Mold in HVAC Systems, Ducts and Window AC Units

Where Mold Grows in an HVAC System

An air conditioner is also a dehumidifier. As warm air passes over the cold evaporator coil, water condenses and drips into a drain pan, then runs out a condensate line. On a hot day a central system can pull gallons of water out of the air. Anywhere that water sits is a place mold can grow.

The usual hot spots are these.

• The drain pan under the coil, when the drain line is slow or clogged and water stands in it.
• The evaporator coil, when it is coated in dust that stays damp.
• The blower wheel and housing, which can pick up damp dust from the coil.
• Internal duct insulation or fiberglass duct board near the air handler, which soaks up moisture.
• Duct runs in hot attics or crawlspaces that sweat on the outside when cold air runs through poorly insulated or leaky ducts.
• Supply registers where cold air meets humid room air and condenses.

Dark spots around a ceiling vent are often dust carried by the air, not mold. Wipe one with a damp cloth. Dust smears gray and comes off easily. Mold tends to return in the same spots within weeks, often with a musty smell when the system starts.

Signs Your System Has Mold

You usually cannot see inside the air handler or ducts, so look for clues.

• A musty or dirty sock smell that gets stronger when the air conditioning or heat turns on, and fades when it turns off.
• Visible growth on the inside of supply registers or the back of return grilles that returns after cleaning.
• Water around the indoor unit, a rusty or stained drain pan, or a drain line that never drips outside when the AC runs.
• A float switch that keeps shutting the system off. This switch trips when the drain pan fills.
• Allergy or asthma symptoms in the home that seem worse when the system runs.

With the power off, open the filter door and look at the filter and the area around it with a flashlight. If your system has an access panel to the coil, some homeowners can see the top of the coil and pan. Do not remove panels that are sealed with foil tape or screwed tight around refrigerant lines.

A musty smell alone does not prove the ducts are moldy. The smell can come from a wet drain pan, a crawlspace, or a return duct pulling air from a damp area. Start with the drain and the filter before you pay for duct work.

What You Can Safely Do Yourself

A lot of HVAC mold prevention is simple maintenance you can do in under an hour.

Change the filter. Check it monthly in cooling season and replace it every 1 to 3 months, or sooner if it looks gray. A clogged filter lets dust coat the coil, where it stays damp. Use the filter rating your equipment maker recommends, since a very dense filter can choke airflow on some systems.

Clear the condensate line. Find the PVC drain line near the indoor unit. Many have a capped T fitting for cleaning. Pour in about a cup of warm water and confirm it drains outside or into the floor drain. If it backs up, attach a wet-dry vacuum to the outdoor end of the line and pull the clog out. Some equipment makers suggest a cup of distilled white vinegar every few months, so check your manual first.

Clean supply registers and return grilles. Remove them, wash them in warm soapy water, dry them fully and reinstall.

Set the fan to AUTO, not ON. When the fan runs nonstop, water on the coil can evaporate back into the house between cooling cycles. AUTO lets the coil drain.

Keep the home dry. The EPA recommends indoor humidity below 60 percent, ideally 30 to 50 percent. A hygrometer tells you if your system is keeping up.

What Needs a Professional

Once mold is inside the air handler or ducts, it can spread spores to every room each time the system runs. Cleaning it well takes tools most homeowners do not have, such as negative air machines, HEPA vacuums built for ductwork and coil cleaning equipment.

Call a specialist when you see any of these.

• Visible growth on the evaporator coil, blower wheel or inside the air handler cabinet.
• Mold inside supply or return ducts beyond the first few inches you can reach.
• Mold on internal duct insulation, fiberglass duct board or flexible duct liner.
• A drain pan that is rusted through or has overflowed into the surrounding insulation or ceiling.

The EPA’s guidance on duct cleaning makes an important point. Ducts made of or lined with insulation that have gotten wet or moldy usually cannot be cleaned effectively and should be removed and replaced. Bare sheet metal ducts can often be cleaned.

The EPA also says duct cleaning will not solve anything if the moisture problem that caused the mold is not fixed. Expect a good contractor to find the source, whether it is a clogged drain, an oversized unit that does not run long enough to dry the air, leaky ducts in a hot attic, or low airflow.

Ask how they will protect the rest of the home during the work and what they will do with any insulated duct that has mold.

Why Foggers and Sprays Are a Bad Idea

Some companies and products promise to fix duct mold by fogging or spraying a chemical through the system. Be cautious.

The EPA’s duct cleaning guidance says to use biocides and sealants in ducts only after the ducts are cleaned, only if the product is registered for use in HVAC ductwork, and only after you understand how it will be used. Few products are registered for use inside ducts. Many that are sold for this use are not.

Here is why fogging falls short.

• It does not remove the mold. Dead mold and dust still cause allergic reactions and still feed new growth.
• It does not fix the water. The drain pan or coil that grew mold will grow it again.
• It can put chemicals in the air you breathe every time the system runs.
• Bleach and harsh cleaners can corrode coils, pans and metal parts, and they are not made for this use.

Encapsulant coatings have a place on cleaned sheet metal in some jobs. They are not a substitute for cleaning. If a contractor recommends a chemical, ask for the product type and its label, and ask what physical cleaning comes first.

Also be wary of low-price whole-house duct cleaning offers that turn into a mold upsell once the technician is in your home.

Window Units, Portables and Wall Units

Room air conditioners grow mold for the same reason central systems do. Water collects in the base pan, and dust on the coil stays damp.

Clean a window unit at the start of each cooling season and monthly while you use it.

1. Unplug the unit.
2. Remove the front grille and wash the filter in warm soapy water. Let it dry fully.
3. Look at the coil behind the filter and, with a flashlight, the blower wheel inside. The blower wheel is the drum-shaped fan with many small blades.
4. Vacuum the coil gently with a brush attachment. Spray a no-rinse coil cleaner and let it drain.
5. Wipe out any slime from the base pan you can reach. Make sure the unit tilts slightly toward the outside so water drains.

If you see black growth on the blower wheel or the foam insulation inside the case, the unit is hard to clean without taking it apart. For most window units, replacing it is cheaper and safer than a deep cleaning.

Portable units have a water tank or drain hose, so empty and dry the tank often. Ductless wall units, often called mini splits, can grow mold on the blower wheel inside the indoor head. A dark speckled wheel and a musty smell mean it needs a professional cleaning.

Step by Step

1. Turn Off the Power

   Set the thermostat to off and turn off the service switch or breaker for the indoor unit. Unplug window and portable units.

2. Check and Replace the Filter

   Pull the filter and look at it and the space around it with a flashlight. Replace it if it is gray or clogged, and write the date on the new one.

3. Inspect the Drain Pan

   If you can see the pan without removing sealed panels, look for standing water, slime or rust. Standing water means the drain line is slow or clogged.

4. Flush the Condensate Line

   Pour about a cup of warm water into the cleanout and confirm it drains. If it backs up, pull the clog out with a wet-dry vacuum at the outdoor end.

5. Clean the Registers and Grilles

   Remove supply registers and return grilles, wash them in warm soapy water, dry them and reinstall them. Wear gloves and an N95 if you see growth.

6. Set the Fan to AUTO

   Switch the thermostat fan from ON to AUTO so the coil can drain between cycles. Check that indoor humidity stays below 60 percent.

7. Clean or Replace Window Units

   Wash the filter, vacuum and treat the coil with a no-rinse cleaner, and clear the base pan. Replace the unit if the blower wheel or inside foam is moldy.

8. Call a Pro for Inside the System

   If you find growth on the coil, blower wheel, inside the cabinet or in the ducts, stop and have a specialist assess it. Run the system as little as possible until then.

Mistakes That Make It Worse

• Paying for duct cleaning before checking the drain pan and line. The mold source is often a clogged drain, and ducts will be moldy again within a season.
• Fogging or spraying bleach into the ducts. It does not remove mold and can damage coils and pans.
• Running the fan on ON all day in humid weather, which puts coil moisture back into the air.
• Cleaning moldy duct board or lined duct and putting it back in service. Insulated duct with mold usually has to be replaced.
• Keeping a window unit with a moldy blower wheel running in a child’s bedroom.
• Ignoring a float switch that keeps shutting the system off. It is telling you the pan is full.

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