What Does Mold in Ductwork Look Like?
Mold in ductwork is one of the more concerning things a homeowner can discover — and one of the harder ones to identify without professional help. Here’s what to look for, what causes it, and what to do if you find it.
What Mold in Ducts Looks Like
Mold in ductwork typically appears as:
- Black, green, or grey spots on duct surfaces, register covers, or visible duct walls near register openings
- Fuzzy or powdery patches on insulated flexible duct material — mold grows readily on the organic-based insulation
- Dark discolouration around register covers, particularly on ceiling registers where condensation can form
- Visible growth inside the register boot (the metal box connecting the duct to the register opening) — easily checked with a flashlight
However, not all discolouration in ductwork is mold. Rust, mineral deposits, and dark dust can look similar. The definitive test is professional inspection or sampling — visual identification alone can be misleading.
The Smell Is Often the First Indicator
Before you see mold, you’ll usually smell it. A persistent musty, earthy, or mildew-like odour coming from your registers — especially when the furnace or AC runs — is the most common first sign. Unlike the dusty smell that’s normal at the start of heating season, a musty smell doesn’t clear after a few cycles. It returns every time the system runs.
According to the EPA, mold in HVAC systems distributes spores throughout the living space with every cycle — making a persistent musty smell worth investigating promptly rather than dismissing.
What Causes Mold in Edmonton Ductwork
Moisture at the Evaporator Coil
The evaporator coil in your air conditioner or heat pump gets cold during operation and naturally attracts condensation. The drain pan beneath the coil collects this water. If the drain pan clogs or the drain line backs up, standing water creates ideal mold conditions directly in the air path. This is the most common source of mold in HVAC systems.
Humidifier Issues
A whole-home humidifier with a failing water panel or improper drainage can introduce excess moisture into the duct system. Humidity levels above 50–55% in ductwork create conditions for mold growth on duct surfaces.
Condensation on Cold Duct Surfaces
Poorly insulated ducts running through cold spaces (attics, crawlspaces, unconditioned basements) can develop condensation on the exterior that eventually infiltrates the duct system. In Edmonton’s climate, this is particularly relevant for ducts in attics during the heating season when significant temperature differentials exist.
What to Do If You Find Mold
Small amounts of surface mold on register covers or the accessible face of register boots can sometimes be addressed with an antimicrobial cleaner and improved moisture control. Mold inside the duct system — visible on duct walls, near the evaporator coil, or throughout multiple registers — requires professional remediation.
Standard duct cleaning addresses dust and debris but isn’t designed as mold remediation. If mold is suspected or confirmed inside ductwork, a professional assessment is the first step — a technician can determine the extent of growth and recommend the appropriate course of action.
Noticing musty smells or visible discolouration in your registers? Get a free quote for an assessment from Total Duct Cleaning or call (780) 670-0400.
