AC, duct and vent mold in Orlando

Why AC systems grow mold here

An air conditioner cools by running house air over a cold coil. Water condenses on the coil, drips into a pan and runs out a drain line. That's how it pulls humidity out of the house, and in Orlando it's doing a lot of it: the airport averages a 70°F or higher dew point on 137 days a year, and July and August average dew points of 73.8°F and 74.5°F. Mold grows where that water collects and the system is dark and dusty.

• Supply vents and registers. Cold air hits a warm, humid room and the grille sweats. Black specks on a vent are often surface mold or dust on condensation. Common when the house is too humid or the ducts are poorly insulated in a hot attic.
• The air handler and coil. A dirty coil and a pan that doesn't drain hold water all summer.
• The drain line. Algae and slime clog the condensate line, the pan overflows, and the ceiling or closet floor under the air handler gets wet. One of the most common Orlando water damage calls.
• Ductwork. Flex duct in a 130°F attic with a torn jacket sweats inside the insulation. Ductboard can hold growth in its fibers.

Days with a dew point of 70°F or more, by month

Oversized, set high, or off

Three habits cause most AC mold in Central Florida houses. An oversized unit cools the house in ten minutes and shuts off before it has pulled much water out of the air, so the house is cold and clammy. A thermostat set to 82°F or higher to save money, or the system switched off while the owners travel, lets indoor humidity climb past 60 percent. And a closed-up house in August with the AC off for two weeks can grow mold on furniture, closets and shoes. The EPA's guidance is to keep indoor relative humidity below 60 percent, ideally 30 to 50. Keeping an Orlando house dry.

What the work involves

1. Find the cause. A clogged drain, a rusted pan, a torn duct, an oversized system, a missing return. This part is HVAC work and may need a licensed air conditioning contractor.
2. Clean or replace the parts that hold growth. The coil and pan get cleaned. Flex duct and ductboard with growth inside usually get replaced rather than cleaned. Metal ducts and registers can often be cleaned.
3. Clean the closet and anything the water reached. Wet drywall under an overflowing pan is a remediation job of its own, and over 10 square feet it takes a Florida mold remediator license.
4. Fix the humidity. A properly sized system, a whole-house dehumidifier, or both.

Be careful with a quote that only "fogs" or sprays the ducts. Treating a surface without removing the growth or fixing the water is the paying-twice problem in another form.

Black dust or mold?

Not every dark spot on a vent is mold. Dust, soot from candles and dirt pulled through gaps around the register look similar. Surface growth on a sweating vent wipes off with detergent and water and comes back in the same spot. If several vents are spotting and the house smells musty, that points at humidity or the system, not the vents.