A furnace in Lake City earns its keep about twenty nights a year. Here is who those nights belong to.

Start with the two bills

The electric bill that lands in Lake City in early September is the largest of the year for almost every household, whether it comes from FPL inside the city or Clay Electric in the county. A furnace does nothing about that bill. The air conditioner does, and if yours is old, that is the page to read first.

The bill that lands in February is a different story. On a house heated by a heat pump, a cold stretch with several nights in the 20s means the electric strip heat ran for hours, and strips are the most expensive heat there is. That bill is where the furnace conversation legitimately begins, and it begins only for some houses.

Who actually benefits

Columbia County sees its first freeze somewhere between late November and mid-December, several more through February, and a hard freeze into the 20s in most winters. Call it twenty nights that need serious heat. For three kinds of household, a gas furnace handles those nights better than anything else:

1. Houses on Lake City Utilities natural gas, mostly the older neighborhoods around Lake DeSoto and downtown, where the line already runs to the meter and a furnace is the natural heating half of a straight-cool system.
2. Acreage homes on propane toward Fort White, Columbia City and Lulu, where the tank is already there for the stove or water heater.
3. Households that dislike heat-pump heat, which comes out of the registers at a lower temperature than furnace air. Some people never get used to it. A dual-fuel system, heat pump for mild days and furnace for cold ones, solves that without running strips.

Everyone else, which in this town is most people, is better served by a good heat pump with properly sized backup heat. We will say so.

Retrofit into a 1970s ranch versus a new Forest Cove build

The older block ranches that make up the heart of Lake City’s housing were built around a furnace closet or a garage air handler with a flue path that may or may not meet current code. Retrofitting a modern sealed-combustion furnace into one usually means a new vent run and sometimes a new return. It is routine work, but it is not a swap.

New construction in Forest Cove, Arbor Greene and Hunter Oaks is a different problem: those houses are tight, and a furnace in a tight house needs its combustion air brought in deliberately. On a new build we almost always recommend a sealed-combustion unit that draws outside air directly, and we size the whole system before the drywall goes up when the builder lets us.

Talk through the furnace question

The duct problem a furnace inherits

A furnace moves more air at a higher temperature than a heat pump does, and the flex duct in a 1980s attic was rarely sized for either. Leaky supply runs, crushed sections and an undersized return will make a new furnace loud, short-cycling and uneven. On every retrofit we test the duct before quoting the equipment, because a furnace on bad duct is an expensive way to heat an attic.

Sizing: smaller than the old one, almost always

Furnaces installed in this region twenty or thirty years ago were routinely oversized, partly because installers used rules of thumb and partly because a big furnace felt like insurance against the one cold week. A furnace that is too large hits the thermostat setpoint in minutes, shuts down, and starts again, which is hard on the heat exchanger and leaves the far bedrooms cold. The load calculation we run accounts for Lake City’s real winter design temperature, the insulation that is actually in the attic and the leakage we measured in the ducts. The number that comes out is usually a size or two below the nameplate on the unit coming out.