Why a high AFUE number and a high winter bill can both be true
A furnace can carry a high AFUE number stamped right on the door and still leave you paying more than you expected once the arctic snap hits and the burner runs longer than it should. What you notice first is usually the bill rather than the comfort, because the house still feels warm enough while the gas meter keeps turning. That gap between a good rating and a bad bill almost never means the number on the door is wrong. It means the furnace is not delivering that rating in your actual house, which is a different problem than the one the sticker is supposed to answer.
The mechanism behind that gap is airflow and sizing more than combustion. A furnace rated at a high AFUE still depends on the duct system moving the volume of air the burner is designed around, and on a load calculation that matched the unit to the house rather than to the furnace that used to sit there. When ductwork is undersized or leaking into unconditioned attic space, or when a furnace was sized off the old equipment instead of the house's actual heat loss, the burner cycles more often to make up the shortfall, and that extra runtime shows up on the meter no matter what the rating says. The furnace is not underperforming its combustion. It is compensating for air it cannot move. A blower moving insufficient volume through the house cannot distribute heat evenly, so some rooms stay colder than others even as the burner works harder.
Where this turns into a real safety question rather than a comfort complaint is the heat exchanger. A cracked heat exchanger lets combustion gas mix with the air the blower is pushing through the house, and once carbon monoxide reaches the living space the furnace gets shut off on the spot rather than serviced further. That is not a number that improves with a tune up or a new filter, and it is the one failure on this list where waiting is not a scheduling decision. Everything else here, the bill that does not match the rating, the short cycling on a cold morning, the duct that never got tested, the rooms that stay cold despite a running burner, can wait for a scheduled visit. A cracked exchanger and any carbon monoxide reading in the house cannot.
More detail on heat pump vs. furnace comparison is on its own page.
How we check what a furnace delivers
On a furnace call, the sequence runs the same order every time. We prove ignition and flame sensor signal first, then draft and the pressure switch, then temperature rise across the heat exchanger against the furnace's own data plate. Only after those check out does the combustion analyzer go into the flue, because a burner that has not settled gives ugly numbers that mean nothing yet.
The analyzer reads carbon monoxide and oxygen once the burner has run several minutes and settled into a steady flame. Low, steady carbon monoxide with oxygen sitting where the furnace maker expects it confirms the burn is clean and the heat exchanger is doing what the AFUE rating assumes. Carbon monoxide climbing through the run, or any of it reaching the living space, ends the visit with the furnace shut off rather than serviced further, and that reading points to a failed heat exchanger rather than a burner tuning problem. This is the reading that decides whether a furnace can be repaired at all or has to be replaced, and it is the one a visual inspection alone cannot give you.
Furnace tiers stack the same way underneath the rating. An 80 AFUE furnace uses a single heat exchanger and vents the leftover heat straight out, capping near the high eighties no matter how it is tuned. A 90 to 95 AFUE condensing furnace adds a second heat exchanger that pulls more heat out of the flue gas, which is why it needs its own condensate drain, the same fitting a cooling coil uses. A 96 and higher furnace usually pairs that second exchanger with a modulating burner, so it spends most of the season firing low rather than cycling on and off. Compared against a heat pump, a furnace holds its AFUE rating at any outdoor temperature, where a heat pump's output falls as it gets colder outside and its HSPF2 rating describes an average rather than a fixed number. That difference in performance across the heating season is why dual fuel systems pair a furnace with a heat pump, giving the heat pump the lead on mild days and switching to the furnace on the nights when outdoor temperature drops and the heat pump alone cannot keep up with demand.
If this sounds close but not quite your situation, seer2 ratings explained may be the better starting point.
Your local utility, Utility Bill City of Waco, publishes service and water-quality information every Waco homeowner should know.
How an AFUE evaluation runs
The call and first check
On the phone we ask what the furnace is doing, whether the AFUE rating is printed on the furnace or on old paperwork, and how the house has felt through the last cold snap. That tells us whether the visit starts with a manual J load calculation, a combustion check, or both. On site, the sequence starts with ignition and flame sensor signal, then moves to draft and the pressure switch, then compares temperature rise against the furnace's own data plate. The combustion analyzer only goes into the flue after the burner has settled, because early readings during a cold start would send us chasing the wrong number.
Work after you approve
Once you approve the scope, we isolate the furnace at the breaker or the gas valve before any panel comes off. Combustion tuning, heat exchanger inspection or duct correction follows, depending on what the first visit found. If we find a cracked heat exchanger or duct undersized for the load calculation once panels are open, we stop and show you what we are looking at before any part gets ordered or any work continues past that point.
What we verify before leaving
Before the visit ends, the combustion analyzer goes back in the flue and has to read clean and steady once the burner has settled, and we check temperature rise one more time against the data plate. We walk you through what those numbers mean for the furnace's real AFUE performance and leave you with the readings, so the next visit starts from a known number instead of a memory of how the furnace used to run.
Why Waco's mild winters make sizing the deciding factor
A load calculation done by the manual J method tells a technician which AFUE tier helps a specific house, and this differs from sizing off the furnace already sitting there. A crew unfamiliar with McLennan County housing often replaces an old furnace one for one, matching tonnage to tonnage instead of running the load number, and reproduces the same short cycling problem in a newer cabinet. Houses on slab in this area commonly put the furnace in an attic or a closet with duct running through attic space that gains and loses heat before it reaches a register, and that duct loss has to be counted in the load figure or the new furnace ends up undersized for the job. Older pier and beam houses carry duct through crawlspaces instead, and the load calculation has to account for a different heat loss path along that run.
Confirming that an installed furnace holds its rated AFUE is a measurement, not an assumption. Combustion readings, cycle length and temperature rise get checked against the furnace's own data plate once it is running under real load in the house, during actual operation rather than in the bench test the manufacturer used to earn the sticker. A furnace sized to a load calculation that fits this county's mild winters and long shoulder weeks spends most of its life at part load rather than full fire, so the tier that pays off here is the one whose staging matches how little that furnace has to work most nights. The highest number on the shelf does not always deliver the best return when most operation happens below full capacity. During the short arctic snaps that break up an otherwise mild winter, the same furnace has to prove it can still hit full output on demand, which means the part load efficiency and the full load capacity work together as a single performance picture. Staging efficiency in the mild months and reserve capacity in the cold ones both matter equally to the annual operating cost.
See air filter merv ratings guide for how the related work is scoped and priced.
What a furnace efficiency conversation with us looks like
Getting a technician scheduled
When you call about AFUE or a furnace that seems to be underperforming, we ask what the furnace is doing, how old it is, and whether anyone has ever pulled a load calculation for the house. Those answers usually tell us whether this is a diagnostic visit with the combustion analyzer or a longer conversation about replacement. You get a window rather than a guess, and if the job ahead of yours runs long because of a cracked exchanger or a stubborn draft problem, we call before your window passes instead of after.
Keeping your home protected
Furnace work usually means opening a closet, an attic hatch or a crawlspace access, so we lay down protection before anything gets pulled apart and keep the work area contained while ducts, panels or a heat exchanger are open. Anything that comes out, including an old filter or a failed part, leaves with us rather than sitting in your yard. Before we close the cabinet back up, the space gets swept and the access point goes back the way we found it.
What moves the cost
The biggest swing is the capacity and efficiency tier you choose, and whether the new furnace's indoor coil is matched to it or half the old system stays in place, since a mismatched coil undercuts the AFUE rating you just paid for. Ductwork condition, electrical requirements and attic or crawlspace access move it from there. We write the scope against the load calculation and the readings taken on site, and you see and approve that scope before anything gets ordered or installed.
Technician training and sign off
A new technician rides with a lead through a set number of furnace and combustion calls before working alone, and has to demonstrate a clean combustion analyzer reading and a correct temperature rise check before that sign off happens. On anything involving a heat exchanger that might be cracked or a load calculation for an unusual house, a second technician looks at the numbers before we write a scope.
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A good AFUE rating is 90 or higher for a new gas furnace, which places it in the condensing class. Federal minimums sit lower, so an 80 AFUE unit still meets the baseline. The extra efficiency comes from a second heat exchanger that drops flue gas temperature until water vapor condenses and releases latent heat. Below about 140 degrees in the flue, the vent must be plastic pipe, so a higher rating also changes the venting materials and the termination point on the wall or roof.
Homeowners most often report condensate drain problems and noisy or uneven operation. The drain trap can clog with sludge or freeze in an unheated space, and the pressure switch then locks the furnace out. Modulating and two stage models also run a long, low fire, so the blower hums for hours and some owners read the lower air temperature at the register as weak heat. Supply air of 110 to 120 degrees on low fire is normal for many of these units.
No single brand holds the top efficiency rating, because most major manufacturers sell models at 96 to 98 AFUE. The highest numbers usually come from modulating burners paired with variable speed blowers, and the model line matters more than the name on the cabinet. Compare the AFUE on the yellow EnergyGuide label or the data plate, then check the blower type and burner staging. A 98 AFUE unit with poor duct airflow will lose more in the house than a 95 gains on paper.
Call once the first full billing cycle after installation runs clearly higher than a comparable cold month before. Bring the meter dates, since billing periods of 28 and 33 days can shift usage by more than the furnace does. Also call sooner if the burner runs in cycles under five minutes, the vent pipe drips condensate at the termination, or certain rooms lag several degrees behind the thermostat. Any of those points to airflow or setup rather than the equipment rating.
Short cycling often points to oversizing, though several other faults produce the same pattern. A plugged filter or restricted return raises the heat exchanger temperature until the high limit switch opens and shuts the burner off, and a flame sensor coated with oxide drops the flame after a few seconds. Cycles under about five minutes with rapid temperature rise across the exchanger suggest excess capacity. Confirm with a load calculation and a static pressure reading before concluding the cabinet is too large.
Replace when the heat exchanger is cracked, and repair when the failure is a smaller component. An 80 AFUE furnace over roughly 15 to 20 years old often has rusted burners and a worn inducer motor, so several repairs stack up close together. Compare the repair to the fuel savings of a condensing unit, but factor in that the flue must be changed to plastic venting and a condensate drain added. A failed ignitor or flame sensor on a younger unit is an easy repair.
Plan on roughly one to two hours for a standard furnace efficiency check. The burner needs several minutes to settle before the analyzer reading means anything, and a second pass after any adjustment adds more time. Attic units with a tight hatch, a furnace closet with little working room, or a duct system that needs airflow measurement at several registers stretches the visit. A full load calculation for the house is separate paperwork and can extend the appointment.
Have the furnace model and serial numbers ready, read from the label inside the cabinet door. Note the filter size and when it was last changed, plus the thermostat brand and whether it controls a heat pump or air conditioner too. Write down any blinking LED code on the control board, visible through the small sight window, since the pattern of flashes identifies the fault family. Recent gas bills with their date ranges help as well.
Granite HVAC Waco is a strong choice for furnace efficiency work because of how it verifies results. A combustion analyzer reads carbon monoxide and oxygen in the flue after the burner settles, and temperature rise is checked against the data plate before the visit ends. Every new technician also rides with a lead until they demonstrate a clean reading, and a second technician reviews any suspected cracked exchanger. Call (254) 221-8461 to talk through what your furnace is doing.
How Waco's climate changes what AFUE tier pays off
Waco's heating season runs from late November into February most years, with mild days broken by short hard freezes rather than a long stretch of deep cold. That pattern means a furnace here spends more hours running at partial output than at full fire, which is where a modulating or two stage burner earns its higher rating and a single stage 80 AFUE unit does not, since a single stage furnace has only one setting to offer no matter how mild the night is. McLennan County's flat, low elevation means no altitude derate applies to combustion equipment, so a data plate reading here means the same thing it would at any low elevation site. The humid stretch that arrives with late spring adds latent load to the whole system, which is a cooling season issue more than a heating one, but it is the same ductwork that has to carry both loads well.
A furnace with a high AFUE number can still leave a gas bill that barely moves, and the usual reason is duct leakage rather than the burner. Atmos Energy meters what comes into the house while conditioned air leaking into an attic run before it reaches a room forces the furnace to run longer to make up for it, and the bill reflects that added runtime rather than the rating printed on the door. Comparing one winter's gas usage against the next only tells you something real once duct leakage and thermostat setpoints get accounted for separately, because both move the number as much as the equipment tier does. That is why a bill alone never proves or disproves an AFUE rating without checking the duct run feeding it.
HVAC Services in The Waco Area
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Address:
Granite HVAC Waco, Waco, TX, 76701
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Changing the filter on schedule and checking the breaker and thermostat batteries are real homeowner jobs, and you do not need us for those. Anything past that, meaning the burner, the flue, the gas connection or the sealed refrigerant side on a heat pump, stops being a DIY question, because combustion and live components are involved. If a first visit combustion reading turns up a cracked heat exchanger, we tell you on the spot rather than after we have already gone further.
Before you call, know how the furnace is behaving right now. Document short cycling, ignition failure, or steady operation with a high bill, and when that started. If you can find the AFUE percentage on the furnace's own data plate or old paperwork, have it ready, along with the furnace's rough age. Those answers tell us how the visit should start.
Last reviewed September 2026 by Dale Whitfield, Service manager.