Why your furnace smells like rotten eggs (and what to do in the next 5 minutes)
If your furnace smells like rotten eggs right now, stop reading and do the 5-minute emergency response in the next section first. Come back to the diagnostic after you\u2019re safe. This is the same script we give every Spokane homeowner who calls us with this symptom.
Marisa K.
Owner / Dispatch Lead, NATE-certified · November 4, 2025 · 6 min read
Quick answer
A rotten-egg smell from a gas furnace is a gas leak. The mercaptan additive in natural gas gives it that distinctive smell. Evacuate the home immediately, call 911 from outside, then call Avista at 1-800-227-9187 to report the leak. Do not flip light switches, use phones, or operate any appliance until the home is cleared.
- Rotten-egg smell = mercaptan in natural gas (or propane).
- First step: evacuate, then call 911, then call Avista 1-800-227-9187.
- Common leak sources: loose fitting, cracked heat exchanger, failed gas valve.
A rotten egg smell in your home is almost always a natural gas leak. This is the emergency-response playbook we give our Spokane customers \u2014 the same steps you\u2019d hear if you called us right now.
The 5-minute emergency response (do this first)
Step 1: Don’t flip any switches, don’t light any matches, don’t use your phone inside. Static electricity or a spark can ignite accumulated gas.
Step 2: Get everyone (including pets) out of the house immediately. Leave the door open behind you to vent.
Step 3: From a safe distance (across the street or at a neighbor’s house), call 911 to report a gas leak. Avista’s emergency line for the Spokane area is also 1-800-227-9187.
Step 4: Do NOT re-enter the home until the fire department or Avista has cleared it and given you the all-clear.
Step 5: Once cleared, call us at (509) 555-1234 for the furnace diagnostic. We’ll dispatch a NATE-certified tech within 60 minutes.
Why gas companies add the rotten egg smell
Natural gas is naturally odorless. Utilities add mercaptan (the same chemical that makes skunks smell) specifically so you can detect leaks. If you can smell it, the leak has reached a level that needs immediate attention.
Spokane’s natural gas is supplied by Avista Utilities. They’re required to odorize gas to detectable levels (typically 1/5th the lower explosive limit). If you can smell it at all, take it seriously.
Some people can’t smell mercaptan — people with anosmia, the elderly, or people with certain neurological conditions. If a household member can’t smell it, install a natural gas detector ($25-50 at Home Depot) as a backup.
What your tech will check after the all-clear
Once the fire department or Avista confirms the home is safe to re-enter, the diagnostic work begins. Most common culprits:
Cracked heat exchanger — the most common cause of rotten egg smell that’s NOT an active leak. The heat exchanger develops cracks over time; combustion gases (including trace sulfur compounds) leak into the airstream. We test with a combustion analyzer, never by sight. Cracked = full furnace replacement, not repair.
Pilot light outage (older furnaces) — the pilot goes out, raw gas builds up near the furnace before the safety valve shuts it off. Usually a thermocouple issue ($129-189) but worth full diagnosis.
Loose gas connection — at the furnace shutoff valve or flex connector. We test with manometer and leak detector solution. If found, we shut off the gas at the valve and re-seat the connection.
Faulty gas valve — the valve that lets gas into the burner box fails to fully close when the furnace shuts off. Replacement is $449-749 plus diagnostic.
Adjacent appliance leak — sometimes the smell is from the water heater, range, or dryer, not the furnace. We check all gas appliances during the visit.
What this will cost (and what insurance covers)
Diagnostic visit: $89 (credited toward any repair you approve). Most leak diagnostics take 30-60 minutes.
If it’s a loose connection or thermocouple: $129-349. Same-day fix.
If it’s a faulty gas valve: $449-749 plus diagnostic. Parts in stock; same-day fix.
If it’s a cracked heat exchanger: full furnace replacement, typically $5,800-9,400. This is usually covered by homeowners insurance as a ”sudden and accidental” event — we provide detailed invoices your adjuster will accept.
Avista does not charge for the emergency response, but they don’t repair your furnace. They restore safe gas service, then your tech does the actual fix.
How to prevent this in the future
Annual furnace maintenance catches most heat exchanger cracks before they’re a safety issue. Our 32-point inspection includes combustion analysis that detects hairline cracks 6-12 months before they’d cause a smell.
Install a carbon monoxide detector (and gas detector) on every floor of your home. Battery-only units cost $20-30; combination smoke/CO units are $30-50.
Don’t ignore slow, intermittent sulfur smells. By the time you can smell mercaptan strongly, the leak has been building for hours. If you smell it even faintly, call us.
If your furnace is over 15 years old, schedule a heat exchanger inspection. That’s the age where Spokane furnaces typically develop the issue.
Rotten egg smell from a furnace is one of the few HVAC emergencies where the right first move is to call 911, not your HVAC company. Get everyone out first, get cleared by Avista, then call us. We\u2019ll be there within 60 minutes of the all-clear.