Last reviewed: September 2026 · By the HVAC Brief Editorial Team · Our sourcing and editorial standards
A furnace replacement is decided by what failed, not by the furnace’s age, and the installation is mostly about the things around the furnace: venting, combustion air, gas supply, condensate and duct static pressure.
One date should shape the decision. Non-weatherized gas furnaces manufactured on or after December 18, 2028 must reach 95% AFUE, which forces condensing equipment and a different venting arrangement.
When replacement beats repair
Three findings justify replacing rather than repairing, and none of them is a birthday.
- A cracked or failed heat exchanger. This is a combustion safety fault, not a cost question.
- A second major failure in a season. A control board plus an inducer motor on an older furnace approaches the cost of the machine.
- The furnace is wrong for the house. Chronic short cycling from oversizing is not fixed by new parts.
Routine wear items, igniters, flame sensors, capacitors, are repairs. Our page on what actually fails on a furnace sorts the two.
What the installation actually involves
Swapping the box is the quick part. These are the items that decide whether the new furnace performs and passes inspection.
| Item | Why it matters |
|---|---|
| Load calculation | Sets the output. Replacing like for like repeats any existing oversizing |
| Venting category | An 80% furnace vents into a flue or chimney; a 95% condensing furnace vents through plastic pipe, usually out a side wall |
| Combustion air | A sealed combustion furnace draws outdoor air; an atmospheric one takes it from the room and needs adequate supply |
| Gas piping | The line must be sized for the input rate; a larger or additional appliance can outgrow it |
| Condensate drain | Condensing furnaces produce acidic condensate that needs a drain and sometimes neutralising |
| Electrical | Dedicated circuit and a service switch |
| Duct connection and static pressure | A new blower on old ducts can reveal a restriction the old furnace masked |
| Permit and inspection | Gas appliance work is permitted in most jurisdictions |
If the old furnace shared a chimney with an atmospheric water heater, moving to a sidewall-vented condensing furnace leaves that water heater alone in an oversized, cold flue. Draft suffers and condensation can form inside the chimney. It has to be handled in the same job, usually by relining the chimney or replacing the water heater. Ask about it before the quote is signed, not afterwards.
Sizing, and the temptation to match the old one
The industry procedure is a room-by-room load calculation, ACCA Manual J, with Manual S for equipment selection. A furnace sized to the old unit’s input inherits decades-old assumptions about a house that has probably since been insulated or had windows replaced.
Oversizing produces short cycles, uneven temperatures and more wear. Our sizing guide covers what a real calculation asks for.
What to settle before signing
- The AFUE and model number of the specific furnace, and whether it is condensing.
- Where the flue will terminate, and what happens to anything else on the old chimney.
- Whether the gas line and electrical supply are adequate as they are.
- What happens to the ductwork, and whether static pressure was measured.
- Who pulls the permit, and whether commissioning readings will be recorded: temperature rise, gas pressure and a combustion check.
Our contractor checklist covers licence, warranty and payment terms.
The 2028 standard, and whether to wait
The standard applies to the date a furnace is manufactured, not to when it is installed. An 80% furnace built before December 18, 2028 remains legal to sell and install after that date, so nothing forces an early replacement.
What it changes is the future: if your furnace is near the end and your house has no practical sidewall venting route, the conversion is easier to plan now than to discover in an emergency in January. See the 95% AFUE standard for the detail.
What it costs
No federal series measures installed furnace prices. What is measured is the factory-gate trend, and gas furnace shipments are down 5.1% year to date through July, which tells you about demand rather than about your quote.
Our page on what HVAC replacement costs explains which parts of the price are measured federally and which are not.
Frequently asked questions
When should a furnace be replaced?
When the heat exchanger has failed, when a second major component fails in the same season, or when the furnace is the wrong size for the house. Age alone is a weak signal: routine wear parts like igniters and flame sensors are repairs, not replacement triggers.
What does furnace installation involve?
A load calculation, the furnace itself, the correct venting for its efficiency class, combustion air, gas piping sized to the input rate, a condensate drain on condensing models, electrical supply, the duct connection, and a permit with inspection. Commissioning should record temperature rise, gas pressure and a combustion check.
Do I need to replace my furnace before the 2028 rule?
No. The 95% AFUE standard applies to furnaces manufactured on or after December 18, 2028, not to installations. Equipment built before that date stays legal to install. The rule matters for planning if your house has no easy sidewall venting route.
How much does a new furnace cost?
No federal source measures installed prices, so any published range is a vendor survey. Make quotes comparable instead: same model number, same venting scope, same duct work, and ask whether static pressure was measured before the equipment was selected.
Methodology and limitations
Efficiency standards are quoted from 10 CFR 430.32(e). Installed base is EIA RECS 2020 table HC6.1. Shipments are AHRI’s July 2026 release. Load calculation procedures are ACCA standards, which we name but do not reproduce.
- We publish no installed prices, because no federal series measures them.
- Venting, gas sizing and permit requirements vary by jurisdiction and by appliance.
- Nothing here is installation guidance for a specific house.
Sources
- Code of Federal Regulations, 10 CFR 430.32(e), furnace efficiency standards.
- US Energy Information Administration, Residential Energy Consumption Survey 2020, table HC6.1.
- Air-Conditioning, Heating, and Refrigeration Institute, July 2026 shipment release.
The HVAC Brief is an independent trade publication. We do not sell equipment, take manufacturer advertising, or accept payment for placement in our research. Corrections welcome.
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