Central air installation cost: what you are paying for

A home with a forced-air furnace usually already has the ducts and blower that central air needs. The installer adds an outdoor condensing unit, an evaporator coil on top of the furnace, a refrigerant line set, a condensate drain and a thermostat connection.

Existing ducts are not always ready for cooling. Ducts sized for heat alone can be too small for the airflow an AC needs, and leaky or uninsulated runs in an attic waste cooling. Ask the contractor to inspect the ducts and state in the quote whether any need to be sealed, insulated or enlarged.

A home heated by a boiler, baseboards or radiators has no ducts. Adding central air there means a new duct system as well as the AC, plus an air handler to move the air. This Old House says new or major ductwork can add several thousand dollars to a project.

The price depends heavily on the house:

• Layout. A single-story home with an open attic or basement is the easiest. Finished ceilings and multiple stories mean more cutting and patching.
• Duct length and number of rooms. More supply and return runs mean more material and labor.
• Location of the ducts. Attic ducts need insulation. Ducts inside the conditioned space perform better and are harder to fit in an existing house.
• Electrical capacity. An older panel may need a new circuit or an upgrade.

The guide to ductwork replacement and installation cost covers the duct part of the job in detail.

If adding ducts would mean tearing into finished walls and ceilings, compare the quote with a ductless system. A mini-split puts a small indoor unit in each zone and connects it to an outdoor unit through a narrow line, so no ducts are needed. It suits additions, older homes and houses where only a few rooms need cooling.

The trade-off is one indoor unit per zone, which adds up in a large house. See mini split cost by number of zones for how the price scales.

Ask each contractor for a written quote with:

• The exact model numbers and capacity of the equipment or part being installed.
• Labor, materials and refrigerant listed as separate lines.
• Permit and inspection fees, and who pulls the permit.
• Removal and disposal of the old equipment.
• The parts warranty and the labor warranty, each with its length in writing.

For a home without ducts, also ask for a duct layout drawing, the number of supply and return registers, and who repairs drywall and paint after the ducts go in. Those items are where quotes for the same house differ most.