Updated October 2026. If your carbon monoxide alarm goes off, get everyone outside now and call 911 from fresh air. Don’t go back in, and don’t restart the furnace, until responders say it is safe. A gas or oil furnace can leak carbon monoxide (CO) when its heat exchanger cracks or its vent is blocked, and CO has no smell, so a working CO alarm is your best warning.
CO alarm going off? Do this now
- Get everyone out, including pets, into fresh air.
- From outside, call 911. Then call your gas utility: Puget Sound Energy (PSE) 1-888-225-5773 or Cascade Natural Gas 888-522-1130.
- Stay out until responders tell you it is safe to go back in.
Smell rotten eggs? That is natural gas, not CO. See what to do if you smell gas near your furnace.
This guide explains how furnaces make carbon monoxide, the warning signs, what a cracked heat exchanger means, where Washington requires CO alarms, and who to call. For a repair once the home is safe, see furnace repair.
What is carbon monoxide, and why can a furnace make it?
Carbon monoxide is a colorless, odorless gas made when fuel burns. Gas and oil furnaces burn fuel, so they make some CO as part of normal operation, and the flue or vent carries it outside (CDC). It only becomes dangerous when it escapes into the home: through a crack in the heat exchanger, a blocked or disconnected vent, or burners that aren’t burning cleanly.
Electric furnaces and heat pumps burn no fuel, so they don’t produce CO. Other appliances in your home still can: gas water heaters, ranges, fireplaces, generators and cars in an attached garage.
It is a real risk here. When Washington passed its CO alarm law, the Legislature noted that after the December 2006 storms more than a thousand people in the state were seen in emergency rooms with CO poisoning symptoms, and eight died (RCW 19.27.530, notes).
Signs of carbon monoxide from a furnace
Symptoms in people
You can’t see, smell or taste carbon monoxide, and it can kill within minutes; the CPSC, citing the CDC, says it kills more than 400 people in the U.S. every year (CPSC). Because the early signs feel like a cold or flu, Cascade Natural Gas lists the pattern to watch for:
- Flu-like symptoms without a fever.
- Everyone in the home feeling ill at the same time, sometimes pets too.
- Feeling better when you are away from home and worse when you come back.
If anyone has symptoms, get everyone outside and call 911. Medical questions are for responders and doctors, not a furnace company.
Signs at the furnace
These don’t prove CO is leaking, but each is a reason to book a furnace check:
- Soot or scorch marks around the furnace or the vent pipe.
- Rust, water or a loose joint where the vent pipe leaves the furnace.
- A furnace that keeps shutting itself off on a safety control.
- Your CO alarm chirping or showing a reading, even briefly.
CO alarm going off: what to do, step by step
- Get out. Everyone and every pet, right away. Don’t stop to look for the cause.
- Call 911 from outside. Firefighters can measure CO levels and find the source.
- Count heads. Make sure everyone who was home is out.
- Wait for the all-clear. Only go back in when responders say it is safe.
- Call your gas utility. PSE says it will do a free on-site investigation if you think a gas appliance is producing CO, with minor repairs for a fee (PSE). Cascade customers call 888-522-1130.
- Don’t run the furnace until it is checked. If the fire department or utility shuts off or tags the furnace, have it repaired before using it. For after-hours help with no heat, see emergency furnace repair.
What causes a furnace to leak carbon monoxide?
| Cause | What happens |
|---|---|
| Cracked heat exchanger | The metal chamber that keeps combustion gases away from your house air develops a crack, so flue gases can mix into the airflow. |
| Blocked or disconnected vent | A bird nest, leaves, snow, or a pipe joint that has come apart sends flue gases back into the home. |
| Burners out of adjustment | Dirty or poorly adjusted burners don’t burn the gas cleanly, which makes more CO. |
| Draft problems | A failed draft inducer or a vent that isn’t drawing lets gases spill instead of leaving through the flue. |
Every gas repair we do ends with a carbon monoxide check before we leave.
Cracked heat exchanger: repair or replace?
A cracked heat exchanger is one of the most serious furnace problems, because it is the part that separates combustion gases from the air you breathe. It is found during a repair visit, when a technician opens the furnace and inspects it.
A cracked heat exchanger on an older furnace is almost always a replace. Replacing the heat exchanger alone is a large job, and on a furnace past its middle years the rest of it is often near the end too. On a newer furnace, check the manufacturer’s parts warranty first: if the heat exchanger is still covered, that can change the math.
- Repair visit: dispatch fee $119; diagnostic $72 standard or $130 deeper, listed separately. You approve an upfront price before any work begins.
- Replacement: estimates are free. Get a price range in about 30 seconds with our instant quote tool, then a technician confirms the exact price on a free site visit. See furnace replacement.
- Rebates: Cascade Natural Gas offers $1,600 for a 95% AFUE furnace and $2,400 for 98% AFUE, for installs on or after March 15, 2026. PSE pays rebates for switching to a heat pump rather than for a new gas furnace. See rebates.
For the wider decision, see furnace repair vs. replacement.
Where to put carbon monoxide alarms (Washington rules)
Washington’s residential code requires CO alarms outside each separate sleeping area, near the bedrooms, and on each level of the home, installed to the maker’s instructions. If a fuel-burning appliance is in a bedroom or its attached bathroom, an alarm goes inside that bedroom (WAC 51-51-0315).
| Location | Required? | Tip |
|---|---|---|
| Outside each sleeping area | Yes (WA code) | In the hallway close enough to wake sleepers |
| Each level of the home | Yes (WA code) | Including a basement with the furnace |
| Bedroom with a fuel-burning appliance | Yes (WA code) | Inside the bedroom itself |
| Near the furnace | Covered by “each level” | Not right beside it; Cascade suggests about 15 feet from gas appliances |
| Near the garage door | No | Cascade suggests about 15 feet from the door to the garage |
Test alarms monthly and replace batteries once a year (CPSC). Replace the alarm by the date on its label. Combination smoke and CO alarms are fine. For whole-home placement of smoke and CO alarms, see how to plan smoke detector installation. We install carbon monoxide alarms; ask our electricians.
Washington’s CO alarm law in plain English
- Who needs them: residential buildings, including rentals and newer homes. New buildings had to have them from January 1, 2011, and other residential buildings by January 1, 2013 (RCW 19.27.530).
- The exception: owner-occupied single-family homes lived in before July 26, 2009 are exempt, until they are sold.
- Selling your home: the seller must install CO alarms to the state building code before the buyer can move in.
- Renting: tenants are responsible for maintaining the alarm, including batteries.
- Permitted work: when permitted alterations, repairs or additions are made to an existing home, CO alarms are checked at inspection. Work on non-fuel-burning plumbing, mechanical or electrical systems is exempt (WAC 51-51-0315).
How to prevent furnace carbon monoxide
- Have fuel-burning heating checked every year. The CDC recommends a yearly inspection. Our furnace tune-up ($230, or $180 for Home Comfort Club members) covers the flue and piping, burner assemblies, gas pressures and safety controls. It keeps the furnace running well, but it is not a CO test. If you are worried about CO, put in CO alarms and book a repair visit.
- Keep vents clear of leaves, snow and nests, inside and out.
- Never heat the home with an oven, grill or generator, and never run a generator indoors or in a garage.
- Replace aging equipment. If your furnace is old and needing repairs, see signs your furnace is about to fail.
Who to call
| Situation | Call |
|---|---|
| CO alarm sounding, or anyone with symptoms | 911, from outside |
| You think a gas appliance is making CO | PSE 1-888-225-5773 or Cascade 888-522-1130 |
| Smell of rotten eggs | Leave, then your gas utility or 911 from outside |
| Furnace repair after the all-clear | Home Comfort Alliance, by phone or online |
Furnace carbon monoxide FAQ
What are the signs of carbon monoxide from a furnace?
A CO alarm sounding, headaches or flu-like symptoms without a fever in more than one person, and feeling better away from home. At the furnace, soot, rust or water at the vent pipe are reasons to have it checked.
What should I do if my carbon monoxide alarm goes off?
Get everyone out, call 911 from outside, and stay out until responders say it is safe. Then call your gas utility, and don’t use the furnace until it has been checked.
Can you smell carbon monoxide from a furnace?
No. Carbon monoxide has no smell. A rotten-egg smell is natural gas, which has an odorant added; that is also a reason to leave and call the utility or 911 from outside.
Can a furnace leak carbon monoxide when it is not running?
A furnace that isn’t burning fuel isn’t making CO. If an alarm sounds while the furnace is off, the source may be another fuel-burning appliance, a generator or a car. Treat any alarm the same way: get out and call 911.
Does a cracked heat exchanger always leak carbon monoxide?
Not always, but a crack is the opening that lets combustion gases reach your house air, so it is treated as a safety problem. A furnace with a cracked heat exchanger shouldn’t be used until it is repaired or replaced.
Should I repair or replace a furnace with a cracked heat exchanger?
On an older furnace it is almost always a replacement. On a newer furnace, check the manufacturer’s parts warranty first, because it may cover the heat exchanger. Replacement estimates are free.
Where should I put a carbon monoxide detector near my furnace?
On the same level as the furnace, but not right beside it. Cascade Natural Gas suggests about 15 feet from gas appliances. Washington code also requires alarms outside each sleeping area and on every level.
Are carbon monoxide alarms required in Washington homes?
Yes, for residential buildings under RCW 19.27.530, outside sleeping areas and on each level. Owner-occupied single-family homes lived in before July 26, 2009 are exempt until they are sold.
Do I have to install CO alarms before I sell my house in Washington?
Yes. The seller must install CO alarms to the state building code before the buyer can move in.
Will PSE come out if I think my furnace is making carbon monoxide?
Yes. PSE says it does a free on-site investigation when a gas appliance may be producing CO, with minor repairs for a fee. If anyone has symptoms, call 911 first.
Can an electric furnace or heat pump make carbon monoxide?
No. They don’t burn fuel. Other fuel-burning appliances in the home, a generator or a car in an attached garage still can.
When is it safe to turn my furnace back on after a CO alarm?
Only after responders say the home is safe and the furnace has been checked and any problem repaired. If the utility shut off or tagged it, don’t turn it back on yourself.
Need your furnace checked after a CO scare?
Once responders say it is safe, our NATE-certified technicians find the cause and give you an upfront price before any work begins. Book by phone or online.
Get an estimate
A local comfort advisor will call you to set up your visit.
Sources: RCW 19.27.530; WAC 51-51-0315; CPSC, CO alarms; CPSC, carbon monoxide; CDC, furnace safety; PSE, carbon monoxide; Cascade Natural Gas, safety tips; Cascade Natural Gas, emergency information. Checked October 2026.