The single most common piece of bad advice in Florida home ownership is “your AC is fine, the thermostat reads 72.” In this state, what a thermostat reads has almost nothing to do with whether the air conditioning system is doing its actual job.

Air conditioning in Florida is dehumidification by accident. The system was designed to cool air. The reason your home is habitable at 70-90 percent outdoor relative humidity is that, as the AC cools indoor air below its dew point, water condenses on the evaporator coil and runs out the condensate drain. The cooling is intentional. The dehumidification is a side effect of the cooling cycle being long enough for that condensation to happen. If the AC reaches the thermostat setpoint and shuts off before the coil has run wet long enough to drag the dew point down, the home stays cool but humid. That is short-cycling. And short-cycling, more than any other single mechanical factor, is what drives indoor mold growth in Florida homes.

What short-cycling actually looks like

A properly-sized residential AC in a Florida home should run in long, slow cycles, often 30 to 90 minutes continuous on a typical summer afternoon. That long run keeps the coil cold and wet, indoor relative humidity drops into the 45-55 percent range, and mold has no biological growth window.

A short-cycling system runs in much shorter bursts. Five minutes on, 20 minutes off. Ten minutes on, 30 minutes off. The thermostat is satisfied (the air is cool), the homeowner thinks the system is working, but the coil never gets wet enough or stays wet long enough to dehumidify. Indoor relative humidity drifts upward into the 60s and 70s. The thermostat still reads 72, but the indoor environment is now perfectly suited to mold colonization on drywall paper, baseboards, behind furniture, and inside HVAC ductwork.

Why short-cycling is common here specifically

Five reasons short-cycling is endemic to Florida residential HVAC:

  1. Oversized systems. The standard residential AC sizing convention (1 ton per 600 sq ft, or per 400 sq ft in older homes) is a rule of thumb developed for drier climates. In Florida, with year-round dew points and heavy latent heat loads, that rule routinely results in oversized systems. An oversized AC drops the thermostat to setpoint in 8 minutes when a properly-sized system would have taken 45.
  2. Attic-mounted air handlers. Florida HVAC convention puts the air handler in the unconditioned attic, where ambient temperature in summer can be 130°F. The supply ducts run through that same hot attic. The system has to overcome heat gain on every cycle, but if the building envelope is leaky enough, the thermostat gets satisfied quickly anyway, and short-cycling sets in.
  3. Single-stage compressors. Most residential ACs in Florida are still single-stage: they are either off or running at full output. A two-stage or variable-speed system can run at partial load for long stretches and dehumidify properly; a single-stage system either short-cycles or doesn’t.
  4. Tighter envelopes than the system was sized for. Code-driven envelope improvements (better windows, better insulation, tighter air-sealing) made Florida homes more efficient over the last 20 years. But many homeowners replaced their AC with the same-size unit they had before. A 4-ton system installed in 2003 on a leaky house is now running on a tighter 2024 envelope and short-cycling chronically.
  5. Vacation and seasonal homes. When a snowbird leaves Naples for the summer and sets the thermostat to 80, the AC short-cycles to maintain that temperature while indoor humidity climbs to 70+ percent for five months straight. The home grows mold quietly while the owner is in Connecticut.

The dehumidification gap, quantified

The number that matters is sensible heat ratio (SHR). An AC’s SHR is the fraction of its cooling capacity spent on temperature reduction (sensible) versus dew-point reduction (latent). A system running at SHR 0.85 is doing 85 percent cooling, 15 percent dehumidification. In Florida summer conditions you want SHR closer to 0.7, meaning 30 percent of the system’s capacity is going to removing water from the air.

An oversized short-cycling system effectively runs at SHR 0.95 or higher; the cooling happens but the dehumidification doesn’t. The thermostat is happy, the homeowner is comfortable in the short term, and the building is gradually pickled in 65 percent indoor RH all summer.

Where the mold actually grows

Indoor mold needs four things: spores (always present), food (cellulose, drywall paper, dust, organic surfaces), temperature (60-90°F, year-round in Florida), and moisture. The moisture comes from indoor RH above ~60 percent. That’s the only variable a homeowner controls, and HVAC dehumidification is the lever for it.

In a short-cycling Florida home, mold appears predictably at:

  • Behind furniture against exterior walls. The wall is cooler than ambient because of conduction; relative humidity at the surface rises into the 70s or 80s; mold colonizes the drywall paper in the dead air space behind the furniture.
  • Inside closet corners that share an exterior wall. Same mechanism. The closet has no airflow.
  • Behind baseboards and below built-ins. Floor-adjacent surfaces are coolest; sub-floor moisture migration meets the cool surface and condensation begins.
  • Inside HVAC plenums and supply ductwork. The duct interior is cold and the surrounding attic air is hot and humid; if any humid air leaks past the duct seal, it condenses on the cold interior surface.
  • On attic-side ceiling drywall near the air handler closet, especially where the condensate line passes through.
  • Inside the air handler itself, on the blower wheel, evaporator coil fins, and drain pan.

An HVAC health check is specifically built to surface these findings non-destructively, with moisture meters, thermal imaging, and accessible-component inspection.

How to diagnose short-cycling at your house

You don’t need a meter. Two quick checks:

  • Indoor RH. Put any $15 hygrometer in the middle of the house, away from windows and supply vents. Read it at 4-6 PM on a typical summer day, when latent load peaks. If the reading is above 55 percent with the AC running, your dehumidification is failing. Above 60 percent, mold is actively favored.
  • Cycle time. Watch your AC for an hour on the same day. If it’s running in cycles under 15 minutes, it’s short-cycling.

If both checks come back bad and the home has no visible mold yet, the situation is reversible. If both come back bad and the home has been like that for two or more summers, you may already have hidden growth that hasn’t surfaced visually. That’s when a full mold inspection with thermal imaging and moisture mapping is the right next step.

How to fix short-cycling

Not all fixes are equal:

  1. Standalone dehumidifier. Cheapest and fastest. A whole-house dehumidifier installed in the conditioned space pulls the indoor RH down to 50 percent regardless of what the AC is doing. Doesn’t fix the AC; works around it.
  2. Right-size the next AC. When the current system needs replacement, have a Manual J load calculation done. Resist the upsell to oversize “just to be sure.”
  3. Upgrade to variable-speed. Variable-capacity compressors run long and slow, which is exactly what Florida needs.
  4. Improve duct sealing and insulation. If the air handler is attic-mounted, every gap in the supply ductwork bleeds conditioned air into the attic and bleeds hot humid attic air into the supply. Fix the leaks, the system can keep up with the latent load.
  5. Programmable humidistat control. Some modern thermostats can run the AC on humidity rather than temperature, holding indoor RH down even when the thermostat is satisfied.

Why this matters for inspection sequencing

When PureSpec is called for a Florida mold inspection, HVAC evaluation is on the standard checklist regardless of where the visible mold is. The reason: about two-thirds of the active mold growth we document in residential properties traces back, directly or indirectly, to an HVAC short-cycling pattern. The visible spot on the bedroom wall is downstream. The upstream cause is in the air handler closet, in the attic ductwork, or in the system sizing decision someone made 15 years ago.

This is also why CIRS environmental assessments in Florida almost always include an HVAC evaluation. A sensitized patient in a short-cycling home is being continuously dosed at low concentrations all summer; the symptom pattern often doesn’t correlate with one specific event, because the exposure is a slow continuous drip rather than a spike.

What to do today

Buy a hygrometer. Put it in your living room. Check it tomorrow evening. If indoor RH is above 55 percent in summer with the AC running, you have a problem to investigate. Call (321) 324-7756 if you want an inspector who reads the HVAC the way the lab reads the air sample.

Note. This article is environmental and HVAC-mechanical context, not medical or legal advice. PureSpec performs assessment only. We do not service or modify HVAC systems. For HVAC repair or replacement, hire a Florida-licensed HVAC contractor.