Cost of furnace replacement in Illinois (IL)
A 100,000 BTU furnace installed
$5,500to $8,300
That is a standard 80% AFUE model, the federal minimum and what most replacements are. A 96% condensing furnace runs $6,600 to $9,800.
Is the efficient one worth it here?
Costs more
$1,300
Saves a year
$377
Pays back in
3.4 yrs
Illinois burns enough gas over a winter for the efficiency to earn its premium back well inside the furnace's life. At $2.03 a therm and 5,900 heating degree days, the condensing model is the better buy.
Before you replace like for like
A furnace is not the only option. A heat pump does the same job using electricity, and in some states costs less to run — it depends on the gap between gas and electricity prices where you live, which is not the same everywhere.
Compare both in IllinoisCommon questions
How much does it cost to replace a furnace in Illinois?
Between about $5,500 and $8,300 installed for a standard 80% AFUE furnace, or $6,600 to $9,800 for a 96% condensing model. A 1,800 square foot home in Illinois needs roughly 100,000 BTU of output.
Is a high-efficiency furnace worth it in Illinois?
Yes. The condensing model costs about $1,300 more and saves roughly $377 a year on gas at Illinois prices, so it repays itself in about 3.4 years — well inside the life of the furnace.
What size furnace do I need?
About 100,000 BTU for 1,800 square feet in Illinois's climate — roughly 56 BTU per square foot, which reflects 5,900 heating degree days a year. Colder states need more output for the same house.
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Sizing from Illinois's heating degree days for a 1,800 square foot home. Gas price from the US Energy Information Administration, 2026-07. Installed costs are a national estimate spread ±20% — ductwork, venting runs and your region move real quotes more than the furnace itself does. Get three.