Mold Found Inside One HVAC Duct Branch in a Dallas Home

The Situation

The homeowner's HVAC contractor had seen material in a supply duct that appeared to be either microbial growth or accumulated dirt. The homeowner requested independent mold evaluation and testing.

The limited assessment included visual inspection of supply grills, borescope views through grill louvers, inspection of accessible attic HVAC components and duct jackets, indoor/attic/outdoor temperature and humidity readings, one indoor and one outdoor air sample, and a surface swab from the left playroom supply grill.

What Was Inspected

• No supply grills were removed, ductwork dismantled or cut open, or building materials removed. Only interior areas visible by borescope through the grill openings were inspected; the full length of the playroom duct branch was not directly viewed.
• Accessible air-handler surfaces, selected other supply grills, their borescope views, and accessible duct runs were compared with the affected playroom branch. Clean accessible views do not establish the condition of every concealed surface.
• Air samples described the playroom and outdoor comparison locations at the time of collection, not all operating or disturbance conditions.

What We Observed

The left playroom supply grill showed visible growth on its face and louvers. A borescope inserted through its louvers showed growth on accessible interior duct surfaces behind the grill. A surface swab was collected from the grill face, and an indoor air sample was collected in the playroom.

The right playroom grill also showed visible surface growth; borescope viewing showed visible growth on accessible surfaces behind it. Tracing the ductwork found that both playroom grills were supplied by the same single branch. Only the left grill was swabbed.

Other inspected supply grills on the same system had no comparable visible growth, and their borescope views showed no comparable discoloration or growth. The bathroom grill's minor surface discoloration was a separate observation with a clean-appearing borescope view behind it.

What Was Found at the Playroom Vents?

Both playroom supply grills had visible growth on their faces. The left grill's louvers also showed growth and supplied the sole surface swab in this assessment. The right grill was not separately sampled. An air sample was collected in the playroom, but neither grill was removed.

Borescope Inspection Showed Growth Inside the Duct Branch

A borescope passed through the existing louvers of each playroom grill showed visible growth on interior duct surfaces behind both openings. This was a visual finding within one shared branch, not a laboratory swab of the entire duct interior.

The duct was not opened or dismantled. The borescope did not document every surface along the branch's full length.

Moisture / Building Conditions

The attic measured 101.1°F with 38.5% relative humidity and a 71.1°F dew point. Indoors: 75.0°F, 49.8% relative humidity, 55.0°F dew point. Outdoors: 95.2°F, 36.3% relative humidity, 61.3°F dew point.

The damaged outer jacket and insulation could allow exchange between the attic and duct air. The readings and visual inspection supported a probable explanation rather than a direct measurement of leakage.

Sampling Performed

• Surface swab, direct microscopic examination — Face of the left playroom supply grill
• Indoor air sample — Playroom: One sample compared with an outdoor air sample at the time of assessment.
• Outdoor comparison air sample — Exterior

Laboratory Findings

What the Findings Suggested

The swab's Heavy Cladosporium, together with visible growth on the left grill and visible growth behind it, supported fungal growth at the sampled grill. Growth inside the shared branch was directly observed by borescope behind both playroom grills, but no interior-duct material was separately sampled and the entire branch was not inspected.

The playroom air results did not show fungal amplification relative to the outdoor comparison at the sampled time. They cannot establish that no mold was present or predict conditions during HVAC operation or disturbance.

The distribution of visible findings, damaged branch, and attic conditions made air exchange through its jacket and insulation the most probable explanation, not a conclusively established cause.

Why Was Growth Limited to One Duct Branch?

The playroom branch had substantial damage to its outer jacket and insulation, with openings near the air handler and elsewhere along the run. These openings could permit exchange between attic air and conditioned air within the duct.

By contrast, the accessible air handler appeared clean. Other inspected supply grills and their borescope views showed no comparable growth, and their duct runs originated from a different side of the air handler without comparable observed jacket damage. This supports a localized finding, not a claim that the entire HVAC system was contaminated or fully inspected.

Attic and Indoor Conditions

At inspection, the outdoors-ventilated attic measured 101.1°F, 38.5% relative humidity, and a 71.1°F dew point. Indoor readings were 75.0°F, 49.8% relative humidity, and a 55.0°F dew point; outdoor readings were 95.2°F, 36.3% relative humidity, and a 61.3°F dew point.

The attic readings provide context for the report's probable-source assessment. They do not prove that air passed through the damaged jacket or that a particular amount of moisture entered the duct.

How Was the Bathroom Grill Different?

A bathroom supply grill had a small amount of surface discoloration consistent with microbial growth. Its borescope view showed no growth or discoloration in the accessible duct immediately behind it. The assessment interpreted the grill-face condition as consistent with localized condensation or humidity, not growth originating in the playroom branch or in that bathroom duct. The bathroom grill was not separately sampled.

Three Different HVAC Inspection Findings

Other published inspections provide useful contrasts: one found growth on the exterior of AC condensate piping without visible growth inside accessible ducts; another confirmed growth on accessible surfaces inside a supply plenum. Here, the observed interior condition was limited to accessible portions of a single damaged duct branch. These are separate properties, not stages of one system-wide problem.

What Homeowners Can Learn From This Case

Discoloration at a vent does not establish that every duct or the air handler has mold. Here, borescope views differentiated a shared playroom branch with visible interior growth from other inspected runs and a bathroom grill with surface-only discoloration.

An investigation can compare vent faces, visible duct interiors, branch routing, attic conditions, jacket and insulation integrity, targeted surface findings, and time-specific air samples. A lower indoor air result did not undo the direct visual and grill-swab findings in this case.

Questions About This Case

Related Services

• Dallas mold inspection
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