A furnace tune-up, a diagnostic service call, and a replacement estimate are booked for different reasons and end in different kinds of paper. Ask for the wrong one and you pay for a second visit; in a few situations, the wrong one means the furnace kept running when it should not have. So this guide starts with the situations that are not a booking decision at all, then matches what you are seeing to the visit that can answer it—what to ask for, and what not to assume.
What this covers. A natural-gas furnace in a house. The safety rules below come from a gas regulator, the Consumer Product Safety Commission, the U.S. Fire Administration, and furnace manufacturers, and they are written for natural gas. Propane and oil furnaces, dual-fuel systems, electric furnaces and heat pumps have their own rules and are not covered here. This is not a troubleshooting guide or a maintenance checklist.
First: the situations that are not a booking decision
You smell gas — the rotten-egg smell. The California Public Utilities Commission's instruction is to "leave the area on foot then call your pipeline operator or 911". On the way out, do not create a flame, "turn electrical appliances on or off," use a phone "in the potentially gaseous environment," operate a vehicle or garage door, ring a doorbell, or "attempt to control the leak by turning valves". Carrier's gas-furnace guidance says the same: "If you smell gas, do not operate electrical switches or attempt to relight the furnace. Leave your home immediately and contact your gas utility or a qualified professional." Some manufacturer pages say to shut the furnace down first; the regulator's list of things not to do includes turning electrical appliances on or off, and this guide follows the regulator.
A carbon monoxide alarm sounds. CPSC's steps, in order: "Immediately move outside to fresh air. Call your emergency services, fire department, or 911." Then "do a head count to check that all persons are accounted for. DO NOT reenter the premises until the emergency services responders have given you permission." Follow what the responders tell you. Two CPSC rules govern what comes next. If "the source of the CO is determined to be a malfunctioning appliance, DO NOT operate that appliance until it has been properly serviced by trained personnel." And if you are allowed back in and "your alarm reactivates within a 24 hour period," repeat the steps and "call a qualified appliance technician to investigate for sources of CO from all fuel burning equipment and appliances." Whichever of those applies to you, say when you book that an alarm has sounded and what the responders told you, and do not let the visit be scheduled as a routine tune-up.
Someone has symptoms that fit carbon monoxide. CPSC lists headache, fatigue, shortness of breath, nausea and dizziness at lower exposures, and mental confusion, vomiting, loss of muscular coordination and loss of consciousness at higher ones. Its instruction when there is no alarm, or the alarm has not sounded: "get outside to fresh air immediately"; "Leave the home and call your fire department to report your symptoms from a neighbor's home"; and "contact a doctor immediately for a proper diagnosis. Tell your doctor that you suspect CO poisoning is causing your problems." If the doctor confirms CO poisoning, CPSC says to have a qualified service person check the appliances for proper operation before reusing them.
Smoke or fire. Get out and stay out. The U.S. Fire Administration's rule is to go to your outside meeting place, stay there, and call the fire department from there.
A burning smell without smoke or fire, or soot you can see without opening anything. Carrier says to "call for professional service immediately" if you "notice soot accumulation" or "detect burning odors"; separately, a contractor quoted on Carrier's page says that where a problem involves electrical components the furnace should be shut off and a professional contacted. Leave the cabinet closed: AHRI's rule is never to operate the furnace without the front-panel door in place (quoted below), and Carrier's is that "internal mechanics should be handled by a professional."
A professional has told you the heat exchanger is cracked. A cracked heat exchanger "can leak combustion gases into your home" (Trane), and Carrier's instruction is to "stop using the furnace and have it professionally inspected; replacement may be necessary." Keep it off at the thermostat. Do not try to confirm or rule out the crack yourself: AHRI says the cracks a contractor checks for "may not be visible to the naked eye," and Trane says the signs "can also be the same as other furnace problems, so it's essential that you call a licensed HVAC company to evaluate the situation and make the necessary repairs." What you need next is the finding in writing and your documented options— Trane's position is that it "wouldn't recommend trying to repair a cracked heat exchanger" and would "replace it or even replace the entire furnace," which are two different purchases. Replacement estimates for a new furnace come after that, not instead of it.
Two rules hold throughout. Carrier: "any repair involving gas lines, electrical components, or internal mechanics should be handled by a professional". AHRI: "Never operate the furnace without the front-panel door properly in place." And turning heating off at the thermostat is only a control command—not permission to touch a gas valve, a disconnect, or wiring, and in the gas-smell case not an instruction at all.
The three visits: what to ask for, and what not to assume
Contractors combine these visits in different ways, so the names below are what you ask for, and the scope you actually get is whatever the contractor states and the invoice confirms.
A tune-up: the furnace works, and you want it checked before the season
There is a national standard for what a maintenance inspection minimally includes—ACCA 4 QM-2019 (reaffirmed 2024), "Maintenance of Residential HVAC Systems," described by ACCA as "a nationally recognized, manufacturer-endorsed set of minimum tasks". ENERGY STAR's maintenance checklist lists what a professional heating check-up includes, among them "all gas (or oil) connections, gas pressure, burner combustion and heat exchanger"; CPSC adds that the annual inspection should "check chimneys and flues for blockages, corrosion, partial and complete disconnections, and loose connections." ENERGY STAR and AHRI both say annually, before the heating season.
What to ask for: a check to that scope, and a written list of what was inspected and what was found. ACCA's announcement of the standard describes the output as "recommended corrective actions that may be needed to remedy identified faults," which the consumer "can compare"—so ask for the faults and the recommended actions separately.
What not to assume: that a fault found on a tune-up has been repaired, or that a recommendation is a decision. Two cases matter:
- The finding is a safety finding—a cracked heat exchanger, a combustion or venting fault, a carbon monoxide reading. Then it is real: for a cracked heat exchanger, Carrier's instruction above applies; for any of them, follow the technician's instruction on whether it runs. Get the finding in writing, and get your options in writing, before anything is bought.
- The recommendation is "it's old, replace it," with no documented fault. The Federal Trade Commission's consumer guidance treats pressure "for an immediate decision" as a warning sign. Ask what was found, in writing. If the answer is age or "it won't last," the next visit is an evaluation, not an estimate—see below.
A diagnostic service call: the furnace has a problem, and you want to know what it is
Book this for a reported symptom—no heat, short cycling, a noise, an unexplained smell that is not gas or burning, a furnace that runs and never catches up—when none of the situations above applies. American Standard's description of a repair visit is a fair picture: "They'll run diagnostics on the thermostat, examine the flame sensor to check if it's dirty or faulty, and determine if the system is improperly sized for your home."
What to ask for: the fault, in writing, and the options for it—repair, or replacement if the fault is one the manufacturer treats that way (see the next section).
What not to assume: that the maintenance tasks above were done. A technician sent to find one fault is not necessarily inspecting the heat exchanger, venting and gas connections. If you want both, ask for both when you book, and expect the invoice to show both.
A replacement estimate: a condition has been diagnosed, and you are deciding what to buy
An estimate is a proposal for new equipment. Book it after a diagnosis or an evaluation has put a specific finding in front of you. The manufacturers frame replacement as a condition plus a history—Carrier: "when a furnace is older and requires frequent repairs, particularly when facing a major repair such as a failed blower motor or cracked heat exchanger"; American Standard: "15-20 years old or older," repairs "becoming more frequent and costly," "major component failures, such as a cracked heat exchanger". When age is the only thing anyone has mentioned, ENERGY STAR's trigger is the one to use: "If your HVAC equipment is more than 10 years old or not keeping your house comfortable, have it evaluated by a professional HVAC contractor." That is an evaluation, not a purchase. In this guide's terms, an evaluation is booked as a tune-up-scope inspection that ends in a written statement of the furnace's condition and any faults—ask for it in those words, so that what arrives is an inspection and not a sales visit.
What to ask for: more than one. The FTC's guidance is to "get multiple estimates," each describing "the work to be done, materials, completion date, and the price," and not to "automatically choose the lowest bidder." Comparing them is the job of What Should an HVAC Replacement Quote Include?, which also covers the written contract and cancelling one signed at home.
What not to assume: that the estimate tells you anything about the furnace you have. If the recommendation to replace arrived with the estimate rather than before it, ask for the finding it rests on, in writing.
Match what you are seeing to the visit
| What is happening | Do this first | Then book | What that visit is for |
|---|---|---|---|
| Rotten-egg smell | Leave on foot, touching no switches or phones inside; call the gas utility or 911 from outside | Nothing until the utility has cleared the house | The utility determines whether there is a leak; a furnace visit does not |
| Carbon monoxide alarm | Outside to fresh air; 911; head count; do not re-enter until told | What responders instruct; if an appliance was the source, its service before use (CPSC); if the alarm reactivates within 24 hours, a technician's investigation of all fuel-burning appliances (CPSC) | Which appliance, if any, is the source |
| Headache, nausea, dizziness, confusion in the house | Outside to fresh air immediately; call the fire department from a neighbor's home; contact a doctor immediately and say you suspect CO poisoning (CPSC) | If CO poisoning is confirmed, a qualified service person checks the appliances before they are used again (CPSC) | Which appliance, if any, is the source |
| Smoke or fire | Get out, stay out, call the fire department from outside (USFA) | Nothing until the fire department says it is safe | — |
| Burning smell with no smoke or fire, or soot you can see without opening anything | Call for professional service immediately (Carrier); leave the cabinet closed (AHRI) | That service visit | A finding on the cause |
| A professional says the heat exchanger is cracked | Stop using it (Carrier) — off at the thermostat; do not try to see the crack (AHRI, Trane) | Your options in writing: exchanger replacement or furnace replacement (Trane) | Which of the two you are buying |
| It works; it is autumn; it has not been checked this year | — | A tune-up to the ACCA 4 QM / ENERGY STAR scope | A written condition report: faults found and recommended actions |
| No heat, short cycling, noise, an unexplained smell that is not gas or burning, or it runs and never catches up | — | A diagnostic service call | A finding on the fault, and the options for it |
| A tune-up ended with "replace it," no fault documented | Ask what was found, in writing | An evaluation (ENERGY STAR): a tune-up-scope inspection ending in a written condition finding | Whether there is a condition, or only an age |
| A diagnosed fault the manufacturer treats as replacement-level, or an evaluation that says so | Decide whether replacement is on the table | Replacement estimates, plural, then the quote guide | What to buy, from whom, on what terms |
What to tell whoever you call
- which of the three you want, and, for a diagnostic visit, the symptom and when it started;
- whether a carbon monoxide alarm has sounded, and when;
- the fuel, and the model information if it is safely visible without opening anything;
- when the furnace was last professionally checked, if you know; and
- if you have already been told something is wrong, what you were told and whether you have it in writing.
A finding in writing is what a second contractor can look at, what an estimate can be based on, and what keeps a sales visit a proposal you are free to compare.