Furnace replacement in Arlington.

Owner-occupied houses in Arlington are older than in any other Northern Virginia county or city, the median one built in 1960. About 6 in 10 owners heat with gas, and in the older neighborhoods the furnace usually sits in a basement the house was built with. Westover’s 1940s houses stand on cinder-block foundations over a basement, and all 104 of Glebewood Village’s rowhouses, built in 1937 and 1938, have one too.

Central air reached most of those houses long after they were built, so the air conditioner and the furnace in an Arlington basement often have little in common but the ductwork. One may be five years old and the other twenty-five. That’s when replacing the furnace alone makes sense: keep a young outdoor unit and the coil that goes with it, and put the money into the half that’s failing. In some Cape Cods an attic air handler cools the upstairs on its own, and a new basement furnace never touches it.

The County’s gas permit rule is below, and it holds even for a same-size swap in the same spot. On top of it, Arlington strongly suggests the contractor doing the work hold the permit, and lets an owner hold a residential permit for only one address every two years, which rules out a landlord handling the paperwork house by house.

Efficiency is the real choice in a basement like these. A standard furnace keeps venting up the old masonry chimney; a high-efficiency one sends its exhaust out through a wall in PVC, and if your gas bill comes from Washington Gas, it pays its Virginia customers a rebate on a furnace rated 95.1 percent AFUE or better. In Maywood and the County’s other local historic districts, ask before a new vent goes through an outside wall, since exterior changes there can need a certificate of appropriateness.