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    hvac troubleshooting

    Gas smell at the furnace

    Common symptoms: gas smell; rotten egg smell; natural gas leak; propane smell; smell gas near furnace

    Stop and call a pro if:

    • gas smell
    • mercaptan odor
    • rotten egg

    Step-by-step diagnostic flow

    1. Step 1

      Do you smell gas (rotten egg / sulfur / sour) inside or near the furnace right now?

      Natural gas and propane are odorized with mercaptan so a real leak smells unmistakably like rotten eggs or sulfur. If you smell it, this is not a DIY workflow.

    2. Step 2

      When did you smell gas, and is the smell gone now?

    Possible outcomes

    Leave the building now. Call the gas utility from outside.

    high confidence

    An active gas leak is an explosion and asphyxiation hazard. The diagnostic equipment to find it (combustible-gas detector, soap-test, manometer) is utility/pro gear, not homeowner gear.

    Safe next steps
    • Get everyone (people and pets) out of the building
    • Don't flip light switches, don't use the phone inside, don't start a car in an attached garage — any spark can ignite a gas/air mix
    • From outside, call the gas utility's 24/7 emergency line (it's on your gas bill) or 911
    • Don't go back in until the utility or fire department clears the building
    Gas leaks are life-safety, not DIY. Federal and state code make natural-gas piping repairs licensed work

    Intermittent gas smell — still treat as a leak

    high confidence

    Intermittent gas smells often mean a small leak at a fitting, valve packing, or appliance connector that the utility leak-survey can find with a CGI meter.

    Safe next steps
    • Schedule a free leak check with your gas utility — they treat reported smells as a priority response
    • Don't try to find it with a lighter or match
    • Don't apply soap-and-water yourself unless you're a trained tech
    • If smell returns and is strong, leave and call from outside
    What to document for a pro
    • Time of day smelled
    • Location of smell (basement, near furnace, near meter)
    • Whether furnace was running

    Could be first-of-season dust burnoff, but don't assume

    low confidence

    Dust burning off the heat exchanger at the first heat of the season smells like hot dust, not like gas/sulfur. If you specifically smell rotten egg, it's not dust.

    Safe next steps
    • Switch to the 'Burning smell on first heat of season' workflow if the smell is dusty/burnt
    • If it's specifically rotten-egg / sulfur, treat as a leak and call the utility
    • When in doubt, call the utility — it's a free check

    If you're unsure — call the gas utility for a free leak check

    medium confidence

    Mercaptan is designed to be obvious. If you're noticing something faint that may or may not be gas, the utility would rather check and find nothing than miss a real leak.

    Safe next steps
    • Call the gas utility's customer service line and ask for a leak check
    • Avoid striking matches or candles, lighting cigarettes, or operating high-spark switches near the suspected area
    • Open a window if it's safe to do so (don't disturb the suspected source)
    Diagnostic guidance only. If unsure, stop and call a licensed professional — gas, electrical, and refrigerant work is hazardous to untrained users.

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