Bad Ducts = Bad Air

If your home has forced-air heating or cooling, the ducts that move that air can also move dirty air into your living space. Ducts either supply conditioned air to rooms or pull house air back to be reheated or recooled, and both sides run under pressure. When ducts leak or aren't sized and balanced right, they can draw in air from attics, crawlspaces, basements, or garages — spaces that often hold mold, rodent droppings, insulation fibers, pesticides, radon, or fuel vapors. A common trouble spot is a disconnected return duct in a crawlspace, which can pull in all of that plus whatever else is down there.

Leaky ducts near a water heater, furnace, boiler, or fireplace create another risk: enough negative pressure can stop exhaust gases from rising up the flue, pushing them back into the house instead. That's called backdrafting, and it can raise carbon monoxide levels, which at high amounts is deadly and at lower levels can cause headaches and fatigue. Leaks are common at unsealed joints, disconnected sections, older sheet-metal "panned" returns built from floor joists, and gaps around air handlers and filters. Leaky ducts also let in humid outside air, which can raise indoor humidity and encourage mold, especially if the air conditioner is oversized and shuts off before it fully dehumidifies the air.

Properly checking and adjusting a duct system after installation — called commissioning — involves measuring airflow, static pressure, and leakage to confirm supply and return air match up. It's a step worth asking about if you're getting new HVAC equipment or ductwork installed.