Ductless mini splits in Oakton.

North and west of Jermantown Road, Fairfax County’s plan describes the Difficult Run headwaters as a place of steep slopes, with homes typically on more than an acre each. A house set into a slope often has a lower level that opens onto the yard on the downhill side, and plenty of those got finished long after the builder left, with a register or two tapped off the main trunk overhead. That floor tends to run colder than the rest of the house most of the year, because the thermostat sits a floor or two up. Mounting a head on that level lets it keep its own temperature.

Nearly half of Oakton’s homes date from 1980 through 1999, when the room above the garage was in a lot of floor plans. That room has a garage under it, a roof right on top of it and three exterior walls, and it usually sits at the far end of the longest supply run. Turning the whole system down to cool it overcools every other room; a mini split handles that one and leaves the rest alone.

Lots that size sometimes hold a building that never had ducts: a detached garage, a workshop, a studio or a pool house. A single zone, with its outdoor unit right beside the building, heats and cools it with nothing run from the house but power. The electrical decides much of that price: the building needs a circuit big enough for the unit, and a long run back to the main panel adds to the job.

Closer to Chain Bridge Road and Blake Lane, in the condo buildings and in townhouse clusters like Oak Marr Courts and Oakborough Square, a mini split starts as a question for the association. The condenser has to go on a pad or a wall bracket, the copper runs along the siding, and both are usually the association’s call before anything gets priced.