In many homes, not much. That is the part most duct cleaning companies leave out, and it is worth saying plainly before anything else.
The EPA does not recommend duct cleaning as routine maintenance performed on a schedule. A layer of dust on the inside walls of your ductwork is not, by itself, causing a problem.
But there are specific situations where leaving a system alone does lead somewhere, and those are worth knowing.
What Actually Accumulates Over Time
Ductwork collects material from the air moving through it and from the house around it: household dust, fabric fibres, skin cells, pet hair and dander, pollen tracked in on shoes and clothing, and lint.
The returns collect faster than the supply side, because they pull air inward continuously.
Most of this settles and stays put. That is why a system can go years without cleaning and still work exactly as it did.
When Leaving It Alone Becomes a Problem
Moisture gets in
This is the one that matters most. If water enters the duct system through a leak, condensation, or a humidity problem, settled dust gives mould something to grow on.
The EPA is direct about this: if the conditions causing growth are not corrected, it returns after cleaning. The moisture source has to be fixed, not just the result.
Summers in the DMV are humid enough that condensation around ductwork is not unusual, particularly where ducts run through unconditioned basements or crawlspaces.
Pests find a way in
Rodents and insects get into duct systems through gaps, damaged sections and unsealed penetrations. This leaves droppings and nesting material, and it is one of the specific conditions EPA guidance names as a reason to clean.
Construction dust fills the system
Drywall dust, sawdust and insulation fragments from a renovation settle deep into supply and return runs. Plastic sheeting slows this down but rarely stops it. Unlike ordinary household dust, this arrives in volume and all at once.
Buildup gets heavy enough to be visible
At a certain point, material starts coming out of the registers when the blower starts. If you can see a puff of dust at startup, or dark buildup around the vent covers that comes back within days, the system is past the point where it holds what it collects.
Something comes apart
A disconnected duct inside a wall or ceiling, a crushed flex run, or a torn section means the system is drawing from or discharging into an unfinished space. This is a repair problem rather than a cleaning problem, but it often gets noticed during a cleaning inspection.
What Does Not Happen
Worth being clear about these, because they get claimed often.
Your energy bill does not steadily climb because of dust in the ducts. Heating and cooling costs are driven far more by equipment efficiency, insulation, thermostat settings and the building envelope.
Household dust does not mostly come from the ducts. It comes from inside the house and from outside air, which we cover in more detail in our guide on whether dirty ducts cause a dusty house.
And duct cleaning does not treat allergies, asthma or any other medical condition. We will not tell you it does.
So How Do You Know When It Is Time?
Not by the calendar. By the condition of the system.
Take a register cover off and look inside with a flashlight. Plain metal with a light film is normal. A matted layer, loose fibres, visible debris or drywall powder is not.
Think about the history. Renovation in the last year? Just moved in with no record of what was done before? Pets that shed heavily? Any of these move the answer toward having it looked at.
And notice what the system is doing. Dust puffing from a register at startup, a musty smell when the blower runs, or buildup that keeps returning around the same vents are all signs worth acting on.
If you want to handle the accessible parts first, see what you can clean yourself.
Getting It Looked At
If any of the situations above apply, an inspection tells you what is actually in the system rather than leaving you guessing.
We provide air duct cleaning in Washington DC and across the Maryland suburbs. We look at the system before recommending anything, and if it does not need cleaning, that is what you will hear.
Call 888-509-3828 or request an estimate.