Immediate danger: what to do
If you smell gas — a sulfur or rotten-egg odor — leave the home immediately with everyone inside. Do not operate light switches, appliances, garage door openers or phones inside the building. Once you are a safe distance away, call your gas utility or 911.
The same response applies to smoke, visible flame outside the burner compartment, or a sounding carbon monoxide alarm. Move to fresh air first, then call. Online troubleshooting is never the right next step in any of those situations.
The safety controls inside your furnace
Each of these opens for a reason. When one trips repeatedly, the answer is to find the cause, not to defeat the control.
- Pressure switch — proves combustion gases are venting before gas flows
- Limit switch — shuts the burners down if the furnace overheats
- Rollout switch — trips if flame escapes the intended burner path
- Door switch — prevents operation with the access panel removed
- Flame sensor — closes the gas valve within seconds if combustion is not confirmed
Never bypass a safety control
Jumpering, taping down or removing a safety switch to restore heat is the single most dangerous piece of advice circulating about furnaces. A bypassed pressure switch can allow a furnace to fire into a blocked vent. A bypassed limit switch can allow sustained overheating. A defeated flame sensor can allow gas to flow without combustion.
The same applies to gas valve adjustment, burner modification, heat exchanger work and live-voltage testing. These are not tasks with a careful homeowner version.
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Carbon monoxide alarms
Carbon monoxide is colorless and odorless. The only reliable detection is a working alarm. Install CO alarms on every level of the home and near sleeping areas, test them monthly, replace batteries as recommended and replace the alarms themselves at the end of their rated life — sensors expire even when the unit still powers on.
Our carbon monoxide furnace safety guide covers symptoms, placement and response in detail.
Keeping the area around the furnace safe
- Maintain clear space around the furnace; do not use the area for storage
- Never store gasoline, paint thinner, aerosols or other flammables near the equipment
- Keep combustion air openings unobstructed
- Clear snow and ice from sidewall intake and exhaust terminations
- Do not close off so many registers that airflow is restricted
- Replace filters on schedule to prevent overheating
Signs worth calling about
A yellow or lazy burner flame instead of crisp blue, soot around the furnace or vent, persistent unexplained burning odors, visible corrosion on vent piping, or repeated safety lockouts all warrant professional inspection. So does a furnace that will not shut off. See furnace smells and emergency furnace repair for next steps.