Is Your Furnace Safe? Why Carbon Monoxide (CO) Testing Matters This Winter

When temperatures drop in Portland most people adjust their central air system to heat without a second thought, but as your furnace kicks in, there is one hidden danger: carbon monoxide (CO). This odorless, colorless, and extremely dangerous gas can leak from malfunctioning furnaces and compromise the safety of your home. Every winter, CO poisoning sends thousands of people nationwide to the emergency room—many cases originating from faulty heating equipment. That’s why having a professional furnace inspection that tests for CO before winter isn’t just recommended… it’s essential. Cornel’s Plumbing, Heating & Air puts CO safety at the top of every furnace tune-up, and here’s exactly why it matters. Schedule your CO safety inspection with Cornel’s Plumbing, Heating & Air Today

What Is Carbon Monoxide (CO) and How It Forms in Your Furnace 

Carbon monoxide is a toxic gas produced when fuel—like natural gas, propane, or oil—does not burn completely. A properly functioning furnace should produce minimal CO and send it safely through the flue and out of your home. However, several furnace issues can cause CO levels to rise or leak inside your home:

• Dirty or clogged burners preventing complete combustion
• Cracked heat exchanger, which can allow CO to mix with indoor air flow
• Blocked, rusted, or disconnected flue pipes
• Improperly adjusted gas pressure
• Poor ventilation or airflow restrictions
• Soot buildup, which disrupts burner performance (as seen in Cornel’s “Excessive Furnace Soot” case study)
• These problems often develop slowly over time, which is why annual inspections are crucial.

Why Carbon Monoxide Is So Dangerous

Even low levels of carbon monoxide exposure can cause health problems, and high levels can become deadly within minutes. Symptoms of CO exposure include Symptoms of carbon monoxide exposure can be subtle at first and often resemble common illnesses, which is why CO is so dangerous. Early signs typically include headache, nausea, dizziness or lightheadedness, and shortness of breath. As exposure increases, people may experience chest pain, disorientation or confusion, and an overwhelming sense of unusual fatigue. These symptoms can escalate quickly, making prompt recognition and immediate action critical. Book A Furnace Maintenance Call Now

CO vs. CO₂: Why the Difference Matters

Clients sometimes confuse carbon monoxide (CO) with carbon dioxide (CO₂). CO₂ testing is part of combustion analysis for efficiency. CO testing is a safety procedure that checks for dangerous leaks. For homeowner safety, CO is the critical gas that must be checked during a furnace tune-up.

How Technicians Test for CO During a Furnace Tune-Up

Professional CO testing isn’t just a quick detector check—it’s a systematic process performed with calibrated instruments. At Cornel’s, technicians follow several steps to ensure your furnace is safe:

1. Ambient CO Testing A technician starts by measuring CO levels in the air around your furnace while it’s running and idle. Any detectable CO indoors is cause for further investigation.
2. Flue Gas Analysis Using a combustion analyzer, the technician measures: CO levels inside the flue; Oxygen (O₂) levels; Combustion efficiency; Stack temperature. This test determines if the furnace is burning fuel safely and cleanly.
3. Heat Exchanger Inspection Cracked or damaged heat exchangers are a top cause of CO leaks. Techs visually inspect and use flashlights, mirrors, or cameras to find early signs of failure.
4. Venting and Draft Testing Blocked, rusted, or improperly sloped vent pipes can trap CO indoors. The technician confirms proper airflow and draft strength.
5. Safety Control & Ignition Check Malfunctioning ignition systems and safety switches can cause incomplete combustion. These are tested and calibrated.
6. System Run Cycle Test Finally, the tech runs the furnace through a full heating cycle to verify: Stable burn; No unusual flame color changes; No hazardous CO readings. This meticulous approach is why CO testing should always be left to trained HVAC professionals—not DIY tools.

Warning Signs Your Furnace Might Be Leaking CO

Physical or operational signs

• Yellow or flickering burner flame (A healthy gas flame should be mostly blue.)
• Soot buildup around the burner, vents, or furnace cabinet
• Burning smells, especially metallic or smoky odors
• Rust or white residue on vent pipes
• Excessive condensation on windows or near the furnace
• Pilot light that frequently goes out
• Unusual noises like rumbling or popping during burner ignition

Human symptoms (occur indoors only)

• Headaches that improve when you go outside
• Dizziness or nausea at home but not elsewhere
• Confusion or fogginess

If you experience these signs, leave the home immediately and call emergency services, then schedule a furnace inspection.

CO Safety Checklist for Homeowners

CO Safety Task	Completed?
Install CO detectors on every level of your home	⬜
Replace CO detector batteries annually	⬜
Schedule a professional furnace tune-up with CO testing	⬜
Keep furnace and flue area clear of debris	⬜
Check for visible rust, soot, or discoloration around vents	⬜
Confirm furnace has a strong blue flame (not yellow)	⬜

Preventive Maintenance: How to Avoid CO Problems Altogether

Your furnace can run safely and efficiently for years—but only with consistent care.

Here’s how to reduce CO risks

Get an annual furnace tune-up

This is the #1 way to prevent CO leaks. Professional cleaning, burner tuning, and flue checks are critical.

Replace air filters regularly

Clogged filters restrict airflow, which leads to overheating and incomplete combustion.

Never block furnace vents or returns

Restricted airflow can cause dangerous pressure buildup.

Keep chimneys and flues clear

Bird nests, leaves, and debris are common causes of vent blockages.

Upgrade older furnaces

Systems over 15–20 years old are more prone to heat exchanger cracks and should be inspected closely. Cornel’s technicians take safety seriously and always include CO checks as part of their comprehensive winter tune-ups.

Why Portland Homeowners Trust Cornel’s for Furnace CO Testing

Cornel’s Plumbing, Heating & Air has earned a reputation for precise, reliable furnace safety inspections. Portland homeowners trust them because:

• Every tune-up includes detailed CO testing
• Technicians are licensed, certified, and trained in combustion analysis
• They understand unique Northwest furnace issues like soot buildup, moisture-related corrosion, and older venting systems
• Service is fast, honest, and highly thorough

When it comes to CO safety, “good enough” is never good enough. You want a team that takes your family’s well-being seriously.

Frequently Asked Questions (FAQs)

1. What CO level is considered dangerous? Any indoor CO reading above 9 ppm (parts per million) is a concern. Levels above 35 ppm can cause symptoms, and 100+ ppm is dangerous and potentially life-threatening. Your home should have zero measurable CO under normal conditions.
2. How often should my furnace be tested for CO? Once per year. Ideally, schedule CO testing every fall, before the heating season begins. Older furnaces or systems with prior issues may need more frequent checks.
3. Do CO detectors replace professional furnace CO testing? No. CO detectors protect you from acute danger, but they do not identify small leaks, combustion issues, or heat exchanger cracks. Only professional testing can verify furnace safety.
4. What does a cracked heat exchanger mean for CO safety? A cracked heat exchanger can allow combustion gases—including CO—to mix with indoor air. It’s a serious safety hazard and often requires furnace replacement.
5. Is CO testing included in a Cornel’s furnace tune-up? Yes. Cornel’s technicians perform CO checks, combustion analysis, venting inspections, and safety tests as part of every standard furnace tune-up.

Stay Safe This Winter—Schedule Your Furnace CO Test Today

Carbon monoxide is a silent, invisible danger—but with the right precautions, it’s completely preventable. A professional furnace tune-up with CO testing ensures your heating system is safe, efficient, and ready for Portland’s winter season. Schedule your CO safety inspection with Cornel’s Plumbing, Heating & Air Today Written by: Hunter Johanson, Licensed HVAC Technician (EPA 608 Certified), Cornel’s Plumbing, Heating & Air Conditioning Reviewed by: Jeff Brown, HVAC Manager, Cornel’s Plumbing, Heating & Air Conditioning Last updated: November 17, 2025