Where mould grows · 1 min read
Mould in commercial HVAC systems: the building-wide multiplier
How one damp air handling unit distributes spores to every room it serves, and why duct cleaning alone rarely resolves it.
A localised mould problem stays localised until it reaches the air handling system. After that it is a building-wide problem, and it usually stops being obvious where it started.
Why plant rooms grow mould
Cooling coils condense moisture by design. Drip trays hold water. Insulated duct interiors stay dark and, if a drain blocks or a tray corrodes, damp. Filters accumulate organic dust. Every condition mould needs is present in normal operation.
The distribution problem
Once growth establishes in the AHU or supply ductwork, every hour the system runs pushes spores into every space it serves. Complaints appear on floors with no visible mould at all, which is why building managers so often chase symptoms in the wrong rooms.
Why duct cleaning is not enough
Mechanical cleaning removes accumulated debris from accessible runs. It does not reach flexible branches, plenum spaces or the underside of insulation, and it does not address airborne load already distributed through the building. Diffusion treatment reaches the spaces cleaning cannot, and works best scheduled alongside mechanical cleaning and a fix for whatever is holding water.