Once a year, in the fall, before the first cold snap. ENERGY STAR recommends pre-season check-ups for both heating and cooling systems, which means a gas, oil, or propane furnace gets serviced in the fall and the AC gets serviced in the spring. Heat pumps and electric furnaces can stretch slightly because they don't burn fuel, but yearly service is still the simplest cadence to keep.
Quick schedule
- Gas, oil, propane furnaces: professional inspection once a year, in the fall.
- Heat pumps: service twice a year (spring and fall) because they run year-round.
- Electric furnaces: once a year is plenty.
- Filter change: every 1 to 3 months, not part of the annual visit. See the HVAC filter schedule.
- Schedule the visit by late September in cold-winter climates. Contractor calendars fill up once heat starts going on.
Why annual, not "as needed"
A furnace can run for months in a degraded state and feel normal. The two failure modes that don't announce themselves are a cracked heat exchanger and a flue or venting problem. Both can put carbon monoxide into living space. A combustion analyzer and a visual heat-exchanger inspection are how a technician catches them before they matter.
The other reason is efficiency. A dirty burner, a clogged flame sensor, or a drifting gas pressure all make the furnace burn more fuel for the same heat. Annual cleaning and tuning typically pays for itself in fuel cost over a heating season, and it extends equipment life.
What a real inspection covers
A real furnace inspection is more than swapping the filter. A solid pre-season visit covers:
- Visual check of the heat exchanger for cracks, rust, or scorching.
- Combustion analysis at the flue with a digital analyzer.
- Carbon monoxide reading at the appliance.
- Gas pressure check (inlet and manifold) against the data plate.
- Gas line leak check at unions and the valve.
- Flame sensor cleaning.
- Burner and pilot/ignitor inspection and cleaning.
- Flue and venting check for blockage, corrosion, and pitch.
- Blower wheel and motor check, amperage draw measurement.
- Capacitor reading where applicable.
- Safety controls: limit switch, pressure switch, rollout switch.
- Condensate trap and drain for high-efficiency models.
- Thermostat operation and staging.
If a quote sounds suspiciously cheap, the visit may be a "tune-up" that skips the combustion analysis and heat-exchanger inspection. Ask what's included before booking.
What it costs
A standard residential furnace inspection runs roughly $80 to $200 in most US markets. Annual maintenance plans that bundle furnace and AC visits typically run $150 to $500 a year. Older furnaces (10+ years) and high-efficiency condensing furnaces tend to land toward the upper end because more components get checked. The cost is small compared to a single emergency winter no-heat call, which often runs $200 to $400 just for the diagnostic.
What changes the cadence
- Age. Furnaces under 10 years old can sometimes stretch to every other year. Past 10 years, annual is the right answer.
- Fuel type. Gas, oil, and propane all need yearly combustion service. Electric doesn't have combustion but still benefits from yearly cleaning.
- Heat pump versus furnace. A heat pump runs in both seasons, so a twice-a-year cadence (spring and fall) makes sense.
- Climate. Long heating seasons mean more run hours and faster wear. Short seasons can stretch the visit, but don't skip it.
- Warranty. Many manufacturer warranties require documented annual professional service. Skipping it can void coverage on a $3,000 to $7,000 component.
Signs to call before the next scheduled visit
- Carbon monoxide alarm goes off (get everyone out, then call 911).
- Smell of gas (leave the house and call the gas utility from outside).
- Soot, rust, or scorching around the burner or flue.
- Yellow burner flames instead of clean blue. (Some high-efficiency units look different, but a long-time blue flame turning yellow is a flag.)
- Furnace short cycling (turning on and off in quick bursts).
- New rumbling, banging, or whistling noises.
- Burning smell that doesn't fade after the first 30 minutes of the season. (A short dust burn-off on the first cold day is normal. Persistent burning isn't.)
- Headaches, nausea, or flu-like symptoms when the furnace runs and that fade when you leave the house. These can be CO symptoms.
The CO detector is the second half of this task
An annual inspection is one safety layer. Working carbon monoxide alarms are the other. Code in most US jurisdictions now requires a CO alarm on every level and near every sleeping area for homes with fuel-burning appliances or attached garages. Test them monthly. Replace the unit on the manufacturer's schedule, usually 7 to 10 years depending on the model. Don't rely on the smoke alarm alone. They detect different things.
Good maintenance rhythm
- Yearly in fall: professional inspection for gas, oil, propane, and electric furnaces.
- Twice a year (spring and fall): professional service for heat pumps.
- Every 1 to 3 months: change or check the air filter (separate from the annual visit).
- Monthly: press the test button on every smoke and CO alarm.
- Every 7 to 10 years: replace each CO alarm per the manufacturer.
- Before booking: confirm the visit includes combustion analysis and heat-exchanger inspection, not just a filter swap.
- After every visit: save the service report. Many manufacturer warranties require proof of annual service.