How to Tell If Your Furnace Is Producing Carbon Monoxide

Carbon monoxide is odorless, colorless, and undetectable by human senses. That is what makes it dangerous.

But a furnace producing carbon monoxide is rarely silent about it. It leaves physical evidence — in the burner compartment, on the flames, and in the venting. Those signs are visible to anyone who knows what to look at, and they are the reason a trained inspection catches problems long before an alarm sounds.

Here is what that evidence looks like, and what testing is required to confirm it.

First, the Chemistry in One Paragraph

Natural gas is carbon and hydrogen. Add an ignition source and oxygen, and combustion breaks those bonds and releases heat.

On a good day the carbon finds two oxygen atoms and forms carbon dioxide, which is harmless to human health and leaves through your exhaust vent. For that to happen, combustion products must stay hot and oxygen must be available.

When there is not enough oxygen — or the flames cool before the carbon can bond with it — the carbon has no choice. It remains carbon, or it becomes carbon monoxide.

That is the whole mechanism. Carbon monoxide is not a mysterious defect. It is what incomplete combustion produces, every time.

The Visual Warning Signs

Carbon Deposits

Look for carbon built up anywhere in the system — in the burner area, in the flue, in the vent. Carbon is a product of incomplete combustion. Its presence is direct physical evidence that combustion is not finishing properly.

Flame Appearance

A correctly tuned burner produces a flame with a dark blue interior, a lighter blue exterior, and at most a light orange tip. It should be steady and straight, never wavering or fluttering.

Warning signs on the flames include:

• Flames burning erratically
• White tips on the flames
• Weak flames
• No visible flame on part of the burner
• Inconsistent patterns across burners
• Flames visibly touching the heat exchanger

That last one has a name: impingement. When a flame contacts the heat exchanger, its temperature drops, complete combustion stops, and carbon begins building on the exchanger surface. Impingement is caused by burner misalignment, a dirty orifice, gas pressure that is too high or too low, or the wrong air mixture.

Flame Roll-Out

Flames unexpectedly coming out the front of the appliance indicate serious combustion problems.

You can see the history of it even when the furnace is off. Look for black or rusted areas in front of the burners, burnt wires, and carbon deposits. Homeowners often mention a related symptom without connecting it: they occasionally hear a “boom” when the furnace turns on.

The Tests That Actually Confirm It

Visual inspection identifies suspects. Instruments produce verdicts.

Flue Carbon Monoxide

Regardless of what the visual inspection shows, a test must be performed to verify there is no CO in the combustion gases.

The sample has to be taken from each flue, before additional dilution air mixes into the gases. A furnace with four burners gets at least four separate tests. If a furnace and water heater share a common vent, both appliances must be tested independently and then together.

The widely used field standard is under 100 ppm in the flue, with many programs accepting 100–200 ppm. We hold a tighter line. Field experience across years of testing shows that most units producing more than 25 ppm in the flue can be corrected — so we treat 25 ppm as the threshold for action rather than a passing grade.

The point of the test is not just the number. It is determining why CO is being created, because CO is a symptom of something being wrong with the unit or the building.

Ambient Carbon Monoxide

Before testing any appliance, a baseline reading is taken in the living space. It should be close to zero. If initial readings are measurable, the source needs to be identified.

In no case should exposure exceed 35 ppm.

Sensors are turned on and calibrated in an uncontaminated environment — typically outside the building — before testing begins.

Draft and Spillage

Draft measures the venting system’s ability to exhaust. It is checked with a gauge and paired with a visual inspection of the venting.

Critically, draft standards change with outdoor temperature, because warm air rises and a vent naturally pulls harder when it is cold outside:

Outdoor temperature	Minimum draft required
Above 80°F	0.005 in. W.C.
32°F to 80°F	0.01 in. W.C.
Below 32°F	0.02 in. W.C.

This has a consequence most homeowners never hear: a furnace tested only in cold weather has not really been proven. If draft is weak when it is cold, it will be weaker when it is warm. In Los Angeles, where much of the year sits in the top band, this matters more than it does almost anywhere else.

Separately, all draft diverters should be tested with smoke to confirm combustion products are leaving through the vent. Spillage — combustion products escaping out of the draft diverter into your home — is an immediate hazard, not a deferred one.

Testing Under Worst-Case Conditions

A furnace that passes on a calm day with the windows open has not been meaningfully tested.

Proper protocol evaluates the house under the conditions most likely to create a problem: air handling equipment that moves air outside is switched on — dryers, bath fans, kitchen fans — and doors and windows are closed. Then the appliance is observed for at least five minutes after ignition, which gives it time to establish a draft and reveals how it behaves on a consistent basis.

The Cause Is Often Not the Furnace

This is the part that surprises people.

Across a thousand combustion safety inspections, the most common finding was not a failed furnace component. It was open return air ducts, present in 22.4% of homes. An open return in or near the furnace room creates a significant depressurization source right where combustion is happening, competing directly for the air the furnace needs.

The full picture from that data set:

Finding	Share of homes
Open return air ducts	22.4%
Cycling on the high limit	13.5%
Disconnected ducts	9.1%
Gas leaks	8.3%
Carbon monoxide	5.0%
Spillage	2.4%

Other contributors follow the same pattern. Remodeling frequently changes or eliminates the combustion air source — and often adds pollution sources to the same room, like installing a dryer next to the furnace. Long horizontal vent runs that just met standards when installed are prone to rusting out over time. Venting into an unlined chimney can leak as the chimney erodes. In crawlspaces, dirt, rust, and lint block combustion air openings.

Very few installers carry the equipment needed to verify a combustion appliance is safely installed. Many units create CO because of improper setup and testing, with gas pressure, orifice size, and improper venting being the most common errors.

Ordinary Work Can Create the Problem

Anyone doing significant work inside a building should understand the unintended consequences of it.

Changing a furnace filter. Repairing ducts. Adding insulation. Sealing the building shell. Fixing a bath fan. Hanging a door. Changing how a customer uses the thermostat.

Every one of these can alter the operating conditions of a combustion appliance. This is why testing has to happen before work begins to identify pre-existing problems, and again on completion to confirm nothing that was working got disturbed.

What to Do Now

If you are seeing carbon deposits, discolored or unsteady flames, scorching in front of the burners, or hearing a boom on startup, treat it as a live issue and get the system tested.

If you have no symptoms at all, an annual combustion safety test is still the only way to know. The whole point of carbon monoxide is that you cannot detect it yourself.

Our technicians test flue CO at every burner, measure draft against the correct standard for the day’s outdoor temperature, and smoke-test for spillage — then hand you the readings. Book a combustion safety diagnostic, or read about what a complete heating tune-up should include.

If your furnace is near end of life and combustion problems keep recurring, a heat pump removes the question entirely. No flue, no gas valve, no carbon monoxide to test for.