Why does ventilation smell, and how do you fix it?

1. Deposits in the ducts

Grease and organic dust settled on the duct walls eventually start to break down. That is what produces the heavy, stale smell that gets stronger when the system runs. The only fix is mechanical cleaning followed by disinfection.

2. Moisture and mould

A recovery unit, an air conditioner evaporator and a condensate system are permanently damp. Left uncleaned, mould settles there and the smell goes straight into the supply air.

In this case the odour is often strongest in the morning, when the equipment first starts — a useful clue for telling the causes apart.

3. Odours from neighbouring spaces

In apartment buildings and shopping centres smells travel between units through a shared shaft, especially where the system leaks or the balance has been altered. Cleaning only partly helps here — draught and tightness need checking, which is what a camera survey is for.

4. A dried-out trap or refuse chute

The most mundane and most often forgotten cause: the water seal in a rarely used floor drain or trap evaporates and sewer gas enters the room. In apartment buildings a neglected refuse chute adds to it.

What to do first

Masking the smell with air fresheners solves nothing. The order that works: find the source, clean mechanically, disinfect, and only then ozonate if the odour has soaked into materials. Ozone without cleaning buys a temporary effect.