How to Fix Musty Smells & Odors From Heating System Air Ducts

Musty smells in your house should be always treated as a warning sign.


Although musty smells area lways associated with moisture presence, eliminating it in some cases requires total redesigning of the forced air heating system.

This particular case applies to the houses with a forced air heating system metal air ducts embedded into the concrete slab (scroll to the bottom for a couple of other scenarios). With a plastic or cement air distributing pipes musty smell is rarely a case, unless some water leaks develop on pipe connections or cracks start appearing due to uneven settlement.

So if you have a forced air heating system, your house is build on concrete slab with no basement or crawlspace underneath, and air registers are located on the floor or along the baseboards – the following might answer your concern about some musty smell coming from heating system air ducts.

Musty smell in your house - forced air heating system floor air supply openingMusty odor from air ducts - wet and contaminated interior of the forced air heating system ducts installed under the house concrete floorDepending on your house location, water table, drainage system around the property, whether you have a sump pump or not, the air ducts embedded into the concrete slab might be acting as a drain tile.

They could pick-up water from the ground through the pipe connections and forced air heating system would distribute moisture through the house.

Musty smell from air ducts - high water line visible on interior walls of an air duct installed under the concrete floor.Musty odor from a forced air heatings system - interior of the heavily corroded metal air duct installed under the concrete slabOver time, those metal air ducts corrode under the concrete slab when exposed to moisture and slowly disintegrate. All kinds of insects might enter through the holes and contaminate your property. Mold will usually start growing in that dark, moist environment and its spores easily distributed with the circulating air contaminate your house.

Musty smell in your house.

The solution to that musty smell caused by heating system air ducts embedded into or installed under the concrete slab without ruining your house floor:

  • Musty smell from air ducts in a single story house with an attic and a forced air heating system installed on the floor level.

Furnace air supply side would have to be disconnected from the floor opening and extended into the attic, new air ducts distributed through the attic area with registers open into each room previously supplied from the floor. This might not be that easy if you have a downflow type forced air furnace and no room for the side or rear air duct that would run into the attic.

New furnace might be the only solution to eliminate musty smell, still cheaper than cutting the concrete floor and replacing those old, corroded air ducts.

  • Musty smell from air ducts in a two story house with lower level partially or fully supplied from the floor registers.

Disconnect section of the air duct entering concrete slab… the rest depends on the house layout. You’d have to find the easiest way to install air supply for the house section that was using floor vents.

The cheapest way to block those disconnected forced air heating system registers, would be to pour some concrete into each opening.  You could also use expanding foam and top it with concrete after it dries out.

Musty smell from a forced air heating system ducts installed in a crawlspace or attic area

  • If you are experiencing this musty smell from the heating system air ducts, and part or entire forced air heating system is located in the crawlspace or attic, your answer will be most likely in one of those areas. There might a moldcontaminated attic and gaps in the return air ducts or filter compartment are acting as a vacuum, pulling musty air with mold spores into the air circulating system.

  • Same condition applies to the area under the floor of your house. You could have a flooded crawlspace , humid / contaminated with mold, and musty smelling which would be picked up by loose, separated forced air heating system ducts, gaps around the floor penetrations, etc.

Musty smell in your house should be always treated as a warning sign. Now, pick up that floor register cover, look into the attic, open the crawlspace hatch.

Those might be the first steps you have to take to get rid of that nasty, musty smell in your house!

  1. JJ Henry says

    When my husband and I moved into an older house it had a really bad musty odor. We could not figure out what was causing the odor. We tried doing repairs, but nothing worked. The neighbor across the street (also with an older house) told me about a odor eliminator that she uses. It’s called Room Shocker and it worked great! It is environmentally friendly too! You can find it at

  2. Antoinette says

    Hi- I’m at a loss right now because my parents went away on a cruise and I’m stuck taking care of the house. I have a house with no basement/crawl space with the ducts built into the concrete slab. After big storms, like this week’s Hurricane Irene, our bottom floor vents get filled with water. I used a wet/dry vac to suck up the water (it was about 2 inches). This process took two days because I didn’t have electricity for some time. However, now there is a mildew smell in the house when I run the fans and I am afraid of mold. Is there anywhere I can spray something to get this smell out? My parents just bought a new air conditioning unit (A-coil and central air condenser) about a week ago. There is an attic space in the upstairs master bed room. Is there a way I can get to the ventilation up there to spray something to get the smell out? Any help would be appreciated.

    1. CTH Experts says

      Hi Antoinette,
      The longer the air ducts located under the slab are wet, the higher possibility of their contamination with mold. Since this isn’t the first time the air ducts got flooded there might be mold already. It feeds off of the moisture and prefers dark areas making wet in-ground air ducts a perfect mold growing environment.
      Not having standing water inside the visible portion of the air duct does not indicate that there’s no water / moisture along some lower sections of the ducts / you might not see it after removing the register(s). If the grounds are heavily saturated this might be the case and even constantly running the fan won’t help in drying it nor removing that musty air duct smell.
      The best option would be to consult an HVAC contractor about eliminating floor air ducts / registers, rerouting them to the ceiling / wall, and sealing-off the floor openings with concrete or expanding foam. Might not be an easy task to do but if your parents plan to stay in this house for a while, it would completely eliminate musty smell and air duct flooding problem.
      Spraying some scented aerosol into the air duct will only help to eliminate this musty smell for a short period of time, which is only as long as this scent stays active. Longer lasting solution would be to attach scent emitting devices (just like the one you put in your car) or scented wipes on / under each of the air supplying registers.
      Even if you’re not constantly using air conditioning system, set the fan to ON position (there should be such option on the thermostat) and let it run constantly. The AC will still kick in on its own whenever the thermostat calls for it. Circulating air will speed up drying process lowering possibility of mold growth inside the air ducts.
      There are chemical agents that can be used to kill the mold spores inside the air ducts but complete process involves cleaning the air ducts interior surface first and then applying this chemical onto the cleaned surface. There are also “fogging” devices that can distribute mold killing chemicals with the air stream as well. However, without completely eliminating the source of the moisture and air ducts flooding all types of chemical delivery would be only a very temporary solution and waste of money.
      Let me know if you have more questions.

      1. Cecil says

        Hi Dariusz,

        The advice you gave Antoinette seems right on for my issue too. Have the slab and after all the rains there was a swishing thumping sound from the vents indicating water has creeped in to the ducts. The sound is gone, smell remains so my question is approx how much will this cost to repair? (sealing the ground vents with foam then concrete and have new vents placed in the attic for redirection of air flow). Fan is running and smell is decreasing.
        Thanks in advance

        1. CTH Experts says

          Difficult to estimate without examining your property. Sealing of the floor registers is the least expensive part of the job, all you need is a bag or two of concrete per register and / or some expending foam. Register covers can stay in place so you wouldn’t have to cover those openings with carpeting or whatever floor finish you have. For re-routing the duct work you’d need to bring an HVAC contractor (I’d try at least 2-3) and explain your situation. There might be more than one way of doing it and different contractors may have a few suggestions. If the attic is open and accessible it is just a matter of getting the main air feed up there and installing insulated / flexible air ducts (quicker and less expensive than sheet metal work).

  3. wanda says

    Hi! A couple of yrs ago my husband decided to paint the basement walls. He scraped them down w/o
    wetting them first & DID NOT turn off the A/C sytem. Now we are experiencing a really heavy musty
    smell when the A/C is on. What can we do? We are linited in funds so this will have to be a diy job.
    Thank you.

    1. CTH Experts says

      Hi Wanda,
      I’d need more information to give you some advice on that musty smell from your air ducts. Where are the air ducts located? – ceiling, walls, floor. If under the floor (sometimes partially), is there a partial crawlspace (wet/dry) in the house and they are suspended under the floor frame, or the air ducts are partially under the concrete slab / ground. Did you open the floor level registers and look inside the air ducts for moisture / water (using mirror, flashlight and or simply by taking pictures with digital camera).

      Is your furnace / air handler located on the lowest level and blowing air down, to the air duct system beneath the floor surface, is it in a crawlspace, attic, basement. Do you use central humidifier attached to the air distributing system (for cold season… if you have a cold season at all)? Did you check your air filter?

      When was the last time your AC system (“A” coil compartment) was opened and checked / cleaned by a professional? Can you see condensate draining from your coil when AC is operating? Do you have a sump pump / drainage system in / around your home? Have you recently experience long periods of rain… Describe everything in as much detail as possible. I don’t believe that painting your basement 2 years ago has anything to do with that musty smell. Let me know those details so I have a better “picture” of your situation. Thanks.

  4. sean says

    Are there any foundation worries if you just seal the vents when water has entered as a solution for slab with no crawl space where ducta have water in the floor?

    1. CTH Experts says

      Hi Sean, I’m not sure if I understood correctly… Your house is on a concrete slab (no crawl / no basement) and the air ducts are embedded into that concrete slab / partially (most likely) into the gravel, sand, dirt under the slab. There’s either a high water table or whenever it rains the air ducts fill up with water.

      I’m not sure how much water are you getting but the air ducts probably don’t overflow inside your house? Ideal solution would be to develop a drainage system around the house foundation and divert all that excess of water away from the structure as far as possible before it penetrates the foundation wall.
      This is assuming that the air ducts aren’t constantly flooded which would be the result of a high water table (you’d most likely need a sump pump in that case) or sometimes a broken water line – I’ve seen that a couple of times.

      After the drainage system installation you could seal the vents with concrete or some other material.
      Right now, that water inside the air ducts evaporates (probably) naturally and with the forced air system’s help (I know that it isn’t healthy). If you seal the air ducts without taking care of the drainage issue there might be water standing inside the air ducts constantly, possibly saturating the concrete and elevating moisture levels inside your house (unhealthy too).

      I don’t know how deep your foundation is, what type of a structure it is supporting (single story or more), if your home is in the freezing zone, how is it positioned (terrain surrounding the house), and how much water penetrates your air ducts. Therefore, I can’t really answer your question concerning the foundation. I would suggest consultation with a local drainage contractor and foundation specialist. Let me know.

  5. wilbur says

    well the ducts in the master bedroom must be blocked there is no air coming out of them and the other two rooms to the west have about one inch of water standing in them.well i am puzzled?

    1. admin says

      Maybe there’s a lower spot inside the air duct, not visible for you, where the water level is higher than 1″ / blocking the entire air flow…

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