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Furnace maintenance: a fall checklist for safe heat

A gas furnace burns fuel inside your home, so fall maintenance is about safety as much as comfort. Here is what to do yourself and what the yearly professional inspection should cover.

The short answer

Book a professional inspection every year before heating season – both the U.S. Consumer Product Safety Commission (CPSC) and ENERGY STAR advise it. Make sure you have working carbon monoxide alarms on every level and outside sleeping areas, change the filter, and keep vent pipes and the space around the furnace clear.

If you smell gas or a carbon monoxide alarm sounds, leave first and call 911 or your gas utility from outside.

Safety first

  • Gas smell (often like rotten eggs): do not switch anything on or off. Get everyone out and call your gas utility or 911 from outside.
  • Carbon monoxide alarm, or people feeling headachy, dizzy or sick and better outdoors: leave and call 911.
  • A furnace shut off for a safety reason – a red tag from the utility or a technician – stays off until it is repaired. Never bypass a safety switch.

More in our carbon monoxide safety guide and on the emergency page.

Book the yearly inspection

The CPSC advises having the heating system professionally inspected and serviced every year to make sure it is working properly. ENERGY STAR’s checklist says the same in seasonal terms: check the heating system in the fall. Book early – before the first cold snap, when companies fill up with no-heat calls. Keep every invoice; some equipment warranties ask for proof of maintenance (see the maintenance checklist).

Carbon monoxide alarms

Carbon monoxide has no color or smell. The CPSC says more than 200 people in the United States die every year from accidental carbon monoxide poisoning not related to fire and associated with consumer products. Its advice:

  • Install battery-operated CO alarms, or alarms with battery backup, on every level of the home and outside sleeping areas.
  • Choose alarms that meet the current UL 2034 safety standard.

Each fall, test the alarms as the maker directs, replace batteries, and check the end-of-life date printed on the back. An alarm past that date needs replacing.

Change the filter

ENERGY STAR advises inspecting, cleaning or changing furnace filters once a month. A clogged filter starves the furnace of air; it overheats, and a limit switch shuts the burner down. Start the season with a fresh filter. Our air filter guide covers sizes and which way the arrow points.

Keep the flue, vents and furnace area clear

  • Vent pipes. Standard furnaces send exhaust up a metal flue. High-efficiency furnaces vent through two plastic pipes, often out a side wall. Outside, check those openings for leaves, nests and debris, and plan to keep them clear of snow all winter.
  • Look, do not touch. If a metal flue pipe is rusted through, sagging or loose at a joint, leave it alone and call – exhaust can escape into the house.
  • Clear space. Keep paint, gasoline, cleaning products, laundry and boxes away from the furnace.
  • Registers and returns. Open supply vents and uncover return grilles before heating season starts.

Give it combustion air

A gas furnace needs a steady supply of combustion air to burn its fuel cleanly. If the furnace sits in a closet with a louvered door or vents in the door or wall, do not block or seal them. Before you finish a basement or close in a furnace room, ask a licensed professional how the furnace will get its air. Too little air can lead to poor combustion.

Condensate on high-efficiency furnaces

Condensing furnaces – the high-efficiency kind with plastic vent pipes – make water as they run. It drains through a hose and trap, sometimes to a small condensate pump. ENERGY STAR’s contractor checklist includes inspecting the condensate drain on furnaces. Between visits, look for water around the base, and listen for the pump running after a heating cycle. A clogged drain or failed pump can shut the furnace down. Where a drain line runs outside, it can freeze in a cold snap.

Whole-house humidifier

If your furnace has a humidifier attached, fall is the time to replace its pad (also called a water panel) as the maker directs, open the water supply valve, and set the humidity control. Do not overdo it. The EPA advises keeping indoor humidity below 60%, ideally between 30% and 50%. Water running down windows is a sign it is set too high. See our indoor humidity guide.

What the professional checks

ENERGY STAR lists these items in a heating check-up:

  • all gas (or oil) connections;
  • gas pressure;
  • burner combustion;
  • the heat exchanger;
  • the condensate drain on high-efficiency furnaces.

Many technicians also clean the flame sensor, check the igniter and safety switches, look over the venting, and test for carbon monoxide. Ask for a written list of what was done. If you are told the heat exchanger is cracked, ask to see the evidence – see repair or replace.

Sources

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Questions

How often should a furnace be serviced?

Once a year, before heating season. The CPSC advises an annual professional inspection and service of the heating system, and ENERGY STAR advises checking it each fall.

Where should carbon monoxide detectors be placed?

The CPSC advises battery-operated or battery-backup CO alarms on every level of the home and outside sleeping areas, meeting the current UL 2034 standard. Follow the alarm maker’s mounting instructions.

Can I do furnace maintenance myself?

You can change the filter, keep vents and the furnace area clear, check the condensate drain for water and test your CO alarms. Gas connections, burners, combustion and the heat exchanger need a trained technician.

Why does my furnace smell the first time I turn it on?

Dust that settled over the summer burns off, and a faint dusty smell for a short while is common. A smell that does not fade, smoke, or any gas smell is not – turn it off, and for gas or smoke, leave and call from outside.

Related guides

Furnace not working? What to check firstA furnace that will not start is often stopped by a safety switch doing its job. Here is how to find which one – and when to leave the house instead.Carbon monoxide and your furnace: alarms, warning signs and what to doCarbon monoxide is a poisonous gas you cannot see or smell. Fuel-burning furnaces, boilers and water heaters can produce it when something goes wrong. Alarms and yearly inspections are the main defenses.How often to change your air filter (furnace and AC)There is no single schedule that fits every home. The habit that works is to look at the filter every month and replace it when it is dirty. Here is what speeds that up and how to get the swap right.

Updated October 2026 by the Heating and Cooling editorial team. General information for homeowners, not professional advice – rules and equipment differ. When gas, carbon monoxide, electricity or anyone’s safety is involved, call 911 or your utility. How we write our guides · HVAC glossary

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