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Need help determining source of musty smell in basement-- furnace or duct work possibility?

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For several years now, there has been a musty (moldy grain-like?) smell in our basement that seems to coincide with furnace usage in the fall and winter.  Originally, I thought the smell might be a function of the carpeting (about 4 or 5 summers ago, a carpet cleaner I used broke mid-stream, preventing me from fully removing the water from some heavily stained areas of the carpet). However, if the carpeting or padding developed mold in it, I would think that the musty smell would exist during the summer when it's humid and the ac is running (and it does not).  

Our house has 2 ac-furnace units.  The basement furnace services the basement and 1st floors and has a humidifier attached to it. We have run the furnace for a few weeks now and the smell has returned.  This fall, I am finding the smell to be a good bit worse (stronger) than in prior years.  Additionally, the smell is no longer in just the basement-- I can now smell it on the 1st floor, as well as in the hallway of the 2nd floor.  And my husband, whose poor sense of smell previously couldn't detect the problem, now agrees there is a problem smell, the source of which he hasn't been able to determine.  We do have an HVAC service contract so the units are serviced regularly in the spring and the fall, filters changed regularly, etc.  When I mentioned the smell problem during a spring or fall service appointment a year or two ago, the tech wasn't particularly helpful and, without any additional checking, brushed off the possibility of the furnace and duct work being a potential cause of the musty smell problem (unfortunately, the smell hadn't started up yet so there wasn't any evidence I could direct him to).  

Recently, I have read that furnace humidifiers can pose moisture problems and wonder whether our humidifier (or what it's connected to) is playing a role in this smell problem.  This past March, I discovered a puddle on the floor of our basement that I thought was coming from our water heater.  Upon further inspection, I found that water was leaking out of the humidifier. I turned it off and called our HVAC repair company.  The repairman determined that the filter had been put in backwards the last time (which had to be months prior).  I can't be absolutely certain, but I don't believe the puddled water had been occurring since the fall, although I don't know why it would have surfaced only months later).

Some additional info: our basement is partially finished and we are not aware of any plumbing, sump-pump, etc. problems in it.  

I would be very grateful if you could suggest some possible next steps in trying to determine the source of the problem-- e.g., get HVAC duct and/or humidifier inspection? home inspector assessment? mold inspector? etc. etc.

THANK YOU for any insights you might provide.
asked in Heating System by jpmackay (120 points)
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1 Answer

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Hi JP,

Musty smell in a basement is one of those things that is difficult to assess remotely. Some HVAC technicians don’t like to investigate such problems because it might be time consuming. Often, because of our senses, what may appear as unpleasant / musty smell for some of us, it will be perfectly normal for others.

There are many factors that might be responsible for what you’re describing but without knowing specifics of your HVAC system installation, house drainage, foundation type and condition, plumbing system materials and their condition, it is virtually impossible to pinpoint the source of your problem.

If you didn’t already, please take a look this article on my website - http://www.checkthishouse.com/3258/musty-smell-from-heating-system-air-ducts-forced-air-heat.html

Some of the things to check / tips:

If you furnace stands on the basement’s floor and / or if you have any air ducts under the concrete slab read that - http://www.checkthishouse.com/14/air-return-in-forced-air-heating-ac-systems.html

Furnace blower / air return compartment should be checked for moisture / mold contamination.

If you have air return ports in your basement area you can install dehumidifiers to lower humidity levels

Humidifier units are huge sources of mold and moisture. They require servicing / maintenance on regular basis when used. How often depends on its type – check with the manufacturer. Many of the humidifiers waste tons of water and are difficult to control so it’s a good idea to have an independent humidity sensor in your house to verify the output. Read it here - http://www.checkthishouse.com/6290/room-humidity-and-house-humidifier-energy-savings.html

Check your foundation for any possible seepage, also, just to make sure that your carpeting and padding are not contaminated, lift it up in several spots along the foundation. If you notice any black residue on it (especially on padding), well, you may need to replace it.

Since you have a sump pump, maybe even ejector pump in basement area – seal the covers air tight.

If you have older cast iron or galvanized drain pipes – they often corrode and crack, sometimes along the top on horizontal runs. In such cases you may not see it but it will release moisture and unpleasant odors - http://www.checkthishouse.com/1757/cast-iron-drain-lines-problems.html

You may also have a small plumbing leak above one of the air ducts. This may not be visible from underneath but it could result in corrosion and water leaking into the duct.

Those are all guesses but maybe one of them will be the right one J. 

answered by darekrudy (21,730 points)
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