Guide
AC sizing and humidity in Florida: why bigger is the expensive mistake
Updated
The most costly decision in a Florida changeout is not which brand you buy. It is agreeing to a system a half-ton too big because it feels safer.
Two loads, not one
- Sensible load
- The heat you can feel on a thermometer, coming through walls, windows, roof and from people and appliances. This is what most people mean by cooling.
- Latent load
- The moisture the system has to condense out of the air, from outdoor humidity, infiltration, showers, cooking and plants. In Florida this is a large share of the total, and it is the part an oversized system fails at.
A cooling system removes moisture only while the coil is cold and air is moving across it. An oversized unit satisfies the thermostat quickly, shuts off, and the moisture stays. You get the specific and familiar Florida complaint: the house is cold and still feels sticky, and someone suggests setting the thermostat lower, which makes it worse.
What right-sizing looks like
- A written load calculation. ACCA Manual J is the industry standard method: it accounts for orientation, glazing, insulation, infiltration and occupancy, not just square footage.
- Duct design checked too. Manual D for duct sizing and Manual S for equipment selection are the companion steps. A correct load with an undersized return is still a bad install.
- Suspicion of a straight swap. Matching the tonnage of the outgoing unit copies whatever mistake was made when the house was built or last re-equipped.
- Longer run times, not shorter. A properly sized system in Florida should run long and steadily on a hot afternoon. That is the dehumidification happening.
- Variable-speed or two-stage equipment holds low capacity for long periods, which is exactly the behavior that dries a Florida house out. It is a genuine comfort upgrade rather than a sales upgrade.
Humidity problems that are not the equipment
- Leaking return ducts in the attic. A return leak pulls hot humid attic air straight into the system, adding latent load that the equipment then has to fight. New equipment on leaky returns is money burned.
- Oversized or badly located supply registers that short-circuit air back to the return without conditioning the room.
- An air handler in an unconditioned garage or attic, with an uninsulated cabinet sweating and losing capacity before the air reaches you.
- A blocked or badly sloped condensate drain, which is also the number one cause of emergency shutoffs and ceiling damage in Florida homes.
- Building leakage. Recessed lights, attic hatches and duct chases let humid air in continuously. Sealing is cheaper than tonnage.
The three sentences to say to every contractor
"Please give me the Manual J load calculation in writing, with the sensible and latent split." "Have you tested the duct leakage, and what did you find?" "If the load comes out smaller than my current system, will you quote the smaller system?" A contractor who answers all three well is the one to hire, almost regardless of price.
Manual J, Manual S and Manual D are ACCA industry standards, named here in plain text rather than linked. Ask your contractor for the calculation output, not for the standard.