Low-Level Carbon Monoxide Exposure in North Texas Homes

Carbon monoxide is called the "silent killer" because it's odorless and colorless. But there's another reason CO is dangerous: standard detectors don't alarm at levels that can cause chronic health problems. Your detector is designed to prevent acute poisoning—it won't alert at 10-30 ppm, even though prolonged exposure at these levels causes headaches, fatigue, nausea, and cognitive impairment. In North Texas homes with gas furnaces, water heaters, and attached garages, low-level CO exposure is more common than most homeowners realize. This guide covers detection, sources, and solutions.

Why Your CO Detector Doesn't Catch Low-Level Exposure

Standard residential CO detectors are calibrated to UL 2034 standards, which specify alarm thresholds designed to prevent acute poisoning—not chronic exposure:

CO Level (ppm)	UL 2034 Alarm Requirement	Health Effect
30 ppm	No alarm required for 30 days	Headaches, fatigue with chronic exposure
70 ppm	Must alarm within 1-4 hours	Headache, dizziness after 2-3 hours
150 ppm	Must alarm within 10-50 minutes	Serious symptoms within 1-2 hours
400 ppm	Must alarm within 4-15 minutes	Life-threatening within 2-3 hours

Pro Tip: Your CO detector is designed to save your life in acute poisoning scenarios. It was never designed to alert you to low-level chronic exposure that affects your health over weeks and months.

Symptoms of Low-Level CO Exposure

Low-level CO exposure (10-30 ppm) mimics many other conditions, which is why it's often misdiagnosed:

Chronic headaches (especially in specific rooms or times)
Persistent fatigue that doesn't improve with rest
Flu-like symptoms without fever
Nausea and dizziness
Difficulty concentrating, "brain fog"
Symptoms that improve when away from home
Multiple family members affected simultaneously

The Key Indicator

The most telling sign of CO exposure versus illness: do symptoms improve when you leave the house? If you feel better at work, outside, or on vacation—and symptoms return when you come home—consider CO as a potential cause. This pattern is especially suspicious if multiple household members experience similar symptoms.

Common CO Sources in North Texas Homes

Any combustion appliance can produce carbon monoxide. In DFW homes, the most common sources are:

Gas Furnaces

Furnaces are the primary CO source in North Texas homes. CO production increases when:

Heat exchanger is cracked (CO enters the air stream directly)
Burners are dirty or misadjusted (incomplete combustion)
Flue is blocked or backdrafting (exhaust enters home instead of venting outside)
Furnace is oversized and short-cycling (incomplete burn cycles)
Combustion air supply is inadequate

Gas Water Heaters

Atmospheric water heaters (the standard tank type with an open burner) can backdraft, especially when:

Exhaust fans or HVAC systems depressurize the home
The flue is improperly sized or installed
The water heater is in a sealed mechanical closet without combustion air
The burner or flame arrestor is dirty

Attached Garages

A major but overlooked CO source in DFW homes:

Cars idling in the garage (even briefly) produce significant CO
CO migrates through shared walls, gaps, and door seals into living space
Gas-powered equipment (lawn mowers, blowers) stored after use
Many homes have direct door connections between garage and house

Other Sources

Less common but still relevant:

Gas ranges and ovens (especially if used for heating)
Gas dryers with blocked or crushed vents
Fireplaces with damper problems or chimney blockages
Generators run near the home (never inside garage)

Testing for Low-Level CO

Identifying low-level CO requires specialized equipment beyond standard residential detectors:

Low-Level CO Monitors

Dedicated low-level monitors detect CO starting at 5-10 ppm and provide continuous readings:

Display real-time CO concentration (not just alarm/no-alarm)
Alert at lower thresholds (typically 10-25 ppm)
Log peak readings so you can see what happened while away
Cost: $100-250 for quality units (NSI 3000, Defender, CO Experts)
These are NOT replacements for standard detectors—use both

Professional Combustion Analysis

HVAC technicians use combustion analyzers to test appliances at the source:

Measures CO in the flue gases (parts per million)
Calculates combustion efficiency
Identifies which appliance is producing CO
Tests draft and venting performance
Part of our <a href="/comfort-audit" class="text-blue-600 hover:text-blue-700 underline">Comfort Audit</a> when gas appliances are present

CO Benchmarks: When to Take Action

Use these benchmarks to evaluate CO readings in your home:

CO Level (ppm)	Rating	EPA Guidance	Recommended Action
0 ppm	Ideal	Target for occupied spaces	No action needed
1-5 ppm	Normal	Acceptable ambient level	Monitor if persistent
5-9 ppm	Elevated	EPA 8-hour limit is 9 ppm	Identify and fix source
10-30 ppm	Concerning	Can cause chronic symptoms	Immediate investigation required
30-70 ppm	Dangerous	Acute symptoms begin	Evacuate, ventilate, call professional
70+ ppm	Emergency	Standard alarm threshold	Evacuate immediately, call 911

Pro Tip: The EPA's 8-hour exposure limit is 9 ppm. If your home consistently shows readings above 5 ppm, something is producing CO that shouldn't be—find it and fix it.

The Furnace Connection: Heat Exchanger and Combustion

Because furnaces are the #1 CO source in North Texas homes, understanding how they can fail is important:

Cracked Heat Exchangers

The heat exchanger separates combustion gases from the air circulated through your home. When it cracks:

CO-laden combustion gases mix directly with supply air
Your furnace becomes a CO distribution system throughout the house
Cracks often develop after 15-20 years or from overheating (restricted airflow)
Detection: Combustion analysis with the blower running shows elevated CO in supply air
Solution: Heat exchanger replacement or furnace replacement (replacement usually more cost-effective)

Incomplete Combustion

Even without a cracked heat exchanger, furnaces can produce excess CO through incomplete combustion:

Dirty or misadjusted burners
Inadequate combustion air supply
Flame impingement on heat exchanger
Natural gas supply pressure issues
See our guide: <a href="/learning-center/incomplete-combustion-north-texas-homes" class="text-blue-600 hover:text-blue-700 underline">Incomplete Combustion in North Texas Homes</a>

Backdrafting

When exhaust gases flow backward into the home instead of up the flue:

Caused by negative pressure in the home (exhaust fans, HVAC systems)
More common with atmospheric (natural-draft) furnaces and water heaters
Tight construction in modern DFW homes increases backdraft risk
Detection: Visual inspection during operation, combustion analysis
Solution: Address pressure imbalances, add combustion air, consider sealed-combustion equipment

Solutions for Low-Level CO Problems

Once you've identified a CO source, solutions depend on the specific cause:

For Furnace-Related CO

Our furnace repair service addresses CO issues:

Combustion analysis to quantify CO production
Burner cleaning and adjustment
Heat exchanger inspection (visual and instrumental)
Draft and venting verification
Recommendation for replacement if heat exchanger is compromised

For Water Heater CO

Water heater CO issues often relate to venting:

Check and clear flue obstructions
Verify proper draft during operation
Consider power-vented or sealed-combustion replacement
Ensure adequate combustion air to mechanical room

For Garage-Related CO

Reducing CO migration from attached garages:

Never idle vehicles in the garage (even with door open)
Seal gaps between garage and living space
Weatherstrip the door between garage and house
Consider a CO detector in the garage with relay to alert inside the home

Seasonal Patterns in North Texas

CO problems in DFW homes follow predictable seasonal patterns:

Late October - Early November: First furnace use after summer reveals problems that developed
December - February: Peak CO incident season as furnaces run continuously
Spring/Fall with windows closed: Tight homes accumulate CO more readily
Summer: Lower risk (furnaces off) but water heaters still a factor

Pro Tip: Schedule furnace inspection in early fall before heating season. Combustion analysis should be part of any furnace tune-up or repair.

Bottom Line

Low-level carbon monoxide exposure is a hidden health hazard in North Texas homes, affecting residents who may attribute their symptoms to stress, allergies, or illness. The key indicators—symptoms that improve away from home, multiple affected family members, and chronic headaches or fatigue—should prompt investigation. Standard CO detectors provide essential life-safety protection but won't alert you to chronic low-level exposure. If you suspect a CO issue, add a low-level CO monitor and schedule professional combustion analysis. Our $89 Comfort Audit includes combustion safety testing for homes with gas appliances. For comprehensive IAQ information, see our Complete Guide to Indoor Air Quality in North Texas Homes.

Frequently Asked Questions

If my CO detector hasn't alarmed, can I still have a CO problem?

Yes. Standard CO detectors are designed to prevent acute poisoning, not chronic exposure. They won't alarm at levels below 70 ppm (and may not alarm for hours even at that level). Chronic exposure to 10-30 ppm causes symptoms but never triggers a standard alarm. Low-level CO monitors ($100-250) detect these problematic but sub-alarm levels.

What level of CO is dangerous?

There's no "safe" level of CO exposure—it's a cumulative poison. The EPA's 8-hour limit is 9 ppm. Levels above 10 ppm warrant investigation and source identification. Levels above 30 ppm indicate a serious problem requiring immediate attention. Standard alarms trigger at 70 ppm because that's where acute danger begins, but chronic effects occur at much lower concentrations.

Can a cracked heat exchanger be repaired?

Technically yes, but economically it rarely makes sense. Heat exchanger replacement on furnaces older than 10-15 years typically costs 50-75% of a new furnace price, and other components are also aging. If a combustion analysis or visual inspection reveals a cracked heat exchanger, replacement is usually the better investment than repair.

How often should I have my furnace tested for CO?

Annual combustion analysis is recommended for furnaces over 10 years old. Newer furnaces in good condition can often go 2-3 years between full combustion tests if annual visual inspections show no concerns. Any time you notice symptoms consistent with CO exposure, test immediately regardless of schedule.

Does opening windows eliminate CO danger?

Opening windows reduces CO concentration by allowing fresh air exchange, but it doesn't fix the source. If your furnace or water heater is producing CO, opening windows provides temporary relief while you address the underlying problem. It's not a long-term solution—you need to identify and repair or replace the malfunctioning appliance.

Can gas stoves cause CO problems?

Yes, though typically at lower levels than furnaces. Gas stoves produce CO during normal operation—using a range hood that vents outside (not recirculating) is important. Never use a gas oven or stove for space heating, as this dramatically increases CO levels. If your stove produces visible soot or yellow (instead of blue) flames, have it serviced.