Mini-splits or central AC for a Hamptons house?

Central air: when the ducts are there

A central system is one outdoor condenser, one indoor coil, and the ductwork that already runs through the house. If the ducts are sound and sized for the load, a replacement is the simplest project in HVAC: $3,500–7,500 installed for a standard-efficiency (14.3–16 SEER2) system, $8,000–12,000 for high efficiency (17–20+ SEER2), a 2-ton unit $3,200–6,500, and about 4–8 hours of work. The federal minimum in New York is 13.4 SEER2. Its weaknesses: the ducts, which in many 1990s–2010s South Fork houses leak into the attic (sealing $1,000–2,500; Aeroseal $1,500–6,900), and the rooms at the end of the run that never get cool.

Ductless: when they are not

A mini-split is an outdoor unit feeding one or more wall-mounted heads through a small refrigerant line. No ducts, so it fits the whaling-era houses in Sag Harbor, the village Capes, the finished attic, the addition and the pool house. Costs: $2,000–8,000 for a single zone (a Mitsubishi single zone $3,500–5,500), $5,000–12,000 for 2–4 zones, plus $2,000–7,000 per additional zone; a large Mitsubishi multi-zone $8,000–15,000 or more. Half a day to a day for one zone, 1–3 days for several. Each head is its own zone with its own control, which is the feature people notice most. Its weaknesses: the heads on the wall, the outdoor units to find a home for, and a per-square-foot cost that climbs when a whole large house goes ductless.

Both

The common Hamptons answer. Central air carries the main floors on the existing ducts; a mini-split takes the attic bedroom, the addition, the garage apartment and the pool house. One contractor, one maintenance plan, two kinds of equipment.

Heating

Modern mini-splits and heat pumps heat as well as cool, and cold-climate models hold capacity at 5°F, below the 11.9–15.7°F design temperatures used for eastern Long Island. A mini-split in the main room is often the cheapest way to keep a closed-up second home above 55°F from October to May, with the boiler as backup.

Salt air, either way

The outdoor unit is the vulnerable part of both systems. Near the water, specify a salt-spray-rated or coated coil, raise the unit at least 12 inches off grade, and plan on a low-pressure rinse 2–4 times a year.

How to decide

Sound ducts, whole-house cooling, and a house you occupy in summer: central. No ducts, a few rooms, a historic house you would rather not open up, or an empty winter house that needs cheap heat: ductless. Both: most large South Fork houses. In every case, New York code requires the equipment to be sized from a Manual J load calculation; a contractor who quotes "a 4-ton like you have" has skipped it.