Carbon Monoxide in Furnace: Hidden Dangers and How to Protect Your Family

When winter settles in across Colorado, furnaces work hard to keep homes, offices, and commercial spaces warm. Most of the time, they do that job safely. But if a furnace is damaged, dirty, poorly installed, or not maintained, it can produce carbon monoxide in furnace systems and let that gas enter your building.

Carbon monoxide, often called CO, is dangerous because you cannot see it or smell it. That makes it especially important for Colorado homeowners and business owners to understand the warning signs, common causes, and best ways to prevent a leak. In this refreshed guide, we explain how carbon monoxide in furnace systems develops, what symptoms to watch for, and how to keep your family, tenants, employees, and customers safe.

What Is Carbon Monoxide?

Carbon monoxide is a toxic gas created when fuel does not burn completely. Furnaces that run on natural gas, propane, or oil can produce CO if combustion is not happening the way it should. In a properly operating system, those combustion gases are vented safely outside. When something goes wrong, carbon monoxide can build up indoors and create a serious health risk.

Because CO has no color, odor, or taste, people often do not realize there is a problem until someone feels sick or a detector alarms. Even low levels of exposure can cause symptoms. High levels can become life-threatening very quickly.

How Carbon Monoxide in Furnace Systems Happens

Most furnaces in Greenwood Village, Englewood, Littleton, Highlands Ranch, Parker, and surrounding Colorado communities are designed to burn fuel efficiently and vent exhaust outdoors. Carbon monoxide in furnace equipment usually develops when one of those safety steps is disrupted.

Common causes include:

• Incomplete combustion because the furnace is not getting enough air
• Cracked or damaged heat exchangers
• Blocked or improperly sized vent pipes
• Dirty burners, clogged filters, or restricted airflow
• Poor furnace installation or unsafe repairs
• Backdrafting, which allows exhaust gases to flow back into the building

For example, a commercial property in Denver might have a rooftop vent obstructed by snow, debris, or nesting material. In a home in Aurora, a plugged flue or aging heat exchanger may cause the same problem. In both cases, the issue can lead to carbon monoxide entering the occupied space.

Why Carbon Monoxide Is So Dangerous

Carbon monoxide is dangerous because it binds to hemoglobin in your blood more readily than oxygen does. That means your body cannot carry oxygen to the brain, heart, and other vital organs as well as it should. The result can be mild symptoms at first, then severe illness if exposure continues.

According to the CDC, hundreds of people die from unintentional carbon monoxide poisoning each year in the United States, and many more require emergency treatment. In homes and businesses, the risk often rises in colder months when furnaces run more frequently and buildings stay closed up tighter.

Symptoms of Carbon Monoxide Exposure

CO poisoning symptoms can look like the flu, food poisoning, or general fatigue. That is why they are easy to miss, especially when several people in the same space start feeling unwell at the same time.

Common symptoms include:

• Headache
• Dizziness
• Nausea or vomiting
• Weakness or fatigue
• Shortness of breath
• Confusion
• Blurred vision
• Chest pain in more serious cases

If symptoms improve when you leave the building and return when you go back inside, carbon monoxide in furnace operation or another fuel-burning appliance may be involved.

Warning Signs of a Carbon Monoxide Problem

In addition to physical symptoms, there are several signs that your furnace may not be operating safely:

• Yellow or flickering burner flames instead of steady blue flames
• Soot buildup around the furnace, vents, or draft hood
• Rust on vent pipes or around the unit
• Moisture, staining, or discoloration near the furnace
• Frequent furnace shutdowns or combustion-related error codes
• Musty, burning, or stale odors that suggest ventilation problems

While carbon monoxide itself has no smell, these surrounding signs often point to a combustion or venting issue that needs immediate attention.

Common Causes of Carbon Monoxide Leaks in Furnaces

Cracked Heat Exchangers

The heat exchanger separates the air you breathe from the hot combustion gases inside your furnace. If it cracks, combustion gases can mix with your indoor air. This is one of the most serious furnace-related causes of carbon monoxide in furnace systems.

Heat exchangers can fail over time due to age, corrosion, repeated heating and cooling cycles, or manufacturing defects. During a routine furnace inspection in Littleton, our team once found a small crack in a relatively newer unit. Catching it early prevented a dangerous indoor air problem and saved the customer from a major emergency.

Blocked or Damaged Venting

Your furnace must vent exhaust gases safely outdoors. If the vent is blocked, damaged, disconnected, or not installed correctly, carbon monoxide can linger or back up into the building. This is a common issue after storms, heavy snow, or debris buildup.

We have seen venting problems in homes and small businesses across Aurora and Denver where birds, leaves, insulation, or ice blocked the exhaust path. The furnace kept running, but the CO risk increased because the gases could not leave the system properly.

Poor Maintenance

A furnace that has not been cleaned or serviced regularly is more likely to develop combustion problems. Dirty burners, clogged filters, weak airflow, and worn components can all contribute to incomplete combustion. That means more carbon monoxide in furnace exhaust and a higher risk that it will escape indoors.

Routine maintenance is especially important for commercial buildings, rental properties, and busy offices where the system may run for long hours and support more people.

Improper Installation or Repairs

Even a new furnace can create carbon monoxide problems if it is installed incorrectly. Poor gas pressure, incorrect vent sizing, loose connections, and code violations can all affect how safely the furnace operates. The same is true for sloppy repairs or unlicensed work.

That is why we always recommend working with licensed HVAC professionals who understand Colorado code requirements, manufacturer specifications, and safe combustion practices.

How to Protect Your Family, Employees, and Property

Schedule Annual Furnace Maintenance

Annual maintenance is one of the best ways to prevent carbon monoxide in furnace systems. A trained HVAC technician can inspect the burners, heat exchanger, venting, airflow, and safety controls before small issues turn into dangerous ones.

At Alloy Heating and Cooling, we service homes and businesses throughout the Denver metro area and nearby communities. During a tune-up, our technicians check for signs of combustion trouble, verify airflow, and look for anything that could cause unsafe operation during the heating season.

Install and Test Carbon Monoxide Detectors

Carbon monoxide detectors are essential any time you have fuel-burning appliances. Colorado law requires CO detectors in many residential settings, and every building with a furnace should have them in the right places.

Best practices include:

• Place detectors near sleeping areas
• Install one on every level of the building
• Test them monthly
• Replace batteries as recommended by the manufacturer
• Replace detectors at the end of their service life

For small businesses and commercial properties, detectors should be part of a broader life-safety plan along with routine HVAC inspections and documented maintenance.

Keep Vents and Airflow Pathways Clear

Never block furnace vents, combustion air openings, or exhaust terminations. Outside, check that snow, ice, leaves, or debris are not covering vent pipes or exhaust outlets. Inside, make sure storage boxes, furniture, or cleaning supplies are not restricting airflow around the furnace.

This is especially important in Colorado winters, when snowdrifts and ice can affect rooftop or sidewall venting.

Watch for Furnace Changes

If your furnace starts making unusual noises, cycling on and off more often, producing visible soot, or failing to heat properly, do not ignore it. Those changes may not always mean carbon monoxide is present, but they do mean the system should be checked promptly.

What to Do If You Suspect Carbon Monoxide in Furnace Operation

If your CO detector goes off or you suspect exposure based on symptoms or furnace warning signs, take it seriously.

• Leave the building immediately
• Get everyone, including pets, outside into fresh air
• Call 911 or your local fire department from outside
• Do not go back inside until emergency responders say it is safe
• After the building is cleared, have a licensed HVAC professional inspect the furnace before using it again

We have helped customers in Westminster and across the Denver metro area after detector alarms or symptom complaints led them to evacuate quickly. In more than one case, a blocked vent or failing component was found before anyone was seriously harmed. Quick action makes a difference.

Why Local HVAC Expertise Matters in Colorado

Colorado homes and businesses face unique heating challenges. High altitude can affect combustion, winter weather can block vents, and older buildings may have aging mechanical systems that need special attention. A furnace that seems to be “working fine” can still have a hidden safety issue.

That is why local inspections matter. An experienced Colorado HVAC technician knows what to look for in our climate, our building styles, and our common furnace setups. At Alloy Heating and Cooling, we take a practical, safety-first approach that helps protect families, tenants, employees, and customers.

Frequently Asked Questions About Carbon Monoxide in Furnace Systems

Here are answers to common questions Colorado homeowners, landlords, and business owners ask about carbon monoxide in furnace equipment.

Protect Your Property with Trusted Furnace Service

Carbon monoxide in furnace systems is a serious safety issue, but it is also one that can often be prevented with regular maintenance, proper installation, and working detectors. If you have concerns about your furnace, do not wait for symptoms or an alarm to tell you something is wrong.

Alloy Heating and Cooling proudly helps Colorado homeowners, small businesses, and commercial property owners keep their heating systems safe and reliable. If you need furnace maintenance, inspection, or repair, our team is ready to help.

Frequently Asked Questions