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Stephenson County's housing stock splits right down the middle, half from 1960 or earlier, half newer. A furnace going into one of the older homes is going into a house that has likely seen changes to its ductwork, venting or insulation since it was built.
Installing a furnace in an older home means sizing it to the house in front of you, not to whatever ran there before. Venting and duct sizing that worked for the original system do not always suit the one going in.
NOAA nClimDiv, 1991–2020 normals
A furnace in Stephenson County runs about eight months a year, October through May.
A furnace sized to that stretch earns its keep here, and the wrong one shows its weak points fast.
Stephenson County falls under zone 5A in the 2021 energy code, a cool, humid zone where the furnace carries most of the load. Any gas furnace built on or after November 19, 2015 has to turn at least 80 percent of the gas it burns into heat over the winter. That floor rises to 95 percent for furnaces built on or after December 18, 2028, so the baseline itself is about to move. A furnace bought now still only has to clear 80 percent. One bought after that date won't have the option.
Eight months of heat a year puts real hours on a furnace here. Get the sizing right and keep a reasonable budget for the unit, and it will carry that load without trouble.
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A contractor sizing a furnace for one of these older homes needs to see the house as it stands, not guess from its age. That means a load calculation on the house in front of them, checking the ducts and vents that are there now rather than assuming anything about what the home had when it was built in 1960 or earlier. Skip that step and the new furnace inherits problems that had nothing to do with the old one.
A furnace too big for the house short cycles. It fires, blasts the place with heat, shuts off before it's pulled moisture out of the air or warmed the far rooms evenly, then fires again. That stop-start wears on the ignitor and the blower motor, and the house ends up with hot and cold spots instead of even heat.
Getting the size right means a contractor works out how much heat the house actually loses, room by room, based on wall and window area, insulation, ceiling height and duct layout. That number tells them what the furnace has to replace on the coldest day. Guessing from square footage or matching the old unit skips the part that actually matters.
Estimates for a typical Stephenson 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, and that alone can take a few hours once the ductwork, gas line and venting are disconnected. The contractor sets the new unit, then connects the gas line, the venting and the ductwork to it, checking each connection holds before moving to the next. Electrical comes next, tying the furnace into the thermostat wiring and the power supply it needs to run the blower and ignition. Once it's wired and connected, the system gets fired up and checked: gas pressure, temperature rise across the heat exchanger, and whether the blower is moving the right amount of air. A contractor worth hiring walks the ductwork at this point too, looking for a run that's choked down or a vent that was never quite right, because those show up now or they show up on the first cold night. Before anyone leaves, the thermostat gets set and tested, and the homeowner should get a plain answer on what the controls do and what the first year of filter changes looks like.
