Published: November 3, 2025 | Closer Look Home Inspectors · InterNACHI Certified · Mankato, MN

Carbon Monoxide Risks in Older Minnesota Homes

By Closer Look Home Inspectors | Updated November 2025 | (507) 721-3820

Carbon monoxide is called the silent killer for good reason: this odorless, colorless gas kills approximately 430 Americans annually and sends 50,000 more to emergency rooms. In Minnesota, where heating season extends six months and homes are sealed tightly against the cold, the risk is significantly elevated. Older homes in the Mankato area present the highest CO risk due to aging combustion appliances, deteriorating venting systems, and modifications that affect combustion air supply.

How Carbon Monoxide Forms in Homes

Carbon monoxide is produced whenever fossil fuel burns incompletely. Every fuel-burning appliance in your home produces some CO: furnaces, water heaters, gas ranges, gas dryers, fireplaces, and wood stoves. Under normal conditions, these appliances vent CO outdoors through chimneys and exhaust pipes. CO becomes dangerous when appliances malfunction, venting systems fail, or combustion air supply is inadequate.

Why Older Mankato Homes Are at Higher Risk

Cracked Heat Exchangers

The heat exchanger is the metal barrier between the combustion chamber and the air circulated through your home. It keeps combustion gases, including CO, separate from breathing air. Over years of thermal cycling, heating to over 1,000 degrees during operation and cooling between cycles, heat exchangers develop stress cracks. These cracks allow CO to mix with the home's air supply. A cracked heat exchanger is a life-threatening emergency. Furnaces older than 15 to 20 years are at highest risk.

Backdrafting

Backdrafting occurs when exhaust gases reverse direction and flow back into the home instead of up the chimney. This happens when the house is under negative pressure from exhaust fans, clothes dryers, or tightly sealed construction that starves combustion appliances of adequate air supply. Older homes that have been weatherized, with new windows, added insulation, and sealed air leaks, are particularly vulnerable because the home's tighter envelope reduces the combustion air supply the furnace and water heater need.

Deteriorating Chimney Flues

Older homes often have clay tile chimney liners or unlined masonry chimneys serving the furnace and water heater. These flue systems deteriorate over time, developing cracks and gaps that allow CO to leak into wall cavities and living spaces. When a high-efficiency furnace replaces an old standard furnace but the existing chimney continues to serve the water heater, the reduced exhaust volume may not keep the oversized chimney warm enough to draft properly, causing backdrafting.

Gas Appliance Issues

Older gas ranges without electronic ignition have standing pilot lights that produce CO continuously. Gas water heaters with corroded burner assemblies or restricted combustion air can produce excessive CO. Gas fireplaces and log sets with deteriorated components or improper installation can leak CO into living spaces.

Symptoms of Carbon Monoxide Exposure

CO symptoms mimic many common illnesses, making identification difficult:

• Low-level exposure: Headache, fatigue, nausea, dizziness. Often mistaken for flu.
• Moderate exposure: Severe headache, drowsiness, confusion, rapid heart rate.
• High-level exposure: Unconsciousness, seizures, death.

A telling sign: if symptoms improve when you leave the house and return when you come home, suspect CO exposure. If multiple household members experience simultaneous flu-like symptoms, suspect CO.

Protection Strategies

• CO detectors: Minnesota law requires CO detectors within 10 feet of every bedroom and on every level. Test monthly, replace batteries annually, and replace the units every 5 to 7 years. See our CO detector guide.
• Annual furnace inspection: Have a qualified HVAC technician inspect your furnace annually, including combustion analysis and heat exchanger evaluation.
• Proper venting: Ensure all fuel-burning appliances vent properly to the exterior. Check that exhaust terminations are clear of snow, ice, and debris.
• Combustion air supply: Ensure fuel-burning appliances have adequate combustion air. If your home has been significantly weatherized, a combustion air supply may need to be added.
• Professional inspection: Our home inspection includes evaluation of all combustion appliances for safety, proper venting, and CO risk factors.

What to Do if Your CO Detector Alarms

1. Do not ignore the alarm. Move everyone outdoors immediately.
2. Call 911 from outside the home.
3. Do not re-enter the home until emergency responders declare it safe.
4. Seek medical attention if anyone has symptoms.
5. Have all fuel-burning appliances professionally inspected before resuming use.

Concerned about CO safety in your older Mankato home? Call Closer Look Home Inspectors at (507) 721-3820 for a comprehensive safety evaluation.