Air Duct Cleaning: What It Does and Doesn't Do

How a forced-air system moves dust

A furnace or air handler pulls room air in through return grilles, pushes it through a filter, across a heat exchanger or cooling coil, and back out through supply ducts and registers. The fan housing, the coil and the drain pan under the coil are part of that loop, along with the ducts.

Dust that gets into the loop tends to settle where air slows down or changes direction, and on any surface that stays damp. That is why the EPA's list of what duct cleaning should reach includes far more than ducts: supply and return ducts and registers, grilles and diffusers, heat exchangers, cooling coils, drain pans, the fan motor and housing, and the air handler housing. The EPA warns that leaving any of those out can let the cleaned parts be re-contaminated.

The EPA also points out that much of the dust in ducts sticks to duct surfaces and does not necessarily enter the living space, and that cooking, cleaning, smoking and outdoor pollution can matter more to indoor particle levels than dirty ducts. Our air duct cleaning page covers what that means for a service visit.

Deciding whether to clean: a path to follow

The EPA's guidance can be turned into a short sequence. Work through it before you book anyone.

1. Can you see a problem?Look in a few supply registers and the return grille with a flashlight. If you see substantial growth that looks like mold, signs of rodents or insects, or debris that puffs out when the fan starts, go to step 2. If you see a light coating of dust and nothing else, the EPA says cleaning is probably unnecessary when no one has allergies or unexplained symptoms. Change the filter and stop here.
2. Is it really mold?The EPA notes that only an expert can confirm mold, sometimes with lab analysis of a sample, and that some parts of a system are hard to inspect. If it matters to your decision, ask that the growth be shown to you and consider having it tested.
3. What caused it?Mold means moisture, and pests mean an opening. The EPA says the cause must be corrected before cleaning, or the problem will likely recur. Find the leak, the condensation or the gap first.
4. Clean, replace or both?The EPA says wet or moldy fiberglass insulation cannot be effectively cleaned and should be replaced, and that accessible flex duct may cost less to replace than to clean. Settle this before any cleaning starts.

What a thorough job reaches

The EPA describes several practices that separate a complete cleaning from a quick pass through a few registers:

• Access to the whole system. The provider opens access points so every part of the system can be cleaned and inspected.
• A check for asbestos. The EPA says materials that may contain asbestos must be checked for, and handled only by trained specialists if found. Older systems are the ones where this comes up.
• Dust that leaves the house. Vacuum equipment should exhaust outdoors, or use HEPA filtration if it exhausts inside.
• Gentle tools on soft surfaces. Only soft-bristled brushes on fiberglass duct board and on sheet metal ducts lined with fiberglass.
• Sealed openings. Any access holes cut into ducts are sealed and re-insulated so they are airtight.
• Protection for the house. Carpets and furniture are covered during the work.

The EPA also says moisture-based methods such as steam cleaning should not be used on ductwork.

Checking the work afterward

The EPA gives homeowners a post-cleaning checklist. Several items are things you can look at, or ask to be shown, before the crew leaves:

• Both sides of the cooling coil are visibly clean, and light passes through the coil.
• The drain pan under the coil drains properly.
• The blower compartment and the return plenum are free of debris.
• Access doors are secured with something more than duct tape.
• The system runs properly in both heating and cooling modes.

Ask for photos of the coil, blower and a few duct runs before and after. A coil you cannot see through after a cleaning is a sign the job stopped short.

Run the system for a day and look at the registers again. Debris still blowing out of one branch can mean a section was missed or that dust is still getting into the system somewhere. Either one is worth raising with the provider while the job is fresh.

Sprays, ozone and sealants

Some providers offer chemical treatments. The EPA's points are specific. It states that no products are currently registered by the EPA as biocides for use on fiberglass duct board, and that using any product in a way that does not match its label violates federal law. It describes ozone as a lung irritant whose use in ducts is controversial. On sprayed sealants, it notes that lab tests suggest they may not fully coat ducts, that their long-term safety is not well understood, and that they should never be used on wet duct liner, over actively growing mold or over debris.

Sealing leaks in ductwork to save energy is a different job from spraying a coating inside. ENERGY STAR adds a caution that applies to both: tightening a house or its ducts can, in some cases, make fuel-burning appliances like water heaters backdraft. If a duct sealing project is planned in a house with a gas or oil furnace, water heater or fireplace, ask how combustion safety will be checked.

Keeping ducts clean: moisture comes first

The EPA calls moisture the most effective factor to control. Its prevention list reads like a maintenance plan:

• Repair leaks and water damage promptly.
• Make sure the cooling coil's drain pan slopes toward its drain.
• Insulate ducts that run through attics and crawl spaces, where warm humid air meets cold metal.
• Size air conditioners correctly, since oversized units cycle poorly and remove less moisture.
• Maintain humidifiers strictly by the manufacturer's instructions.

Dust control is the other half: use the highest-efficiency filter your equipment's manufacturer recommends, change it on schedule, make sure air cannot bypass it through gaps, ask for the coil and drain pan to be cleaned during routine service, and seal registers and turn the system off during dusty renovation work.

Ducts are not the flue

Duct cleaning deals with the air you breathe as it circulates. It does not touch the path that combustion gases take out of the house. A gas or oil furnace or boiler sends its exhaust through a vent pipe and a chimney flue or a sidewall vent, and that path has its own failures: soot, fallen debris, corrosion and blockages that can push carbon monoxide back into the house.

The EPA's duct cleaning page makes the same separation in a different way. Whether or not you clean the ducts, it recommends that furnaces, stoves and fireplaces be inspected for proper functioning and serviced before each heating season to protect against carbon monoxide poisoning. For the flue side, see furnace and boiler flue cleaning, oil furnace and boiler flues and carbon monoxide alarms.

Questions homeowners ask

Sources

1. U.S. EPA: Should You Have the Air Ducts in Your Home Cleaned?
2. ENERGY STAR: Identify the problems you want to fix (sealing and insulating)

This guide is general information. It can't diagnose a specific chimney; an inspection can.