No federal rule, and no maker’s document we read, says when to stop repairing a furnace or a central air conditioner. What the primary sources do give is a short list of points to weigh: the system’s age against ENERGY STAR’s prompts (consider replacing an air conditioner more than 10 years old, a furnace more than 15), the refrigerant rules, which still allow R-22 and R-410A systems to be repaired, the efficiency a new system must meet, safety findings such as flooding, and the repair as a share of your own replacement quote. This hub takes them in the order that helps when a big repair quote arrives. We publish no prices: use your own written quotes.
If you smell gas: leave the house now. Don’t switch lights or appliances on or off, don’t use a phone inside, and don’t try to find the leak. From outside or a neighbour’s home, call 911 or your gas utility’s emergency line (PHMSA).
If a carbon monoxide alarm sounds: get everyone outside into fresh air, call 911 or the fire department from outside, and don’t go back in until responders say it’s safe. Headache, dizziness, weakness, nausea, chest pain or confusion that eases away from home can be carbon monoxide (CPSC; CDC). Don’t run the furnace again until a technician has checked it.
General information, not professional advice. Your system’s maker, your manual and a qualified technician decide what’s safe for your equipment, and local codes decide what needs a permit or a licensed installer.
First: is this a safety question, not a money one?
Some findings change the question from cost to safety. A furnace that has been under water is one: don’t use it or try to restart it. Carrier’s owner manual calls a flood-damaged furnace “extremely dangerous” and says a qualified service agency should inspect it and replace the gas controls and electrical parts that got wet, or the whole furnace if needed (80GSFRN-B-03OM, Edition 08/22); Trane and Lennox say the same.
A leaking heat exchanger or vent pipe is another. Rheem’s installation instructions for its 96V furnaces say a leaking heat exchanger “must be replaced” (92-24161-124-08, p. 110). Only a technician can confirm a crack: ask for the finding in writing. Our page on a cracked heat exchanger covers what the technician checks and what to ask.
How old is it, and what does age mean?
No law or federal agency sets a lifespan for a furnace or an air conditioner. The closest figures are the Department of Energy’s own estimates, made when it set efficiency standards, of the average age at which a unit is “retired from service”. An average hides a wide spread: plenty of units go sooner, plenty run longer.
Gas furnaces
For its December 2023 furnace rule, DOE estimated an average of about 21.5 years, 22.5 in its northern region and 20.2 in the rest of the country (88 FR 87502, section IV.F.6). Furnace makers’ consumer pages say 15 to 20 years, and ENERGY STAR’s 15-year line is a prompt to look, not an expiry date. How long a furnace lasts sets the figures side by side and lists what the makers say wears one out.
Central air conditioners
DOE’s 2017 rule put the national average at about 21 years (21.2), from 18.0 years in the hot, humid Southeast to 24.1 in the North and 24.9 in the hot, dry Southwest (82 FR 1786, Table IV-16). Carrier’s own page says 15 to 20 years with proper maintenance, and ENERGY STAR’s “more than 10 years old” is a prompt to review. How long AC units last explains why climate moves the number.
Finding the age
- Check the paperwork: the manual’s record page, the invoice, a warranty registration or a permit record.
- Photograph the rating plates on the outdoor unit and on the furnace or indoor coil. The two parts may not be the same age. If no date is printed, the maker’s customer service can tell you from the model and serial number.
- In the Southeast and Southwest, contractors must keep the make, model, serial number, address and installation date of each split-system outdoor unit they install for at least 48 months (10 CFR 429.142), so a recent installer should have the record.
Age alone isn’t a reason to replace a system that works well. It matters because the older the system, the more of its other parts are as old as the one that failed.
The refrigerant on the plate
For an older AC, the refrigerant often matters more than the age. You may hear that an R-22 or R-410A system has to be replaced. We checked both claims against EPA’s rules, and neither is true.
- R-22. US production and import ended on 1 January 2020, “but you may continue to use HCFC-22 in your existing system for as long as necessary” (EPA-430-FR-20001, February 2020). There are no drop-in replacements. See R-22 refrigerant.
- R-410A. A system you already have can keep running and be repaired. Replacement parts are exempt from the bans on making and selling these products, and those made after the cut-off are labelled “For servicing existing equipment only” (40 CFR 84.58(b)). See R-410A after 2025.
- R-454B and R-32. These A2L refrigerants are allowed only in new equipment designed for them, never as a conversion of an existing system, so a quote that moves you to one replaces the refrigerant-carrying equipment. See R-454B and R-32.
Whatever the refrigerant, only an EPA Section 608 certified technician may open the refrigerant circuit, even to attach gauges (40 CFR 82.152), and EPA asks owners to have leaks found and repaired rather than topped off. When a repair quote is for refrigerant, ask whether it’s a leak repair or just a recharge: AC refrigerant leaks covers the signs and the questions.
Efficiency: what a new system must meet
Furnaces. A new indoor gas furnace must reach at least 80% AFUE today (10 CFR 430.32(e)(1)(ii), read 10 November 2026). AFUE is the share of the fuel’s energy that becomes heat in the house over a typical year. DOE puts older standing-pilot furnaces at 56 to 70% and condensing furnaces at 90 to 98.5% (DOE Energy Saver, Internet Archive copy of 31 May 2026). DOE also says a chimney should be relined when an older furnace is replaced with one of 80% AFUE or more, so ask how a new furnace will be vented. The 95% rule is on the books for furnaces made from 18 December 2028, but it is under legal review and may change (checked 10 November 2026): the Supreme Court vacated the appeals court judgment that had upheld it and sent the case back (No. 25-879, 8 June 2026). Either way it covers furnaces as they’re made, not one already in your house. The AFUE guide has the detail.
Air conditioners. A split-system central AC made since 1 January 2023 must reach at least 13.4 SEER2 nationally; one installed since then in the Southeast or Southwest must reach 14.3 SEER2, or 13.8 SEER2 at 45,000 Btu/h and up (10 CFR 430.32(c)(5) and (c)(6)). That binds installers, not you, and a unit already in your yard isn’t affected. If you know your cooling kWh, the cooling cost calculator estimates the yearly cost at two SEER2 levels.
Weighing the quote
You’ll read rules of thumb such as “replace once a repair reaches half the price of a new system” or the “$5,000 rule”. No federal agency or maker document we read sets them. The repair or replace check puts your own repair and replacement quotes side by side as a share, with the age, refrigerant, flood and efficiency points for your system, with a fuel saving estimate. It gives points to weigh, not a verdict.
Two more facts belong beside the share. How many repairs the system has needed lately: ENERGY STAR lists equipment that “needs frequent repairs” while energy bills go up as a reason to consider replacing. And whether the failed part is still under the maker’s warranty, which can bring the repair down to labor; makers attach conditions, such as a yearly check. Don’t count on a federal tax credit for a system installed in 2026: the IRS says the energy efficient home improvement credit covers improvements made through 31 December 2025 (IRS).
A reading path
- Find the age, then read the lifespan page for your system.
- On an AC, read the page for its refrigerant.
- Get the repair and a replacement quoted in writing, covering the same work.
- Run the repair or replace check with each pair of quotes.
Your next step
Write down for the technician the model and serial numbers from the rating plates, the install date if you have it, any error code, what happened and when, and recent repairs. Ask what failed and why, and whether the part is under warranty. Make sure you have working CO alarms on every level and outside sleeping areas (CPSC). What can’t wait: a gas smell, a carbon monoxide alarm, a burning smell from wiring, or a furnace that has been under water. Get out first where the box above says so, then call.
