Furnace replacement in Great Falls.

Natural gas heats a smaller share of Great Falls than of Fairfax County: about 46 in 100 owner homes here, against 61 countywide. Propane fills much of the gap. About one owner home in twelve heats with propane from a tank in the yard, more than five times the county’s share. Both fuels burn in the same furnace; for a propane house it gets new burner orifices and a gas valve set for propane pressure before it’s lit.

The county’s tax records put the typical house in 22066 at 1985, and half went up in the 1970s and 80s. With fifteen to twenty years the usual run for a gas furnace, the one in a house that age is probably its second or third, and if the last swap came before 2011, it’s due again. The builder’s own furnace is likeliest in the 2000s houses, about one in nine here, and those have now seen seventeen to twenty-six winters.

What makes this a furnace-only job is a cooling side worth keeping. If the outdoor unit and the coil above the furnace are under ten years old or so and cool well, leave them. We read both model plates before pricing, so the new furnace’s blower delivers the airflow that air conditioner was built around. An outdoor unit labeled R-22 dates from before about 2010; then the pair, $8,898 to $23,000, is the price to see.

Three in four houses here are two stories, and the bedrooms often have their own furnace up in the attic. That swap takes more planning. The new furnace has to come through the hatch or the pull-down stair, it sits in a pan with a float switch so a blocked drain shuts it off before water reaches the ceiling, and a high-efficiency model’s condensate line has to be kept from freezing. The basement one is simpler: by the county’s books two in three houses here have a daylight basement, open at ground level on one side, so the old furnace can often go out a door instead of up the stairs.