Some people believe that AC ducts are destined to become moldy. While all AC ducts will likely grow some mold and will become discolored with visible mold or visible dust to some degree, it is not normal or healthy for ducts to become excessively moldy, or to produce a strong mold odor. Mold spores from AC ducts result in health problems and even mold odors in the absence of spores have been shown to cause health problems. Keeping ducts free of excessive mold is important. The information in this blog is intended to provide readers with information on: 1) Some reasons why AC ducts become moldy. 2) The importance of proper sizing of AC units. 3) In addition it draws attention to a very common indoor air quality problem called entrainment. Entrainment is when AC units pull contaminated air from inside attics or walls or other undesirable areas. The following information is taken from a Broward County mold inspection we conducted on 11-17-07 it shows an actual example of how ducts became moldy and gives recommendations at the end.

The property inspected is a one story single family Broward county house built in 1974. Client reported mold growing on AC vent registers. The mold started a few weeks ago. The client reported that a new larger AC unit was installed about 3 months prior to the mold starting. This inspection is for mold conditions at the AC registers only. In the above referenced area the inspector observed a new air conditioner. The serial number on the unit indicates that it is a 4 ton unit, thus is it rated to cool a 2400 sf house. This AC unit is just a little large for this house. The inspector also observed a new condenser / compressor unit. The serial number indicates that it is a 3.5 ton unit thus designed to cool a 2100 sf house. This is also slightly large for the house. You have a newer larger AC unit. The size of this newer AC unit is a major reason for why you have humidity related cladosporium on your AC registers that were previously not moldy. The previous AC unit in use about 3 months ago was smaller than this AC system thus would have been more efficient at drying the houses air before cooling the houses air and shutting down.

If an AC is to large it shuts off after the air is cooled but before the air is properly dried / dehumidified. Air that is not dehumidified and has a humidity above 60%RH to 65%RH can and often does cause humidity problems in Florida homes. The above photo is of cladosporium mold growing in your registers. Another reason why you have mold problems now is because of the two holes in your AC return. These holes in the wall studs allow the AC to pull humidity into the AC each time the AC turns on, this phenomenon is called entrainment and is a common cause of mold and humidity problems. Yes all metal studs have these holes, but it is not typical for these open holes in the studs to be exposed and in such close proximity to an AC return. Where does the humidity in your walls come from? it comes room the attic, the attic in turn gets the humidity from outside via the soffit vents like the one in the above photo. Why did this problem not occur till recently? the older smaller AC was more efficient at drying air as stated above and thus was able to handle the humidity load created by these openings.

More photos of mold on and around your AC registers. ac to dc converter buyNote that the tiny black spots on and around AC registers are very typical examples of cladosporium spheasperium, or cladosporium cladosporidies mold. what causes an ac unit to ice upThis mold problem is caused by elevated humidity in your ducts.how to choose central air conditioning units This is not likely to cause a toxic reaction, but this mold may result in allergic reactions or act as an asthma trigger in sensitive persons. The real concern is if it is growing excessively deep in your ducts then you will have much more mold than you can see on the registers. Mold growth in the ducts can and often causes health complaints. Excessive mold was not observed in your AC unit but you are recommended to have your ducts cleaned in accordance with NADCA standards or replaced.

Note that the tiny black spots on and around AC registers are very typical examples of cladosporium spheasperium, or cladosporium cladosporidies mold. Based on this inspectors observations and conclusions your inspector recommends using good quality foil mastic tape or other methods to seal two openings in your AC return wall studs. Have a AC company install a humidistat and set it to around 50% to 60%RH always maintain indoor humidity between 30% and 60%RH. Most people live with a little mold in the ducts and have no problems. Mold spore levels in your air was very low. No mold odors were detected. Much of the mold appeared to be primarily in the ends of the ducts just behind the registers. Have an AC company clean the mold from the registers and from the ends of the ducts just behind the registers. Duct removal or good professional cleaning in accordance with NADCA standards is recommended if additional hidden mold is discovered inside the ducts during cleaning.

Daryl Watters is president of A Accredited Mold Inspection Service, Inc. He provides home, mold, and indoor air quality investigations in South Florida. He is also the creator of MIR forms designed to aid inspectors in the production of computer generated indoor air quality and mold inspection reports.This is a topic that’s hard for me to cover in one blog post!  I’m coming up on my 20th year as a professional builder and I think that my understanding of HVAC design & execution is something that I’ll always be learning.  So, let’s start with some obvious mistakes that aren’t even visible to most builders or buyers. Mistake #1 – Oversized Systems. I would venture to guess that 75%+ of the homes built in America this year will have HVAC systems that were designed on the first day of HVAC rough-in.  The HVAC sub took the house’s square footage and divided by his sf/ton ballpark number and started to run ducts all in one day.  Here’s an example from my area here in the Hot/Humid South.  

Most AC subs use 500sf/ton of house so a 2000 sf ranch style house would typically get a 4 Ton AC system installed.  The problem with this is that every house has different orientations, different windows/doors, and very different insulation/air sealing than a house built even 5 years ago.  Most houses in Texas have too large an AC unit for the house’s normal loads and so that 4 ton machine in a 2000 sf house might actually be 1 (or even 2) whole ton(s) too large.  Too large of an AC means that on most days the AC will cool the house rapidly (maybe only needing 7-10 minutes to cool) and all those short cycles are terrible for dehumidification, will shorten equipment life, and are using more energy than is really needed to cool the house.  Longer equipment run times are actually better with AC compressors.  See this great blog post by my blogger buddy Allison Bailes for a detailed analysis on this subject. The Smart Builders know that their houses need a true HVAC design done by an ACCA accredited professional.  

I have a third-party HVAC designer do an Energy Model and Manual J (load calculations) on every house I build.  Then I take those to my HVAC contractor to solicit a bid. Mistake #2 – Bedrooms with no Returns Every room in the house that has a duct needs a path for that air to get back to the AC return.  In the past, Builder have relied on the under-cut of the door to provide a return path but that’s a rookie mistake.  If you were to calculate the supply & return from a bedroom you’d be surprised at how high you’d need to undercut that door! Smart builders insist on either Ducted Returns in bedrooms or “Jumper Ducts”.  See the picture of a Jump Duct in one of my houses. Mistake #3 – Air sealing & WeatherizationIt’s a rookie builder mistake to let these type of things happen on your houses. The Smart Builders know that trades must complete their work before claddings are installed.  He’s a great flashing tip for you too.  I order a Quickflash HVAC boot for every line set that penetrates my WRB.  

I usually have them onsite for the HVAC subs to install.  They run about $30 each but make a perfect weather-seal at freon line penetrations AND they look alot better than the alternatives. Mistake #4 – Ducts outside of conditioned space Here in the Southern US we don’t typically have basements so you’ll find most HVAC systems either in hot attics or in smelly crawlspaces.  Putting ducts in these locations is a major Rookie Mistake.  Think about this for a minute, your AC cools the air down to 60 degrees only to send that air through ducts in your attic that might run 120+ degrees (and that’s with a radiant barrier installed).  The ducts are insulated to R-8 but there might be a delta of 60 degrees between inside/outside those ducts.  Also, remember that all ducts have some leakage.  Well built new houses might leak 5-10% (older ducts often are tested above 20% leakage).  So, if your 1600 CFM HVAC system has 10% duct leakage you are losing 160 CFM to the hot attic every time your blower turns on.  

This loss also depressurizes your house and that 160CFM deficit is often made up by air blowing in around ceiling registers, under doors, and backdrafting through bath exhaust ducts. Smart builders either bring their ducts into the conditioned space, or they move the envelope outward so that the ducts are inside.   There are a myriad of ways to bring ducts inside the envelope but this needs to be thought through before construction begins! Stay tuned for part 3 of my series on Plumbing mistakes… – Risinger Homes in Austin, TX Risinger Homes is a custom builder and whole house remodeling contractor that specializes in Architect driven and fine craftsmanship work. We utilize an in-house carpentry staff and the latest building science research to build dramatically more efficient, healthy and durable homes. Be sure to check out my video blog on YouTube, and follow me on Twitter @MattRisinger You can also check out my new Amazon Store here with Matt Risinger approved items.