Ductwork, airflow, and humidity in Jacksonville

The complaint sounds like a weak air conditioner and is often the ducts: a bedroom that never cools, a bonus room over the garage ten degrees warmer than the hall, a house that reads 75 °F and feels damp, a system that runs all afternoon and never shuts off. On the First Coast the ducts usually run through an attic that reaches 130 °F in July, and every leak on the return side pulls that air into the system.

The right first step is a measurement, not a bigger unit.

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What the houses here do

By era, because the era decides the ducts.

• Springfield, Riverside, Avondale, San Marco, Ortega, St. Augustine's Lincolnville: 1900s to 1930s houses retrofitted from window units, with the air handler in a closet, one undersized return, and ducts added later through whatever space was there.
• Arlington, Lakewood, the Beaches, Davis Shores: 1950s and 1960s block houses, often with the air handler in a hall closet or a garage and ducts in a low attic; the original metal trunk may still be in service.
• Mandarin, the Southside, Orange Park, Argyle: 1975 to 1995 houses with flex duct in a vented attic, two or three systems in, and the flex crushed, kinked, or disconnected at the boots.
• Nocatee, St. Johns, Oakleaf, Fleming Island, Bartram Park: 1998 and newer, builder-grade flex sized to the drawing, a bonus room on the end of the longest run, and the first duct test the house has ever had.

What a contractor measures

Ask for these before anyone proposes a larger system.

• Static pressure at the air handler: high static means the ducts or the filter are choking the blower.
• Airflow per register and total, against the design; a duct leakage test with a blower door or a duct blaster where the numbers do not add up.
• Return size against the tonnage: many older First Coast houses have less return than the system needs, and the contractor should measure it rather than assume.
• Attic conditions: duct insulation, crushed runs, disconnected boots, the plenum, and whether the ducts can be sealed, repaired, rerouted or replaced.
• Humidity: indoor relative humidity through a cooling cycle. A system that is oversized or leaking cools the air before it dries it, and the answer may be a right-sized system, a variable-speed blower, or a whole-house dehumidifier, not more tonnage.

The work

Sealing (mastic, not tape) and insulating existing ducts, adding a return, replacing crushed flex, rebuilding a plenum, or a full duct replacement. A full replacement is permitted mechanical work in the City and the counties; sealing and repair of existing ducts usually is not, and the contractor should say which yours is. Ask for a before-and-after leakage number so the work can be checked.