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3 Places Mold Can Hide in Your Home Air Conditioning and Heating System

Posted 03-28-16 in Air Conditioning

Every home has some mold. And exposure to mold has been linked to upper respiratory tract symptoms, aggravating allergies and other breathing problems.

But you know what's worse than mold growing on old food in your refrigerator? Mold in your AC and heating system, which can spread that mold EVERYWHERE else in your home.

Here are 3 of the most common places we find mold in an AC and heating system and what you can do to prevent mold growth there.

On your air conditioner’s evaporator coil and drain pan

How mold grows here: The evaporator coil (the part that cools your air) and drain pan inside of your air conditioner have both of the necessary ingredients for mold growth: moisture and food (dirt and dust).

Moisture forms on the cold evaporator coil when warm air blows over it, just like how moisture forms on the outside of a cold glass of water on a hot summer day.

diagram of AC split system
Split AC system with evaporator coil labeled as “cold coils”. Photo source: HowStuffWorks

The moisture on the coil then drips into the condensate drain pan, allowing the moisture to drain outside of your home or into your main sewer line.

Condensate drain line from evaporator coil
Condensate drain line from evaporator coil. Photo source

During the whole process, dirt and mold spores from your home’s air are also collecting on the coil and dripping into the drain pan.

Now the mold spores have a home, food and moisture to grow and thrive.

How to prevent mold growth here: First, check the air filter once a month and change it if it’s dirty. Keeping the filter clean allows it to catch some of the airborne dirt and dust, preventing your evaporator coil from getting too dirty. However, a typical fiberglass filter will not capture mold spores. You would need an air purifier for that.

Second, don’t skip your annual air conditioner tune-up from a professional AC service technician. A proper tune-up includes cleaning the dirt and other contaminants from the evaporator coil and making sure the condensate drain line is clear.

Finally, you can install a UV germicidal light over the evaporator coil to kill mold spores and bacteria there before they grow.

In your whole-home humidifier

How mold forms here: Most whole-home humidifiers work by passing air over a wet humidifier pad. Water from the pad evaporates into the air, helping alleviate problems with dry air in the winter.

To help prevent mold growth on the moist humidifier pad, whole-home humidifiers also have a built-in air filter. The filter traps the airborne dirt and debris, denying the mold spores a food source.

But if the filter is not regularly replaced, dirt can collect on the humidifier pad, turning your humidifier into a hiding place for mold.

How to prevent mold here: Make sure you follow your humidifier manufacturer’s instructions for maintenance. Many have filters and pads that you need to replace regularly.

In your air ducts

How mold forms here: When your air conditioning and heating system is running, dirt and mold spores from your home’s air are constantly circulating through the ducts. (And it’s made worse by leaky ductwork that pulls in dirt and mold spores from your attic.)

But your air ducts will really only start growing mold when there’s a regular source of moisture. Some sources of moisture in ducts can include:

  • Roof leak—If your roof has a leak and your air ducts are in the attic, water may soak your ducts every time it rains.
  • Oversized AC unit—Large air conditioners won’t run as long and therefore won’t remove much humidity from your home’s air. That’s a problem in humid Atlanta. The humidity can then condense on your metal vents and become a breeding ground for mold.
  • Plumbing leak—A hidden plumbing leak can let water into your ducts, creating a great environment for mold growth.
  • Inadequate bathroom venting—When you take a hot shower, that steam needs to go somewhere. Ideally, it should be vented outside by an exhaust fan. Otherwise it can collect on vents or registers in the bathroom.

How to prevent mold here: There are 2 steps to reducing the chance of mold in your air ducts: get rid of any signs of moisture and reduce the amount of dirt in the ducts.

To reduce the amount of dirt/debris in the ducts:

  • Install an air purifier in your heating and cooling system. This is a highly efficient air filter that removes up to 96% of all the dirt from the air that passes through it (including small mold spores).
  • Seal your ducts to keep them from sucking in dirt and debris from your attic, basement or crawlspace.

To eliminate moisture from your ducts:

  • Clean up any water problems near air ducts as soon as you find them. If you have a leaky roof, fix it. If you have leaky pipes, fix them.
  • Use your bathroom vent fans. Install some if you don’t have them.
  • If you get a new air conditioner, make sure it’s properly sized and installed. (Here’s how to find a good AC installer in Atlanta)

And if your ducts already have mold in them, The Environmental Protection Agency (EPA) recommends getting them cleaned—after you’ve fixed the source of moisture.

Further reading:

Coolray is your Atlanta-area home comfort expert with specialists in heating, air conditioning, air quality and plumbing. Have more questions? We’d be happy to help – just contact us online.

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