Furnaces and air conditioners are dated machines with a typical service life, not mysteries. Gas furnaces often run 15 to 25 years. Central air conditioners often run 10 to 15 years, less in hot, coastal, or hard-start climates. A house can have a new condenser and a 20-year-old air handler, or a new furnace and an original evaporator coil full of the wrong refrigerant. "HVAC replaced" without a year and a component list is not information.

The listing will say the house has heat and air. Every house on the market claims that, including the one whose furnace was red-tagged last winter and reset for the open house.

What it is

The heating plant (gas furnace, boiler, heat pump, electric strip) and the cooling plant (split system, package unit, heat pump). Age is on the data plate: a serial number you can decode, or a manufacture date printed on newer equipment. R-22 air conditioners are the ones that matter historically. Production of that refrigerant stopped for new equipment years ago. Recharging an R-22 system is expensive and temporary. If the outdoor unit is from before about 2010, ask.

Boilers and steam systems in older housing are a different trade. Do not let a standard "HVAC tune-up" be the only look at a 90-year-old boiler.

How to spot it in the listing

Utility-room photos, if they exist. The condenser is often in a side-yard photo by accident. A rusty outdoor unit on a leaning pad, or a condenser that is a different color era than the house's "new" interior, is a clue. Remarks: "newer HVAC," "roof unit 2019," "furnace unknown." Prior winter listings sometimes mention that heat was not on. Compare. The inspection checklist should include "read the data plates" as a line, because inspectors do it and buyers never ask for the photos of the plates.

Permits tell you more than adjectives. A mechanical permit in 2018 is a fact. "Serviced regularly" is not.

How to spot it at the showing

Open the furnace door or ask the inspector to. Photograph the data plate. Turn the heat on and the cool on, even if the agent says "let's not, it's mild." A system that short-cycles, blows barely cool air, or smells like scorched dust the entire visit is telling you something. Listen at the condenser. A scream is a failing motor or compressor, not "old houses make noise." Look at the filter. A filter black with dust in a "renovated" house means the system was not part of the renovation. That is the flip pattern: the surfaces are new and the machine is not.

Condensate leaks around the air handler stain the platform and rust the cabinet. That stain is a maintenance failure that becomes a ceiling stain one floor down.

The real risk

A mid-winter failure, an R-22 system that cannot be economically recharged, and a flue that is not drafting (a safety issue, not a comfort issue). Carbon monoxide from a cracked heat exchanger is the reason the inspector looks at the furnace with a mirror and the reason you do not ignore a red tag. Efficiency is the smaller issue. A 20-year-old 80% furnace costs more to run than a new 95% unit. The bill difference rarely pays for the replacement by itself. The replacement happens because the old one dies or the heat exchanger is condemned.

Insurance and lending

Insurers sometimes ask the age of the furnace and the roof in the same breath, and some will require the heat to be working to bind a policy on a vacant house. They do not generally refuse a house for a 16-year-old furnace that runs. Lenders require the house to have a permanent heat source. FHA and VA appraisers test that it functions. A non-working furnace in January is a repair condition. A working old furnace is your future capital expense, not the lender's. If the flue is shared with a water heater in a way current code does not love, the HVAC replacement may force a water heater conversation at the same time.

Typical cost ranges

Typical ranges, not quotes.

Service call and a combustion test: often $150 to $400. Worth it when the unit is old and you want more than the general inspector's glance.
Furnace replacement, standard efficiency to high efficiency, ordinary house: commonly $4,000 to $8,000 installed. High-efficiency venting changes (PVC flues) add cost in a house that was not built for them.
Air conditioner and coil replacement: often $5,000 to $12,000, more for larger tonnage or difficult access.
Full system, furnace plus air conditioner: commonly $8,000 to $16,000 in many metros, higher in expensive labor markets and for heat pumps with air handlers.
Boiler replacement: often $6,000 to $15,000 or more.

Get model numbers. A bid that says "replace HVAC" without tonnage and AFUE is not a bid.

Pass, negotiate, or investigate

Investigate by reading the plates and running the system. Age over about 15 years on cooling or 20 on heat is a planning item even when it runs. Negotiate when the unit is dead, red-tagged, still on R-22 and already failing, or the inspector documents a cracked heat exchanger. A credit in the typical range above is normal. A seller who offers a home warranty instead of a credit is offering a phone number with exclusions. Pass only when the system is dead, the house has other capital items of the same size, and the price pretends everything is new. An old furnace that runs, disclosed as old, in a fairly priced house is not a pass. It is a line in the budget. The red-flag guide is for the red tag, not for every condenser with dust on it.

Old HVAC and Furnaces: Age, the Data Plate, and the Winter You Don’t Want