Furnace on water damaged OSB platform.  The return plenum is located beneath the platform.

Furnace on water damaged OSB platform. The return plenum is located beneath the platform.

A common practice in multi-story apartment and condominium complexes is to house the furnaces in closets that are located on the patio or balcony for each unit. The closets are aligned vertically in each building. Although the orientation is convenient for plumbing access, numerous water damage issues are inherent to this type of HVAC set-up.
Water damage from plugged condensate drain tubes or malfunctioning condensate drain pans is common. Slimy biological growth in PVC tubing and drain pans pan causes overflows. Current or historical overflows from the pans will appear on the sheet metal plenum under the A-coil. Drips and mold growth from malfunctioning units above a closet are commonly apparent as drip stains or growth on the ceiling and walls of the closet.
Water damage caused by freezing and thawing of the AC coils is common in apartment HVAC systems. Filter changes are often left in the hands of uninformed tenants. Plugged filters prevent sufficient air flow to the coil, causing it to freeze. As the coil warms up, the ice thaws, releasing large amounts of water into the return plenum, floor system of the apartment, furnace platform, or furnace closet below the damaged unit.
Another issue associated with stacked HVAC closets is the installation of the furnace on a platform, which is commonly constructed from plywood, drywall, or oriented strand board. When wet, all of these materials support the growth of mold and bacteria, which can produce various toxins. Additionally, the glues and resins in these materials can be liberated when wet.

Ceiling of furnace closet showing water entered from HVAC  closet above.

Ceiling of furnace closet showing water entered from HVAC closet above.

The return plenum in furnace closet systems is located beneath the platform, with the underside of the platform being the top of the plenum, a concrete slab serving as the bottom, and the walls of the platform forming the sides of the plenum. In addition to being difficult to clean, a water damaged common plenum provides both a contaminant source and transport pathway.  Dust mites, insects and their fecal material, garbage, cigarette butts, contaminated chunks of paper, and rodent feces are just a few of the treasures that have been recovered from plenums under furnace platforms.
When damage does occur, the underlying cause of water must be corrected. This involves replacing plugged drain tubing, cleaning drip pans, cleaning coils, and changing filters. For consistency, preventive measures in the form of routine inspections and HVAC maintenance should be the responsibility of the management company or maintenance staff.
Addressing the consequences of the water damage, whether it manifests as biological growth or deteriorated building materials, is imperative to appropriate HVAC hygiene and good indoor air quality. Contaminants on water damaged plywood, OSB, and drywall in the furnace closet and on the furnace platform can cause adverse health effects, whether the growth is active or historical. Once dried, mold and bacteria engage in survival mechanisms that can include toxin production and increased sporulation. When dry, the contaminants are easily liberated into the airstream.  Mold remediation is required when building materials in furnace closets become contaminated. The remediation involves two components (1) removal of the mold or contaminated building material and (2) addressing spores that are released from areas of actual growth. Engineering controls, containment barriers, HEPA filtered air filtration devices, personal protective equipment, HEPA vacuuming, and damp-wiping are all essential for safe and effective remediation in water damaged furnace closets.

Return grille located inside of the apartment behind the furnace platform.  Stains show repeated wetting fo tack strip.

Return grille located inside of the apartment behind the furnace platform. Stains show repeated wetting fo tack strip.



  1. Anthony Denatala Says:

    Live it everyday. Just did a few air tests and inspection on an apartment that had water damage from 3rd floor straight through second and down into first. The second floor was our client. The apartment complex cleaned up the water and the only mitigation consisted of 1dehu and a fan for 1day, carpet cleaning and kilz/paint for coverup.
    2nd floor had carpet cleaned; first floor got new carpet. The problem being, well, I guess I don’t have to tell you.

    Now the tenant questioned the cleanup but also questioned how her health was going to be affected. When she listed her symptoms it was like reading a book written about a persons daily struggle living with mold. The client swore she didn’t look it up on the Internet. Now, she also started talking about prior to the water loss and how she and her daughter got frequent yeast infections: nose, ears, eyes, throat, and other areas. Your article Hits the Nail on the Head, people should pay attention and educate themselves about the ill effects of our environments. Water damage, excessive humidity, excessive dust and dirt, uncleanliness, dirty laundry pile ups, unmitigated water losses, improper water cleanup, mildew, mold, bacteria, HVAC system maintenance and cleaning, pest control, pest control aftermath cleanup, crawlspace air quality, …….. Ok, ok, ok. I’ll stop now. Spread the word. Educate as many as possible. Help people live comfortably, without infection from environmental contaminants and live peacefully within the places we call home. Our homes should not be what kills us.

    • Sanit Air, Inc. | CleanliNEST™ Says:

      Thanks for taking the time to comment Anthony. It seems like I only get calls from very sick people with symtoms similar to what you describe. The toxigenic and multi-system inflammatory responses are more common than most people (and most docs) can imagine. Even though medical and scientific knowledge to support the link between contaminants in water damaged buildings and illnesses has increased rapidly over the past five years, the naysayers continue to claim any opinions other than their own are based on “junk science.” I am not ready to throw in the towel yet – just had a doc tell one of my clients that I saved his daughter’s life. Not looking for pats on the back, but it does make up for the stresses associated with trying to do the right thing.

      • sickbuildinggroup Says:

        As you may have seen in my posts on LinkedIn I work alongside a few Doctors whom determine that patients do not get better unless they assure that their living environments are free of contaminants. We have been blessed in that while we inspect and mitigate homes, provide solutions to reduce indoor contaminants, and sustain environemntal controls by providing yearly re-inspections the Doctors who refer us provide us with treatment for the exposure of contaminants we inundate ourselves with daily. The ability to help people definitely returns itself three fold. Saving lives is what we do, we don’t necessarily pat ourselves on the back as our job can be very detrimental to our own existance but we do enjoy helping others and in doing so learn how to help ourselves through caring. If you are ever interested in our Doctors treatment plan let me know and I will elaborate.

  2. tony705 Says:

    very good post. we get calls all of the time from tenants with this problem.

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