Musty smell from your vents? Five causes and what duct cleaning fixes

A musty smell from your vents comes from one of five places: moisture in the return ducts, a dirty evaporator coil or air conditioner drain pan, a dirty humidifier, a dead rodent or a nest in the ductwork, or dust burning off the furnace the first time it fires in fall. Duct cleaning fixes the first, the fourth and the fifth outright and clears the way for the second and third. A wet coil or a cracked drain pan needs an HVAC technician as well.

Below is each cause, what it smells like, how to check for it, and whether a duct cleaning will deal with it.

1. Moisture in the return ducts

What it smells like. Damp basement, wet cardboard, old towels. It is strongest when the fan first starts and fades a little as the system runs, and it is worse in humid weather.

Why it happens. Return ducts pull air from the house back to the furnace. In many Toronto houses they run through the basement, along a cold exterior wall or under a slab, and the metal is cool enough for humid summer air to condense on it. Dust that has collected in the duct gets damp, and damp dust is where mould and bacteria grow. Health Canada's mould page says that mould may not be visible but may have a musty odour, and advises keeping indoor relative humidity between 30 and 50 percent.

How to check. Pull a return grille off and look at the first metre of duct with a flashlight. Dark streaks, a felt-like layer of grey dust, or visible condensation on the metal in summer all point here. Check the humidity in the basement with an inexpensive hygrometer.

Does duct cleaning fix it? Yes, for the smell. Our 12-step house process pressure-cleans every return run and the main trunks, then applies Benefect, a botanical disinfectant, through the system. That removes the damp dust the smell is coming from. The moisture itself needs a dehumidifier, better basement drainage or sealing the duct joints. The US EPA's page on duct cleaning puts it plainly: controlling moisture is the most effective way to prevent biological growth in air ducts, and if the conditions that caused mould growth are not corrected, it will recur.

2. A dirty evaporator coil or AC drain pan

What it smells like. Sour, slightly sweet, sometimes like dirty socks. It shows up when the air conditioning runs, often within a minute of the compressor starting, and goes away when the system switches to heat.

Why it happens. The evaporator coil sits above the furnace in the supply plenum. When the AC runs, the coil is cold and wet, and it pulls moisture out of the air which drips into a pan and down a drain line. Dust that lands on a wet coil stays wet. If the drain line is partly blocked, water stands in the pan and grows a film. Everything the furnace blower moves passes over this coil, so whatever is growing on it goes straight into every room.

How to check. Remove the furnace filter and look up into the plenum with a flashlight if there is an access panel. Standing water in the pan or a slimy coating on the coil fins is the sign. Water on the floor beside the furnace in summer means the drain is backing up. The EPA page recommends checking cooling coils and drain pans for standing water and making sure condensate pans drain properly.

Does duct cleaning fix it? Partly. The furnace base and blower are steps 8 and 9 of our process, and we clean what is reachable from there. A coil with a heavy biological film, a cracked pan or a blocked drain line is HVAC work. Book the technician first to clean the coil and clear the drain, then book a duct cleaning to remove what the coil has been sending through the ducts.

3. A dirty humidifier

What it smells like. Stale water, like a vase that has not been changed. It appears when the furnace runs in winter and is strongest near the furnace itself.

Why it happens. A bypass or fan-powered humidifier wets a paper or foam pad and lets the furnace air pick up moisture as it passes. If the pad is not replaced each season, minerals and bacteria build up on it, and the water reservoir in a drum-style unit grows the same film as a neglected AC pan.

How to check. Open the humidifier cover. The pad should be pale and stiff. If it is grey, crusted or soft, that is the source. Smell the pad directly if you want to be sure.

Does duct cleaning fix it? Yes, if you add it. Humidifier cleaning is a $90 add-on on a house visit and covers the housing, the pad frame and the water tray. Replace the pad each fall. If the smell has been there a while, the supply duct downstream of the humidifier will have picked some of it up, and the duct cleaning takes care of that.

4. A dead rodent or a nest

What it smells like. Different from any of the others. A dead mouse smells sweet and rotten and gets much worse over several days before it fades. A nest smells of ammonia and dry grass. Both are concentrated at one or two registers rather than through the whole house.

Why it happens. Mice get into ducts through gaps where the trunk passes through the foundation, through an uncapped register during a renovation, or through a dryer vent hood with a broken flap. Squirrels and birds are more common in attic runs and bathroom exhaust ducts than in furnace ducts.

How to check. Walk the house with the fan running and find the register where it is strongest. Pull the grille and look. Droppings, shredded insulation or seed husks confirm it. If the smell is in one bathroom only, check the exhaust fan housing.

Does duct cleaning fix it? Yes. This is one of the clearest cases for it. The EPA page lists ducts infested with vermin as one of the situations where cleaning is warranted. Our process puts the whole system under suction from the truck, pressure-cleans every run and sanitises with Benefect. We also seal the access points we cut. Finding and closing the gap the animal came through is on you or a pest control company, and it is worth doing before the cleaning so it does not happen again.

5. Dust burning off at the first furnace start

What it smells like. Hot dust, faintly like a space heater or a toaster. It happens the first evening the furnace runs in October or November, lasts an hour or two, and is gone by the next day.

Why it happens. Over the summer a layer of dust settles on the heat exchanger and burner assembly. The first time the burners fire, that dust scorches. It is normal and it is not dangerous. If it comes back every time the furnace cycles, or smells of plastic or electrical insulation, that is a different problem and you should shut the furnace off and call a technician.

How to check. Time it. A one-evening smell at first start is this. A smell that persists through the week is one of the other four.

Does duct cleaning fix it? A cleaning in late summer or early fall removes the dust on the furnace base and blower before the first start, so there is less to burn off. If this is the only smell you get, it is not on its own a reason to book.

Related runs that carry their own smells

Two other systems get blamed on the furnace ducts. A heat recovery ventilator with a dirty core and a blocked condensate drain smells like cause 1 but comes from the HRV's own supply grilles, often in bedrooms. See our HRV cleaning page. A dryer vent with a jammed flapper lets outside air and moisture into the laundry room, and a dead mouse in a dryer duct is not unusual. See the dryer vent cleaning page. Both are $90 add-ons to a house duct cleaning.

What the cleaning includes

Our residential duct cleaning for a house with one furnace is $399 under 2,000 square feet, $449 to 3,000 and $499 to 4,000, before HST, priced by home size and not per vent. The visit takes about 1.5 hours. Every supply and return run is pressure-cleaned with the truck-mounted vacuum connected at the main trunks, the furnace J-channel, blower and base are cleaned, and the system is sanitised with Benefect. You get before-and-after photos by email and the "Love it or don't pay" guarantee.

If you have a smell and are not sure which of the five it is, get a quote and describe it, or call (416) 548-7833. We are open seven days a week, 6:00 AM to midnight, across Toronto and the GTA.

Frequently Asked Questions

Will duct cleaning get rid of a musty smell?

If the smell is from damp dust in the ducts, a nest or a dead animal, or dust on the furnace, yes. If it is from a wet evaporator coil, a blocked AC drain or a neglected humidifier, the source needs to be cleaned or repaired too, and we can do the humidifier as an add-on.

How do I tell a mould smell from a dead mouse?

Mould and damp dust smell earthy and are spread through the house. A dead animal smells sweet and rotten, is strongest at one or two registers, and gets worse over a few days before fading.

Is the burning smell at first furnace start dangerous?

A hot-dust smell on the first evening the furnace runs in fall is normal and clears within hours. A smell of plastic or electrical insulation, or one that returns every cycle, is not. Shut the furnace off and call a technician.

What is Benefect?

Benefect is a botanical disinfectant. We apply it through the duct system after the mechanical cleaning as step 10 of the house process and on the wall unit in a condo.

Can you clean the AC coil?

We clean the furnace base and blower and what is reachable from there. A coil with heavy film, a cracked drain pan or a blocked drain line needs an HVAC technician. Have that done first, then the duct cleaning.