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Most homes in Clinton County heat with utility gas, at just over half. Electricity comes next, then propane for homes that don't have a gas line nearby. If your old furnace ran on gas, putting in another gas furnace is usually the straightforward call. The venting, the gas line and the layout of the ductwork are already built around that choice. Swapping fuel types means more than a new furnace, so check what your house is already set up for before you compare options.
NOAA nClimDiv, 1991–2020 normals
Half the homes in Clinton County were built in 1977 or earlier, half after.
Either way, a standard size furnace matched to your square footage and ductwork is what fits.
Clinton County is zone 4A, a mixed climate that asks real work of both heating and cooling. Any furnace going into a home here has to meet 80.0 percent AFUE at minimum, the federal floor for non-weatherized gas furnaces today. That's already the baseline on just about anything you'd be quoted. The minimum jumps to 95.0 percent AFUE for furnaces built starting in late 2028, so the standard unit available now won't be the standard unit available in a few years. If you're weighing a standard-efficiency furnace against a high-efficiency one, the choice is yours to make, not the code's.
Seven months of heat a year, down to 25 degrees on a winter night, is well within what a standard furnace handles. Nothing about Clinton County calls for oversizing or for paying up for a higher tier than you need. Get the size matched to your house and move on.
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The rating on the label tells you how much of the gas it burns turns into heat for your house, and how much goes straight out the flue as exhaust. A higher rating means less of that gas is wasted, but it also means a different kind of furnace. Getting more heat out of the exhaust before it leaves takes a second heat exchanger and a drain for the condensation that forms. That changes how the furnace vents too, usually PVC pipe out a sidewall instead of a metal flue up through the roof. Once it's installed, that rating doesn't drift downward. A furnace doesn't lose efficiency with age the way a car loses mileage. What hurts it is a dirty filter, a blower working harder than it should, or a fouled flame sensor. Keep it maintained and it burns gas at close to the same rate the day you retire it as the day it went in.
A furnace too big for the house turns on, blasts heat fast, and shuts off again before it's pulled moisture out of the air or warmed things evenly. That short cycling wears on the ignitor and the blower motor both, since they take the most stress at startup. A furnace sized right runs longer, steadier stretches and wears slower doing it.
A contractor works out the right size with a room by room load calculation, called a Manual J, that accounts for the square footage, the windows, the insulation and how the ductwork is laid out. It's not a matter of matching whatever furnace is already in the basement. If that one was sized wrong to begin with, or the house has since had windows replaced or insulation added, the old tonnage isn't a safe guide.
Estimates for a typical Clinton County home, from EIA household energy data, Illinois fuel prices and NOAA climate normals. Your home will differ.
Installation day starts with the old furnace coming out, along with the venting that served it if the new one vents differently. Expect the work area, usually a basement or closet, to be cleared and protected before anything gets cut or disconnected. The gas line gets shut off and tested once the new furnace is connected, and the electrical and thermostat wiring gets hooked in. If the venting changes from a metal flue to PVC, that means new piping run to a sidewall, which takes more time than reusing what's there. Ductwork gets connected to the new unit, and any transitions that don't match up get rebuilt rather than forced. Before anyone leaves, the furnace gets fired up, checked for correct gas pressure, and run through a cycle to confirm it heats and shuts off the way it should.
