Why the bigger air conditioner made the Interlachen cottage wetter
Interlachen, seventeen miles west of Palatka on SR 20, sits among more than two dozen lakes, and the cottages around them are closed up for weeks at a time. Humidity, not heat, is what damages them.
Two cottages on the same lake
Both are about twelve hundred square feet, frame construction from the 1960s, screened porch facing the water, three small bedrooms, pine-paneled walls, used by the owners most weekends in spring and fall and a week or two in summer. Both sit on a sand lot, on a well, on Clay Electric rather than the FPL lines that serve Palatka itself. Both had the same problem when the owners bought them: a musty smell on arrival, mildew on the north walls, swollen doors in August.
Cottage A fixed it the obvious way. The previous three-ton system was “too small,” the owner reasoned, since the house never felt cold enough on arrival. A four-ton unit went in. It cools the house from 85 to 76 in about fifteen minutes. The mildew is worse than before.
Cottage B got a two-and-a-half-ton system, the size a load calculation produced, a humidity-sensing thermostat set to hold 80 degrees and 55 percent while the house is empty, and sealed returns. It takes forty minutes to cool the house on arrival. The walls are dry and the smell is gone.
Why the big unit lost
An air conditioner removes moisture only while it runs, and only after the indoor coil has been cold for a few minutes. A unit sized well above the house’s load reaches the thermostat setting quickly and shuts off, having removed very little water. The still-wet coil then evaporates its water back indoors while the compressor rests. The temperature is right; the humidity climbs. In a closed cottage the cycle repeats all day while nobody is there, and the house sits at 76 degrees and 70 percent humidity, which is precisely the condition mildew likes.
The smaller unit runs longer per cycle, the coil stays cold long enough to wring water out, and the humidity drops with the temperature. In a humid climate, long run times are the goal, not the problem.
What each costs to run, in words
Cottage A’s four-ton unit draws more power every time it starts, starts far more often, and still leaves the house damp, so the owner also runs a portable dehumidifier that fills and shuts off within a day of departure. The combined bill is noticeably higher than the neighbor’s.
Cottage B’s smaller unit runs in long, efficient cycles, and because the thermostat holds the house at 80 while empty it runs a fraction of the hours a 76-degree setting would require. The summer bill is modest, and there is no dehumidifier to buy, empty or replace.
Add what Cottage A spends on repainting the north walls every couple of years and replacing a mildewed mattress, and the difference stops being about the power bill.
The settings that matter for an empty house
- Thermostat mode: cooling, set to 78 to 80 while away. Not off.
- Fan: AUTO. On ON, the blower returns the coil’s moisture to the rooms during every off cycle.
- Humidity setpoint: 55 percent on a humidity-sensing thermostat; the system runs a longer, slower cycle when humidity is high even if the temperature is satisfied.
- Returns: sealed at the grille and the air handler so the system is not pulling damp crawlspace or porch air.
- Sliders and windows: closed and latched. One slider left cracked for the cat undoes everything above.
- Bathroom and kitchen fans: off while away; they pull humid air in through every gap.
A season-by-season calendar for a lake cottage
| Season | What the house faces | What to do |
|---|---|---|
| March to May | Pollen on the coil, first humid weeks, weekend use | Spring maintenance: coil rinse, drain treatment, float test, thermostat check |
| June to September | Peak humidity, long empty stretches, afternoon storms and surges | Hold 80 and 55 percent while away; check the house every two weeks or have a neighbor read the hygrometer |
| October to November | Drier air, cooler nights, heavier use | Raise the empty setpoint slightly; fall check of the drain and heat mode |
| December to February | Occasional freezes, little cooling | Heat mode tested; the humidity problem rests |
Signs the house is losing the battle
A musty smell at the door. Fog at the bottom of windows in the morning. Doors that stick. Dark spots on the north wall and in closets. A hygrometer, which costs less than lunch, that reads above sixty. Any of these in a cottage near Interlachen, Hollister or the lakes toward Melrose is a sizing or settings problem, and almost never a reason to buy a bigger unit.
Where the right size comes from
Not from the old unit’s nameplate and not from square footage alone. A load calculation accounts for the cottage’s orientation, glass, insulation, porch shading and the air it leaks, and it almost always comes out smaller than what the previous installer put in. We run it before quoting any lake house, and we will show you the number.
Lake-house humidity questions
What indoor humidity should a closed lake cottage hold in summer?
Fifty to fifty-five percent is the target; under sixty is acceptable. Hold above sixty for several days running and mildew starts on cool surfaces, and above sixty-five it spreads fast. The cottages we are called to in August are usually sitting at seventy or higher.
Would a standalone dehumidifier in the living room do the job?
For a small cottage it helps, provided it drains to the outside or a sink rather than a bucket that fills and stops. A whole-house dehumidifier plumbed to the air handler does it better and quieter, and a right-sized system with a humidity-sensing thermostat often makes either one unnecessary.
Can I just shut everything off when I leave to save money?
You can, and you will spend what you saved on repainting and replacing soft furnishings. A system holding the house at 80 degrees and under 60 percent humidity costs little to run and protects everything inside. Shutting it off in July is the most expensive option there is.
Does the lake itself raise the humidity inside the house?
Only through the air the house takes in: open porches, sliders left cracked, leaky returns pulling crawlspace air. The lake raises the outdoor moisture a little; the house’s openings are what bring it in. Seal the house and size the system, and the lake stops mattering.
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