Photograph of rust and leakage inside of an air conditioner Moldy or Wet Air Handlers
Air Conditioning Air Handler Condensate Leaks, Causes of Mold Problems in HVAC Blower Compartments & Ductwork

  • BLOWER LEAKS, RUST & MOLD - CONTENTS: Mold contaminated air conditioning ducts or air handlers: how to find, cure, and prevent moldy ductwork and HVAC systems. Air Conditioning air handler blower fan unit (AHU) leaks and risk of toxic mold. Air conditioning air handlers - Fan Coil Unit Inspection, Diagnosis, Repair, Replacement
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Air handler blower unit leaks & mold hazards: this air conditioning repair article discusses the problem of A/C condensate leaks into air conditioning system air handler units, blower units, or AHU's, (also called fan coil units).

We include the air conditioner blower fan, air conditioning system filters in this discussion, and we explain the causes, cures, and prevention of air handler condensate or other moisture leaks that lead to rust, damage, and mold contamination in the air conditioning system. The evaporator coil and problems of frost build-up in the air handler are also reviewed.

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BLOWER LEAKS, RUST & MOLD - Air Conditioner Air Handler Rust, Condensate Leaks, Wet Insulation, & Mold Hazards

Photograph of leaks and rust in an air handlerThe photograph shows quite a bit of rust on components inside this air handler. More significantly were stains indicating that condensate had blown off of the evaporator coil or otherwise spilled into the air handler cabinet where it had wet fiberglass insulation there.

[Click to enlarge any image]

Wet fiberglass and other wet insulation material form a potential mold reservoir right in the air path.

If an air conditioning system also lacks a good air filter and thus if it also has become loaded with organic house dust and debris, combined with water where we don't want it this material also forms a possible growth medium for toxic or allergenic mold inside the air handler or duct work.

Photograph of air conditioner condensate overflow inside of an air handler Photograph of wet moldy air conditioner fiberglass insulation

These two photographs show more clearly that condensate has been overflowing the condensate collector inside the air handler.

This air conditioning condensate water has gone where we don't want it: onto the fiberglass insulation and into the duct system. The moldy looking material growing on the foil face of the insulation was a rather common Cladosporium cladosporoioides mold which we often find inside air handlers and duct systems that have been wet like this.

The right hand photo is a close up showing evidence that the fiberglass insulation has been wet in this area - note the rust stains? We often find more problematic molds in the Aspergillus sp. genus where fiberglass insulation has been wet, even when no mold was visible to the eye.

Mold on HVAC Air Supply Registers

By simple visual inspection (by the naked eye) it can be difficult to know if the black or gray debris on air supply registers is moldy crud or common house dust (see Stains HVAC Supply Registers). . Our photos below show debris deposited on HVAC ceiling air supply registers in a Kentucky building.

In response to a history of a building leak event and odor complaints as well as the appearance of mold on some building walls, we analyzed samples of the dust from the ceiling air supply register shown at left. At right you can see that the dominant particle in the sample was Cladosporium sp. C. sphaerospermum spores were also present in this sample.

Mold on an air supply register (C) Daniel Friedman Photograph of wet moldy air conditioner fiberglass insulation

While Cladosporium sp. is the most common mold found on earth and while it's just about everywhere, for some people this is an allergenic mold. We don't want it being blown around by our air conditioning system nor its growth improved by mishandling of HVAC condensate.


and/or see Mold Atlas & Particles List for a description of the health effects and air quality complaints associated with various kinds of mold.

Don't panic even if you do find small amounts of mold in an air supply register. Even in a healthy building we might expect condensate forming on ceiling air supply registers in some conditions, and thus an accompanying growth of small areas of mold. If that's the extent of mold contamination, ordinary household cleaning procedures are sufficient.

Watch out: if the building leak history or complaint history suggest that a larger mold reservoir could be present, including other genera/species of mold, further investigation is warranted.

Photographs of mold growing right on an air conditioner squirrel cage blower fan are in our article
at Mold Growth in Air Handlers.

More information about the potential of problematic mold growth in fiberglass and more example photographs of this event can be read at Mold in Fiberglass in Insulation at this website.


Continue reading at PARTICLE & MOLD LEVELS in DUCTWORK or select a topic from the More Reading links shown below. or select a topic from the More Reading links shown below.


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