Gas Furnace Safety and Carbon Monoxide in Indiana Homes

Last February, a friend in Broad Ripple called me at 6 a.m. Her carbon monoxide alarm wouldn't stop beeping. She'd just had a new furnace put in the year before, so she figured it was a false alarm. But when the fire department showed up, they found CO levels high enough to make a grown man dizzy. The problem? Snow had piled up against the sidewall vent outside, blocking the exhaust. The furnace kept running, pushing carbon monoxide back into the house.

That story isn't rare around Indianapolis. We get freezing temps, drifting snow, and older homes with quirky venting. Furnace carbon monoxide problems don't announce themselves—the gas is colorless and odorless, and you can't taste it. That's why so much of this comes down to prevention and knowing what to look for.

How a Furnace Can Leak Carbon Monoxide (Even a New One)

A gas furnace burns natural gas. If the burners are clean, the gas is adjusted right, and the exhaust vent is clear, the byproduct is mostly water vapor and carbon dioxide—not carbon monoxide. But when something's off, CO shows up. The usual culprits:

• Cracked heat exchanger. The metal separates over time from heating and cooling. A crack can let combustion gases mix with the air blowing through your ducts. It's often invisible unless you use a camera or do a combustion test.
• Blocked flue or vent. Snow, ice, leaves, bird nests, or even a fallen dryer vent cap can clog the exhaust path. High-efficiency furnaces with PVC pipes on the side of the house are especially prone to drifting snow in an Indianapolis winter.
• Backdrafting. If your house is tight and you run a strong kitchen exhaust fan or bathroom fan while the furnace is on, the exhaust can get pulled back down the chimney instead of going outside. That's classic backdrafting—and it can happen in older homes with masonry chimneys (think Meridian-Kessler, Irvington, or Fountain Square).
• Dirty burners. When dust or rust builds up, the flame doesn't mix with air properly. That incomplete combustion makes CO.

So yes, a furnace that seems to run fine can still leak CO. Newer furnaces aren't immune—if the installer didn't size the vent right, or the termination is too close to a window, CO can sneak back in. No matter the age, an annual safety check catches most of this.

The Indianapolis Twist: Snow, Chimneys, and Tight Houses

Around here, the risks aren't just mechanical. Local weather and housing stock play a role.

Snow blocks sidewall vents. High-efficiency furnaces in newer homes across Carmel, Fishers, Westfield, and Noblesville often have two white PVC pipes sticking out the side of the house. One pulls in combustion air, the other pushes out exhaust. If drifting snow buries the intake, the furnace starves for air and the flame gets lazy. If the exhaust pipe gets covered, CO backs up into the home. Last January, I got a call from a homeowner in Zionsville whose furnace kept shutting off. The exhaust pipe had an ice cap on it from a freeze-thaw cycle. Easy fix, but only after the CO alarm did its job.

Old brick chimneys without liners. Indianapolis has a lot of 1920s and 1930s homes with brick chimneys. Over time, the mortar erodes and the flue gets rough. Without a proper liner, exhaust gases can seep through cracks into wall cavities or back into the living space. A homeowner in Speedway told me his gas bill was higher than usual and his CO detector near the furnace would occasionally chirp. The chimney had multiple gaps. We recommend a liner or a switch to a direct-vent furnace in that situation.

Tight houses with strong exhaust fans. Newer homes or remodeled older ones often have spray foam insulation and sealed ductwork. That's great for energy bills, but it means the house can't naturally draw enough air for the furnace to vent properly. If someone runs a 600 CFM range hood while the furnace is on, the house goes negative pressure and the chimney becomes a straw pulling exhaust back down. I've seen this in Greenwood and Avon homes where a new kitchen exhaust fan caused all kinds of CO headaches until we added a makeup air vent.

Attached garages pulling fumes inside. If you idle a car in an attached garage, even with the overhead door open a bit, CO can seep through the door seal or get pulled into a return duct. That's why we never recommend running a car in the garage, even in winter. Same goes for gas water heaters or fireplaces in the garage—make sure they vent outside, not into the garage.

What a Professional Actually Checks for Furnace Carbon Monoxide

When HVACPros comes out for a furnace CO inspection, we don't just wave a hardware-store meter near the register and call it done. We do the full combustion safety test.

First, we measure ambient CO in different rooms—especially near the furnace and upstairs bedrooms. Then we run the furnace and put a combustion analyzer in the flue. That tells us oxygen, carbon monoxide, carbon dioxide, and stack temperature. A good furnace should have CO in the flue below 100 ppm, often under 50 ppm. If it's climbing above that, something's wrong.

We also check the heat exchanger with an inspection camera. A crack might not be visible to the naked eye, but the camera can spot hairline separations. Then we test the draft. If the flue pressure isn't right, we look for vent sizing issues, blockages, or backdrafting. Finally, we verify the vent termination outside—no snow, leaves, or improper clearance from windows and doors.

This isn't just about the furnace. As a general contractor, we look at the whole house. Is the mechanical room getting enough combustion air? Are return ducts pulling from places they shouldn't be? Sometimes the fix is as simple as adding a louvered door or sealing a return duct that was leaking garage air. We've seen all of it.

Repair or Replace? Straight Talk on a Cracked Heat Exchanger

If we find a cracked heat exchanger, the furnace is unsafe to run. Period. Some companies might offer to patch it, but that's a temporary band-aid. The metal is fatigued—cracks will come back, and you're gambling with CO every time the furnace cycles.

So when does replacement make sense? If the furnace is over 15 years old, the heat exchanger is cracked, or the combustion analysis shows CO levels that can't be tuned down, replacement is usually the smart call. But not always. If the only issue is a dirty burner, a clogged condensate line, or a vent pipe that's sagging and causing poor draft, we can often clean, adjust, and re-test. We'll show you the numbers before and after.

If a tech tells you need a new furnace because of carbon monoxide risk, get a second opinion. Seriously. A reputable company will show you photos of the crack or the combustion readings. If they won't show you evidence, that's a red flag. Check out our post on getting a second opinion on an HVAC replacement quote for more on that.

CO Detectors: Where to Put Them and What the Numbers Mean

You need carbon monoxide detectors even if your furnace is brand new. Here's what I tell homeowners:

• Place one on every level of the house, including the basement. Especially outside sleeping areas.
• Install them at least 15 feet away from fuel-burning appliances to avoid nuisance alarms.
• Test them monthly. Replace batteries when you change clocks in spring and fall.
• Replace the whole unit according to the manufacturer's lifespan—usually 5 to 7 years. They don't last forever.
• Choose a model with a digital readout. That way you can see the peak CO level even after the alarm stops.

What ppm is dangerous? Low levels (9-35 ppm) over many hours can cause headaches and fatigue, especially in kids, elderly, or people with heart conditions. Levels above 70 ppm can cause symptoms within an hour. Anything over 150-200 ppm means get out immediately and call 911. That's why a detector with a digital display is worth the extra few bucks—you get real information, not just a beep.

If your CO alarm goes off, even if it stops after you open a window, don't ignore it. Evacuate everyone to fresh air. Call 911 or the fire department. They have equipment to measure CO and will tell you when it's safe to go back inside. Don't re-enter until they give the all-clear. Then call a professional to find the source. Opening a window might clear the CO temporarily, but it doesn't fix the furnace vent that was blocked in the first place.

Maintenance That Prevents Furnace Carbon Monoxide Problems

The single best thing you can do is get an annual furnace safety check before heating season starts. In Indiana, that means September or early October—before the first freeze.

During a tune-up, we clean the burners, check the flame sensor, test the inducer motor, inspect the heat exchanger, and verify the venting. We also change or clean the air filter. A dirty filter restricts airflow, which can cause the furnace to overheat and stress the heat exchanger—leading to cracks over time.

Outside, keep the vent terminations clear. Trim back shrubs, remove leaves in fall, and after a heavy snow, go outside and check that the PVC pipes aren't buried. If you have an older furnace with a metal chimney, have the chimney inspected every few years. Freeze-thaw cycles in central Indiana can crack the mortar and create gaps.

Also, don't idle the car in an attached garage. Ever. And if you have a gas water heater or fireplace, test for CO around those too. An electric furnace won't produce CO, but your gas water heater, gas dryer, or attached garage still can. Whole-home safety matters.

If you're new to homeownership or just bought a house with an existing furnace, check out our guide on HVAC warranty registration to make sure you're covered if something fails. And if you're ever dealing with a furnace emergency—like a CO alarm at 2 a.m.—take a look at what qualifies as an HVAC emergency. Sometimes waiting until Monday isn't an option.

Myths That Get People in Trouble

"I'll just crack a window." That might lower CO levels temporarily, but it doesn't stop the furnace from producing more. A window is not a fix.

"My furnace is new, so it's safe." A new furnace installed wrong is still dangerous. Bad venting, improper gas pressure, or a blocked termination on a brand-new unit can still create CO.

"I can smell gas, so I'm fine." Natural gas has an odorant added (that rotten egg smell from Citizens Energy gas). Carbon monoxide has no smell. You can't detect CO with your nose. If you smell gas, leave and call the gas company. But CO is a completely different threat.

"CO alarms go off for no reason." They can false alarm from low battery, humidity, or being too close to a stove. But if a digital detector shows a real level, don't dismiss it. Better to have it checked.

What HVACPros Does—and How to Book a Check

If you want peace of mind before winter, schedule a furnace CO inspection with HVACPros. We'll do a combustion analysis, check the heat exchanger, test the draft, and inspect your venting. You'll get a written report with all the readings, plus photos if we find a crack or blockage. No scare tactics, no pushy sales pitch. Just straight talk about what's safe and what isn't.

If you're worried a tech might try to upsell you, read our second opinion guide. We believe in showing you the numbers and letting you decide.

Ready to book? Get a quote or schedule an inspection here. We serve Indianapolis, Carmel, Fishers, Zionsville, Westfield, Noblesville, Greenwood, Brownsburg, Avon, and Plainfield. Same-day appointments are often available during heating season.