Furnace replacement in Fort Belvoir.

Fort Belvoir mail covers two very different kinds of home. Family housing on post belongs to The Villages at Belvoir, which keeps up its own heating, so if you live there, your furnace is a service request to The Villages, not a call to us. Off post, the houses are ours: the 256 in Cook Inlet, on Inlet Cove Drive, Point Replete Drive, Stone Inlet Drive and the courts around them, and the older handful on Backlick Road in Accotink.

Fairfax County’s records date 251 of Cook Inlet’s houses to 2002, 2003 or 2004, and show every one of the 256 hooked up to gas. So a whole neighborhood of gas furnaces lit for the first time inside about three years, and twenty-some winters later they’re reaching the end of what a furnace usually gives, more or less together. If yours is still the original, price the replacement now, on a mild day, instead of the morning it quits. If the air conditioner beside it is the same age, the pair runs $8,898 to $23,000 installed.

Size is worth slowing down for. The county puts the typical detached house at about 3,080 square feet above ground, with a finished rec room in every one of their basements, and the townhouses at about 1,940. A house that big often runs two systems, one for the main floors and one for the bedrooms, and the two furnaces aren’t always the same age. Count the outdoor units out back before we come, and each furnace gets priced on its own condition.

Then there’s the vent. Nearly every Cook Inlet basement is a daylight basement, part of one wall above the yard, so a high-efficiency furnace’s two plastic pipes have a short run outside. Where they come out matters: the January 2016 blizzard left 23 to 31 inches across Fairfax County, and a drift over the intake shuts a furnace down on exactly the night you need it.