensitivity pneumonitis, and allergic rhinitis. Public-health agencies (CDC/NIOSH and Washington State Department of Health) stress solving moisture problems promptly and thoroughly.

Spokane-Specific Notes (Crawlspaces, Rentals, and Codes)

Many Spokane homes have vented crawlspaces. When returns leak, those spaces become “unplanned air suppliers.” Sealing ducts and maintaining low indoor RH are the best defenses. For rentals, Washington’s health guidance for landlords outlines responsibilities to address leaks and ventilation defects that cause moisture problems.

Our Diagnostic Workflow (What “Good” Looks Like)

1. Interview and odor mapping to tie odor strength to fan on/off and time of day.

   
   

2. Humidity and temperature logging in living areas and at returns.

   
   

3. Visual inspection of coils, drain pan slope/outlet, insulation condition, microbial growth indicators, and any UVGI equipment.

   
   

4. Pressure and leakage tests (blower door/duct pressurization) to locate return-side leaks in crawl/attic runs.

   
   

5. Particulate and debris assessment and lint loading at registers.

   
   

6. Moisture tracing (condensate backups, plumbing leaks, rain intrusion).

   
   

7. Plan of correction aligned to recognized standards, including the NADCA ACR Standard.

   
   

For cleaning, we follow NADCA ACR with containment, HEPA-filtered negative pressure, mechanical agitation, and cleanliness verification.

Fix the Source, Then Clean: A Proven Odor-Removal Plan

1) Control Moisture and Replace Unsalvageable Porous Materials

Fix leaks and drainage issues first. Remove and replace wet or mold-contaminated duct liners and insulation. Moisture control is the foundation of lasting odor remediation.

2) Restore Proper Drainage and Clean the Pan/Coil

Verify positive pan drainage and clean the pan/coil assembly. Correct pan pitch, unclog the drain, and scrub away biofilms. Consider adding a float switch to prevent overflows.

3) Seal the Duct System (Especially Returns in Crawl/Attic Runs)

Use mastic or UL-listed foil tape on seams, boots, and return plenums; insulate accessible runs. Sealing prevents odor-laden infiltration and improves efficiency.

4) Balance Ventilation and Airflow

Meet minimum ventilation targets and avoid negative-pressure imbalances that pull in crawlspace air. Ensure bath and kitchen exhausts vent outdoors and are used consistently.

5) Upgrade Filtration (If the System Can Handle It)

Use the highest-rated filter your blower and filter slot allow. MERV-13 to MERV-16 media can remove a high fraction of fine particles that pass through the filter, but only if the system can maintain airflow without excessive static pressure.

6) Hold Indoor Humidity at 30–50% RH

This is the sweet spot to deter mold and dust mites. Use whole-house dehumidification if central AC cycling is insufficient, and run spot exhaust during moisture-producing activities.

What It May Cost (So You Can Budget Wisely)

• Air-duct cleaning (national): Typical homeowner spend ranges roughly $270–$500+, with higher bills for large or heavily contaminated systems.

• Typical Spokane ranges: Many two-story homes fall around $330–$400, varying by vent count, access, and contamination level.

• Seal and repairs: Return-side sealing and minor access fixes can range from a few hundred dollars to more for complex crawlspace runs.

• Remediation of contaminated duct liner: Replacement costs depend on footage and accessibility; budgeting in the low thousands is common for extensive liners.

Priority order for best ROI: fix moisture and drainage → seal returns → clean to standard → upgrade filtration and humidity control.

Maintenance Checklist to Keep Musty Odors Gone

• Replace filters on schedule; inspect coil and drain pan each cooling season for slime or standing water.

• Keep RH 30–50%; use bath and kitchen exhaust, fix plumbing leaks, ventilate damp rooms, and consider a dehumidifier during shoulder seasons.

• Recheck sealed returns and boots annually (especially in crawl/attic runs).

• During wildfire season, consider MERV-13 filtration (if compatible) or a DIY box-fan cleaner for a designated clean room.

• Schedule periodic inspections aligned with NADCA ACR and ASHRAE O&M practices.

CONCLUSION

Musty odors aren’t “air freshener problems”—they’re moisture and airflow problems that often originate in the HVAC path. Confirm the source (especially returns, coils, pans, and porous liners), correct moisture and pressure issues, and then clean to the NADCA ACR standard. Lock in prevention with duct sealing, right-sized ventilation, high-efficiency filtration your system can handle, and tight humidity control (30–50% RH). Do that, and the musty smell goes away—and stays away.

For why one room runs hotter than the rest in summer—and how to fix it—see this guide.

FAQs

1) Is a musty smell dangerous?

A musty odor signals dampness and potential microbial growth. Damp buildings are linked to respiratory symptoms and asthma. Solve moisture issues first, then remediate contaminated materials.

2) Will duct cleaning alone remove the smell?

Not if porous liners are moldy, the drain pan is slimed, or returns leak from a crawlspace. Cleaning helps, but moisture control and sealing leaks are essential to prevent recurrence.

3) What filter should we use to fight odors and particles?

Use the highest MERV your system safely supports. Many residential blowers can handle MERV-13 with proper sizing; confirm static pressure and airflow to avoid performance losses.