AC not keeping up: when the house won't drop below 80

Inland afternoons, coastal nights

The thermostat says 74, the house says 81, and the system hasn't shut off since lunch. Some people turn the thermostat down to 68 hoping it will try harder. It won't. An air conditioner has one speed of trying, or a few on variable systems, and a lower setting only means it runs longer.

Where you live changes what "normal" looks like. Crestview, up by I-10, is the hot-afternoon end of the metro: 108 days a year at 90°F or hotter at Bob Sikes Airport, with a July average high of 93°F. The Destin-Fort Walton Beach airport averages 71 such days and a July high of 91°F (2016 to 2025). But the coast doesn't cool off at night. July lows average 78°F there against 72°F in Crestview.

Inland vs coast, yearly average, 2016 to 2025

So in Crestview or Baker, a system slipping a couple of degrees at 4 pm in July is common, and the test is whether it recovers once the sun drops and the night cools into the low 70s. On Okaloosa Island or in Destin the afternoon is easier but the night gives the system little relief, so a beach unit that is still behind at midnight is the one to worry about.

Normal, and not normal

• Normal: runs most of a hot afternoon, house drifts up a degree or two, catches up by mid-evening, and the air at the vents feels distinctly cold.
• Not normal: the house is still warm at 10 pm, the vent air feels cool rather than cold, the system short cycles, or the house feels damp even when it is cool.
• Never normal: ice on the copper lines or the indoor coil. Long run times should not freeze a healthy system. Ice points to low airflow or low refrigerant.

One local question I saw was a coil that kept freezing all through a family's first summer in a house they had just bought. That's the kind of thing to have looked at early, not after the third thaw.

A useful measurement to ask for is the temperature drop between the air going into the return and the air coming out of a supply vent. It tells the technician quickly whether the system is doing its job or whether the problem is the house or the ducts, or just the sun.

Cheap things to check first

• The filter. A clogged one starves the coil of air, and so can a very restrictive high-rated filter the system wasn't designed for. Try a fresh, mid-grade filter.
• The outdoor coil. Pine pollen and grass clippings inland, salt film near the water. Look at the fins with the power off and rinse gently with a garden hose if they're matted.
• The drain pan. If the float switch is cutting the system off, it looks like the AC "can't keep up." See drain line clogs.
• Closed vents and blocked returns. Furniture over a return grille matters more than people think.
• Sun. Blinds on west and south windows in the afternoon make a real difference, especially in a newer Crestview subdivision house with little shade.

What a technician usually finds

• Low refrigerant from a leak. Adding refrigerant without finding the leak means paying again. Fixr lists leak repair at $200 to $1,500.
• A weak capacitor or failing fan motor that lets the outdoor unit run, but poorly. This Old House puts capacitor replacement at $80 to $400.
• Leaky attic ducts dumping cold air into a very hot attic. See ducts.
• A system that was undersized for the house, or oversized and short cycling, which leaves the air cool but damp. The DOE says size should come from a load calculation, not square footage. See AC sizing.

A brand new system that won't keep up

Someone replaces a 20 year old unit, and the new one struggles the first week. The usual suspects are the refrigerant charge, the airflow setting on the new air handler, reused ductwork that wasn't checked, and sizing. Call the installer back first; a new install should be checked under load. Florida requires a permit to replace a mechanical system, and the City of Crestview and Okaloosa County both say so, so ask whether the final inspection passed. If the installer says the house simply can't be cooled, get a second opinion. See what an installation should include.