Every fall we get calls about furnaces. What we don’t get calls about, until something goes wrong, is the carbon monoxide detector sitting quietly on the wall the whole time. It’s usually fine. But “usually” isn’t the standard you want for a device whose entire job is warning you about a gas you can’t see, smell, or taste.

A Working Furnace and a Dead Detector Look Identical

This is the part that catches people off guard: there’s no visible difference between a furnace operating safely and one slowly leaking carbon monoxide into a room with no working detector to catch it. The furnace still runs. The house still heats up. Nothing looks wrong, because CO doesn’t announce itself the way a gas leak’s smell does. The detector is the only thing standing between a normal heating season and a genuine emergency, which is exactly why a detector nobody’s tested since installation is a bigger risk than most homeowners realize.

Press the Test Button. It Takes Five Minutes.

The test button isn’t decorative. Pressing it and holding it for a few seconds checks that the alarm circuitry, speaker, and power supply are all still functioning together — not just that the unit has power. Manufacturers generally recommend testing monthly, but if it’s been longer than that, this is the moment to catch up before the furnace’s first real cycle of the season. If the alarm doesn’t sound clearly within a few seconds, the unit needs new batteries or replacement, not a second attempt.

Batteries Fail Quietly — Don’t Wait for the Chirp

Battery-powered and battery-backup CO detectors are designed to chirp when the battery gets low, but that chirp assumes the unit’s monitoring circuit is still working well enough to notice. Batteries can also drain faster than expected, especially in units that have been in place for years. If it’s been more than a year since the batteries were changed, replace them now, regardless of what the indicator light says. It costs a few dollars and a few minutes.

Check the Manufacture Date, Not Just the Batteries

Carbon monoxide sensors degrade over time even in a unit that passes its test button check and has fresh batteries. Most CO detectors have a usable life of five to seven years, printed as a manufacture date on the back or bottom of the unit. Past that window, the sensor’s accuracy can’t be guaranteed even if the alarm still sounds during a test. If you don’t know how old your detector is, that’s worth finding out before relying on it through another winter.

What We Actually Check During a Fall Safety Inspection

The detector is the backup. The furnace itself is where a CO problem usually starts, which is why a real fall inspection goes past “does it turn on.” Two things matter most:

• Heat exchanger. It separates combustion gases from the air that circulates through your home. A crack in it — often invisible without the right inspection — lets combustion byproducts, including carbon monoxide, mix into the air your furnace blows into your living space.

• Gas connections. Loose or degraded connections at the gas line, valve, and burner assembly can allow small leaks that don’t always trigger a strong smell, particularly with modern low-odorant gas blends. Pressure and connection checks catch this before it becomes a bigger problem.

These are the two issues that actually cause a carbon monoxide problem in a home with a gas furnace, and neither is something you can check by looking at the unit or listening to it run.

Signs You’re Overdue for Both Checks

• Your CO detector doesn’t have a visible manufacture date sticker, or you can’t remember installing it.

• The furnace hasn’t been serviced since last spring, or longer.

• You’ve noticed a mild burning or metallic smell the first time the furnace runs each season.

• Anyone in the house has had unexplained headaches, dizziness, or nausea that improve when they leave the house.

• The detector has ever chirped and you weren’t sure if it was a low battery or something else.

Any one of these on its own might be nothing. Together, or ignored for more than a season, they’re worth acting on.

If the Alarm Ever Does Go Off

Treat it as real, not as a false alarm to silence and forget. Get everyone outside into fresh air immediately, and don’t go back in until the source has been identified. Call for help before troubleshooting it yourself — carbon monoxide exposure isn’t always obvious in the moment, and it’s not a situation where waiting to see if symptoms develop is a good plan.

Why This Comes Up Every Fall in the Denver Metro

Furnaces here typically sit unused for five or six months between spring and the first cold snap. That’s a long stretch for a combustion appliance and its detector to go untouched. At altitude, gas appliances also burn slightly differently than at sea level, which is one more reason a proper fall inspection — not just a glance at the thermostat — matters here specifically.

Serving the Denver Metro

We provide fall furnace safety inspections and carbon monoxide safety checks for homeowners across Denver, Aurora, Lakewood, Westminster, Thornton, Centennial, Broomfield, Parker, Highlands Ranch, Commerce City, Littleton, Wheat Ridge, Brighton, Englewood, Arvada, Golden, Castle Rock, Greenwood Village, and Northglenn.

Frequently Asked Questions

How often should I test my carbon monoxide detector? Once a month, plus anytime you haven’t checked it in a while before a season change. Replace batteries at least once a year regardless of the low-battery indicator.

Do I need a CO detector on every floor? Yes — at minimum one on each level of the home and near sleeping areas, since CO can accumulate differently depending on air movement and appliance location.

Can a furnace that’s heating fine still be producing carbon monoxide? Yes. A cracked heat exchanger or a gas connection issue can leak CO into circulated air while the furnace continues to heat normally, which is exactly why the detector matters.

Is a chirping detector always a low battery? Usually, but not always — some units chirp to indicate end-of-life. If new batteries don’t stop the chirping, the unit itself likely needs replacing.

What’s included in a professional fall safety check? A heat exchanger inspection, gas connection and pressure check, ignition and burner inspection, and a functional test of your system’s safety controls, in addition to standard maintenance items like the filter.

Test It Before the Furnace Runs

A working furnace paired with an untested detector is the exact combination that turns a routine maintenance item into a real risk. Both take only minutes to check, and both are worth doing before the temperature actually drops.

Book a fall safety check