HVAC Replacement Cost: What Furnace, AC, and Replacing Both Together Actually Run

A standard-efficiency (80% AFUE) furnace, straightforward swap, runs roughly $3,500–$5,500 installed. A high-efficiency (95%+ AFUE) furnace, including the new PVC venting it requires, runs roughly $4,800–$7,500. The full furnace replacement breakdown covers what's included at each tier and what installation day looks like.

A full AC installation with a real Manual J load calculation starts from $3,800, with the final number depending on tonnage, ductwork condition, and efficiency tier. The full AC replacement breakdown covers repair-vs-replace, sizing, and what the written scope should include.

This isn't a bundled discount or a sales incentive — it's a mechanical fact about how the two jobs overlap. Replacing a furnace and an AC on separate visits means paying for two separate mobilizations, two separate permit filings in townships that charge per permit, and often duplicated electrical or disconnect work that could have been done once. Replace them together and a lot of that overlap collapses into a single job: one crew on-site for one day instead of two, frequently one permit covering both scopes instead of two, and one commissioning visit instead of two.

The furnace and the air handler that pairs with a new AC system also interact directly — airflow, ductwork sizing, and in some configurations the blower itself are shared between the two systems. Replacing them in the same visit means sizing both together instead of matching a new AC to whatever the existing furnace's air handler happens to support, which is exactly the kind of guess Manual J sizing exists to avoid.

Most houses had their furnace and AC installed around the same time originally, which means they tend to reach the 15–20 year furnace / 10–15 year AC replacement window closer together than people expect. If one system just failed and the other is already within a couple of years of that same window, replacing both now as one project is a real, defensible decision — not urgency being manufactured, just two systems that were always going to need this conversation close together.

A healthy AC with years of life left doesn't need replacing just because the furnace died. Run the same 50% rule on each system independently: if a repair costs more than roughly half of a new unit, or the unit's age times the repair cost tops $5,000, that system is a replacement candidate. If it isn't, it isn't — a working system with real life left doesn't become a better deal just because a crew is already on-site for the other one.

Not simply the sum of both jobs. A furnace alone typically finishes in a day; an AC installation alone runs one to two days. Done together, most straightforward combined replacements finish in one to two days total, not three or four — because the crew is already on-site, sharing setup and teardown, and the electrical and disconnect work that would otherwise happen twice happens once. A high-efficiency furnace needing new PVC venting, or ductwork modifications on either system, can add a half day either way.

A written quote covering both scopes — furnace and AC — with each line itemized separately, not one flat number with no breakdown. A Manual J load calculation for the AC side and a Manual D duct check confirming your ductwork can move air for both new units correctly, not just whichever one is more obviously failing. Permit filing handled as part of the job, whether that's one combined permit or two, depending on your township. Old equipment removal and disposal for both units, and a single commissioning report with final readings for the whole system once it's done — the same written-scope standard either system gets replaced on its own.

Every new central AC installed in Illinois has to meet a federal minimum of 13.4 SEER2 in the North region — equivalent to the old 14.0 SEER standard — effective since January 2023 under an updated testing procedure. On the furnace side, ENERGY STAR's efficiency guidance puts modern high-efficiency units at 95%+ AFUE versus 60–70% for furnaces built before the mid-2000s. Neither number changed because equipment got worse — the North region SEER2 change reflects a stricter test method, not a step backward in what a system can actually do.

Worth being precise about this, since it changes yearly: the federal Section 25C Energy Efficient Home Improvement Credit expired for installations after December 31, 2025, and isn't currently available for a 2026 install, on the furnace or the AC side. Utility-level and state-level rebates are a separate, still-evolving track and sometimes still apply — we check current ComEd and Nicor Gas program offerings directly on your quote rather than quoting a number that may already be stale by the time you read this.

A furnace alone runs $3,500–$7,500 depending on efficiency tier; an AC alone starts from $3,800. Replacing both together usually costs less per system than doing them apart, because the labor, permitting, and sizing work overlap — not because of a discount. Whether that's the right call for your house comes down to running the 50% rule on each system independently, not on which one happens to be dead this week.