Cool but clammy: a Jacksonville house at 73 degrees and 65 percent humidity
A thermostat only measures temperature. In a city where the dew point lives in the 70s for four months, that is half the story, and the half it misses is the one you feel.
The weather the house is fighting
From June through August, Jacksonville’s dew point averages 71 to 73 degrees. By early August nearly every day qualifies as muggy, and the pattern holds into late September. Hurricane season layers weeks of rain on top. Every time a door opens, every gap in the duct, every breath of outdoor air brings in water that the house has to get rid of, and the only appliance doing that is the air conditioner, and only while it runs.
The thermostat does not know any of this. It reads 73 and stops. Which is how a Mandarin two-story ends up cold to the touch and damp to the skin: the cooling has outrun the drying.
Why the AC is not drying the house
The coil removes moisture while it is cold and air is moving across it. Anything that shortens that time or warms that coil leaves water in the house. Ranked by how often we find each cause in Duval County:
- Oversized equipment. The dominant cause in post-2000 subdivisions. A unit too big for a tight house cools the air in minutes and shuts off before the coil has condensed much of anything. Short cycles, cool rooms, 65 percent humidity.
- Leaky return duct pulling attic air. In 1960s Arlington and Westside block ranches, the return side of the duct runs through a 130-degree attic. Any gap sucks hot, wet attic air straight into the system. The house never gets ahead of it.
- Fan set to “on.” The blower runs between cycles and re-evaporates the water sitting on the coil. A one-switch fix.
- A fouled or iced coil. Pollen from February to May packs the coil, airflow drops, and the coil either freezes or runs so poorly it cannot dry. Common in homes that only change one-inch filters twice a year.
- Crawlspace and slab moisture. Riverside, Ortega, San Marco and the lower Southbank lots take on water with tides and storms, and a 1920s floor is not a vapor barrier. The house imports humidity from below.
- Infiltration. Original single-pane windows in the historic districts, an attic hatch that does not seal, a bath fan with no damper. Each is a small leak; together they add up.
- Low refrigerant charge. A coil running warm because the charge is low cools a little and dries almost nothing. Beachside systems with corroded coils reach this sooner.
Readings that mean “call this week”
Buy a hygrometer and put it in the main living space. If it reads above 60 percent for most of a week with the AC running normally, the house has a humidity problem and not just a muggy day. If the air handler closet or garage smells musty, or the supply registers show black speckling, the coil has been wet too long. If a bathroom mirror fogs without a shower, or doors swell and stick in July, the structure is absorbing water. Any of those is enough to call.
What we would change, cheapest first
- Set the fan to auto. Free.
- Change the filter and clean the coil. Low cost; the coil cleaning is part of a maintenance visit.
- Seal the return duct and the attic penetrations. Modest cost, and on a block ranch the single most effective fix we do.
- Fix the charge or the leak. A repair call.
- Slow the blower. On a mildly oversized system, a lower fan speed keeps the coil colder and wetter; a technician sets it against measured static pressure.
- Add a whole-house dehumidifier. It runs on humidity alone, covering the mild, rainy days when the AC barely cycles. The right fix for most tight 2000s houses, and for any riverfront house with a damp crawlspace until the crawlspace is dealt with.
- Replace an oversized unit with a staged one sized by load calculation. The permanent fix on a twenty-year-old oversized system. A two-stage or variable-speed Lennox system runs long and low and dries as it goes.
We serve Jacksonville from the Palm Coast headquarters and quote every one of those steps before doing it. The maintenance agreement that keeps the coil clean is the quiet part of the answer; ask what it includes when you call. [[CONFIRM WITH JERRY]]
Humidity questions from sticky Jacksonville houses
What indoor humidity should a Jacksonville house hold in summer?
Between 45 and 55 percent. Above 60 for days at a time, mildew starts in closets and on the coil; the musty smell follows. A hygrometer from any hardware store will tell you where you stand.
Would turning the thermostat down fix it?
Briefly and expensively. Colder air holds less water, so 70 degrees feels drier than 75, but you pay JEA for the extra cooling and the house still swings damp whenever the system rests. Fixing the drying is cheaper than overcooling.
Does running the fan on 'on' instead of 'auto' help?
It hurts. With the fan running constantly, the water sitting on the coil after each cycle is blown back into the house instead of draining. Set it to auto unless a technician has told you otherwise for a specific reason.
My San Marco house gets musty after every king tide. Related?
Very likely. Water under the house after tidal flooding evaporates up through the floor for weeks. The HVAC fixes help, but a crawlspace that floods needs drainage and a vapor barrier before any equipment can keep up.
Cool enough, still sticky?
Call with the thermostat setting and a hygrometer reading. We'll tell you which of these causes fits before we schedule.