How HVAC Systems Spread Mold Through a House

Air conditioners make water on purpose. That water should leave through a clear drain. When the pan holds water, the cabinet stays filmed with dust and biofilm. The blower does the rest. If the house smells only when the fan runs, start at the cabinet, not at a bedroom vent cover you can wipe in five minutes.

Metal ductwork downstream is often just dusty. It can be vacuumed. Fiberglass liner that is fuzzy and stained usually has to be replaced because the liner itself is the food. Flex duct that held standing water is trash. We will say which one you have instead of selling you a house-wide replacement by default.

Older houses sometimes use a joist bay or a wall cavity as a return. If that bay is also a damp crawl or a moldy stud space, the return is a vacuum for the problem. Panning over a wet basement is the same idea. Filtering the bedroom does nothing while the return is inhaling the cellar.

During remediation we seal returns so demolition dust is not distributed. Afterward, the fan should stay on auto until the mechanical room is actually dry. 'On' is not fresh air. It is the basement, reheated.

A scented fog through the ducts is not cleaning. It is a smell on top of a dirty coil. A better filter helps dust. It does not dry a drain pan. UV lights are a maybe, later, after the mechanical room is dry and the coil is clean — not a first purchase.

Change the filter on a schedule you will keep. If you want a number: look at it monthly in allergy season and replace it when it looks loaded, not when a sticker says a year from now.