Furnace Carbon Monoxide Risks: Warning Signs and What to Do in Denver

A gas furnace makes carbon monoxide every time it burns. A healthy furnace sends that gas up and out through the vent. When something goes wrong, like a cracked heat exchanger, a blocked vent, or poor burning, CO can get into the air your family breathes. This page covers what to do first, the warning signs, Colorado's alarm rules, and how to keep your furnace safe through a Denver winter.

What to do if your CO alarm goes off

Get everyone outside right away, including pets. Don't stop to search for the source.
Call 911 from outside. Firefighters can check the air in your home.
Stay out until the fire department says it's safe to go back in.
Get medical care if anyone has symptoms, even mild ones. For poison questions, the Rocky Mountain Poison and Drug Safety center answers at 1-800-222-1222.
Don't run the furnace or any appliance you suspect until a qualified technician has checked it and any needed repair is done.

If you smell gas instead (a rotten-egg smell), the steps are almost the same: get outside, don't flip switches or use a phone inside, call 911 from outside, then call Xcel Energy.

Can a furnace leak carbon monoxide?

Yes. Natural gas furnaces burn fuel to make heat, and CO is one of the gases that comes from burning. In a working furnace, the burners heat a metal chamber called the heat exchanger, and the exhaust gases stay inside it and leave through the flue or vent pipe. The air that blows through your home passes over the outside of the heat exchanger and never mixes with the exhaust.

CO can reach your living space when:

The heat exchanger is cracked or rusted through, letting exhaust mix with the air the blower moves.
The vent or flue is blocked or damaged, so exhaust can't leave the house. On high-efficiency furnaces with white PVC pipes, snow and ice can block the outdoor pipes.
The furnace isn't burning cleanly, often from dirty burners, a gas pressure problem, or not enough combustion air.

CO has no color, smell, or taste. That's why a working CO alarm matters more than any of the signs below.

Warning signs to watch for

Your CO alarm sounds or shows a reading. Treat it as real and follow the steps above.
Flu-like symptoms that get better when you leave the house, such as headache, tiredness, nausea, dizziness, or shortness of breath. Local fire departments warn that CO symptoms are easy to mistake for the flu.
A yellow, orange, or flickering burner flame instead of a steady blue one.
A flame that jumps or wavers when the blower starts. That can mean air is getting into the burner area where it shouldn't.
Soot, scorch marks, or rust on or around the furnace or vent pipe.
Unusual smells when the furnace runs.
Vent pipes that look disconnected, sagging, or blocked, including buried outdoor pipes after a snowstorm.

None of these alone proves your furnace is leaking CO, and a furnace can have a problem with no visible sign at all. If you notice any of them, turn the furnace off at the thermostat and have it checked.

What a cracked heat exchanger means

The heat exchanger is the wall between the fire and your air. If it cracks, that wall stops doing its job. This isn't something to fix yourself or keep running "until spring."

A technician has to look at the furnace to know for sure. How the heat exchanger gets inspected depends on the age and condition of the unit. If a crack is confirmed, the furnace should not be used. On most furnaces, especially older ones, a cracked heat exchanger points toward replacement rather than repair. Our furnace repair and installation services cover both paths.

Worried about your heat exchanger? Call JC Mechanical at (720) 779-7263.

Colorado and Denver CO alarm rules, in plain language

Colorado law (C.R.S. 38-45-102) requires an operational CO alarm within 15 feet of the entrance to each bedroom in a single-family home that has a fuel-fired heater or appliance, a fireplace, or an attached garage when:

the home is sold, or
work that needs a building permit is done, including replacing a fuel-fired appliance like a furnace, or a bedroom is added.

Rental homes are covered by a separate section of the same law. The Denver Fire Department's best practice is to place CO alarms on each floor, especially near bedrooms and in areas with fuel-burning appliances. Rules can change and cities can add their own, so check current requirements with your city or county. This page isn't legal advice.

JC Mechanical sells CO alarms. Ask about one when you call.

Keeping your furnace safe through a Denver winter

Have the furnace inspected every year before the cold sets in. Fire departments and Boulder County's healthy-home program both recommend a yearly furnace inspection.
Change the filter on schedule. A clogged filter makes the furnace overheat, which stresses the heat exchanger.
Keep vents and outdoor pipes clear, especially after a snowstorm on high-efficiency furnaces.
Never heat your home with an oven, stove, grill, or unvented heater.
Test CO alarms monthly and replace them when the manufacturer says to.
Keep the area around the furnace clear of boxes, paint, and anything that blocks airflow.

What JC Mechanical's furnace safety inspection includes

At JC Mechanical, the furnace tune-up is the furnace safety inspection. It's not a separate service. Each tune-up includes:

Light cleaning of the furnace
Cleaning the flame sensor
Checking amp draw on the motors
Visually inspecting the blower wheel
Adjusting gas pressure if needed
Checking blower speed
Testing the igniter
Checking all wires for loose connections
Checking all plug connections
Light cleaning of the inside cabinet and the outside of the furnace
A heat exchanger inspection (the method depends on the age and condition of the unit)

When we're done, we'll tell you the condition of your furnace and recommend any repairs or further testing it needs. A tune-up is $129, and our maintenance plan is $240 per year. See maintenance and tune-ups for details.

If something's wrong now rather than at tune-up time, the service call is $89 during normal hours (Monday to Saturday, 7 AM to 7 PM). 24/7 service is available. After hours there is a larger fee, which we tell you before we come out.

Frequently asked questions

What should I do if my carbon monoxide alarm goes off?

Get everyone outside, call 911 from outside, and don't go back in until the fire department says it's safe. Get medical care if anyone has symptoms. Don't use the furnace again until a technician has checked it.

Can a furnace leak carbon monoxide?

Yes. A cracked heat exchanger, a blocked or damaged vent, or poor burning can let CO from a gas furnace into your home. A working CO alarm is the only reliable way to know, because CO has no smell.

What are the signs of a cracked heat exchanger?

Common signs are a yellow or flickering flame, a flame that moves when the blower starts, soot or rust around the furnace, and CO alarm activity. Only an inspection can confirm a crack.

What are the symptoms of carbon monoxide poisoning?

Headache, tiredness, nausea, dizziness, and shortness of breath are common early symptoms, and they're often mistaken for the flu. Higher levels can cause confusion. If symptoms ease when you leave home, take that seriously.

Where should CO alarms go in a Denver home?

Colorado law calls for an alarm within 15 feet of each bedroom entrance in many homes. Denver Fire recommends alarms on each floor, especially near bedrooms and fuel-burning appliances. Follow the alarm maker's instructions too.

How often should a furnace be inspected?

Once a year, before the heating season. JC Mechanical's tune-up includes a furnace safety inspection.

Is a yellow burner flame a problem?

It can be. A healthy gas flame is mostly steady blue. A yellow, orange, or flickering flame can point to poor burning or a heat exchanger problem, so have it checked.

Can I keep using a furnace with a cracked heat exchanger?

No. Turn it off and have it evaluated by a technician. A cracked heat exchanger can let carbon monoxide into your home.

Who checks my furnace after the fire department clears my house?

Call an HVAC company before you run the furnace again. JC Mechanical can inspect it and tell you what we find. Call (720) 779-7263.

Had a CO scare, or want your furnace checked before winter?

Call JC Mechanical at (720) 779-7263. We serve Denver and the south metro, including Centennial, Highlands Ranch, Littleton, Lone Tree, and the other cities on our service areas page. You can also request service online.

JC Mechanical Heating & Air Conditioning LLC · Owner Jimmie Ciccone · In business since 2019 · 7600 S Grant St, Unit B4, Littleton, CO 80122 · (720) 779-7263

Sources: Colorado C.R.S. 38-45-102 · Denver Fire Department: smoke and CO detector installation · North Metro Fire Rescue: carbon monoxide poisoning · Boulder County: carbon monoxide · Rocky Mountain Poison and Drug Safety