Carbon Monoxide: The Silent Killer Every Tech Should Know How to Handle

The Truth About CO 

You don’t need massive amounts of CO to create a dangerous situation. While air normally contains about 200,000 parts per million (ppm) of oxygen, just 70 ppm of CO can start causing problems for healthy adults. At 400 ppm, you’re looking at potential unconsciousness and death within a couple of hours of exposure.

Carbon monoxide (CO) is a colorless, odorless, tasteless, and highly toxic gas that’s produced during incomplete combustion. As professional HVAC technicians, we need to understand that even at low concentrations, CO binds to hemoglobin in the blood with an affinity 200-250 times greater than oxygen, preventing oxygen from being transported throughout the body.

Here’s a quick breakdown of CO exposure effects:

• 9 ppm: Maximum allowable concentration for short-term exposure in living environments (ASHRAE standard)
• 35 ppm: Maximum allowable concentration for continuous exposure in any 8-hour period (US federal law)
• 200 ppm: Maximum allowable concentration at any time according to OSHA (can cause headaches, fatigue, and nausea after 2-3 hours)
• 800 ppm: Nausea and convulsions within 45 minutes and death within 2-3 hours
• 3,200 ppm: Headaches and nausea within 5-10 minutes and death within 30 minutes

Why Should You Care? 

Every time you work on a furnace, boiler, or water heater, you’re dealing with equipment that could potentially produce CO. But – unlike smoke – you can’t see it, smell it, or taste it. That’s why we call it the “silent killer.” Without proper testing, you might walk away from a job thinking everything’s fine, while leaving behind a potentially lethal situation.

As professional HVAC technicians, we’re on the front lines of CO safety. The stark reality is that improper combustion analysis or overlooked maintenance can lead to serious health risks or even fatalities. When a crack develops in a heat exchanger, when venting becomes compromised, or when fuel/air mixtures are incorrect, deadly CO can leak into living spaces.

Tools of the Trade 

This is where having the right equipment becomes crucial. Seitron’s lineup, particularly the Novo analyzer, is designed specifically for techs like us who need accurate, reliable readings. 

Here’s what you should be looking for in your CO testing equipment:

1. Ambient CO Detection

• Built-in ambient monitor for immediate safety checks
• Alerts you to dangerous conditions before you even start working
• Should be carried and used on every service call, not just heating system repairs

2. Combustion Analysis Capabilities

• Measures O₂, CO₂, and CO simultaneously
• Helps you dial in that perfect combustion setup
• Calculates combustion efficiency to optimize system performance

3. Data Recording

• Keeps track of your readings for documentation
• Provides evidence of your proper testing procedures
• Covers you legally if questions come up later

There are three major types of professional-grade instruments available for testing carbon monoxide:

1. Ambient Testers – Small handheld units that fit in a shirt pocket. These combine a CO sensor with a digital display and are relatively affordable (approximately $200 USD). While convenient, they cannot test raw flue products or warm air streams.
2. Pump-driven Single Gas CO Analyzers – These handheld units feature a flue probe and an onboard pump to draw in sample air and flow it across the sensor. They’re ideal for ambient testing, sampling warm supply air streams, and testing flue products. Cost range: $450-500 USD.
3. Combustion Analyzers – These comprehensive units measure oxygen content, carbon monoxide, and flue gas temperature. They also calculate CO₂ concentration, excess air, and combustion efficiency. Prices start around $600 and can go up to $2,000 depending on features.

Real-World Testing Protocol

Tools are great, but they aren’t worth much if you don’t know how or when to use them. Let’s break down where you need to test on different equipment:

Before starting any testing procedure, always zero your CO instrument in fresh air. This establishes an accurate baseline for your measurements. Then, follow these testing points:

1. Ambient Air Testing

Walk into the house with your CO meter on and sampling. Any measurement above zero is not normal, as CO is only present as a byproduct of combustion. In homes where people smoke or burn scented candles, readings between 2-6 ppm are common. Anything above 6 ppm should be investigated.

2. Water Heaters:

• Check combustion readings (O₂, CO₂, CO)
• Verify stack temperature
• Measure draft

3. Furnaces (80%):

• Test gas pressure
• Check limit and pressure switches
• Verify proper combustion parameters
• Monitor static duct pressure
• Check mechanical room CO levels
• Test appliance vestibule and burner area (readings should match ambient air)
• Test supply air stream in the plenum (any increase indicates potential heat exchanger issues)

4. High-Efficiency Units (90%+):

• All the above, plus
• Verify condensate drainage
• Check inducer operation
• Inspect venting system for proper installation and operation

The Numbers That Matter 

Here’s your quick reference guide for flue gas measurements:

• Under 50 ppm: Normal for most modern gas appliances
• Up to 175 ppm: Acceptable for some high-efficiency boilers
• 200 ppm: Your absolute maximum before requiring adjustment
• 400 ppm: Red tag territory – shut it down

For different heating systems, here are the typical acceptable combustion results (always follow manufacturer’s specifications):

Gas Fired Power Burners:

• Oxygen (O₂): 3-6%
• Carbon Monoxide (CO): < 100 ppm
• Carbon Dioxide (CO₂): 8.0-11.0%
• Stack Temperature: 275-500°F
• Stack Draft: -0.02 to -0.04 inWC (or manufacturer’s specs)

High-Efficiency Gas Fired 90+ Power Burners:

• Oxygen (O₂): 5-7%
• Carbon Monoxide (CO): < 100 ppm
• Carbon Dioxide (CO₂): 7.0-9.0%
• Stack Temperature: Less than 125°F
• Stack Draft: +0.02 to +0.08 inWC (or manufacturer’s specs)

Pro Tips from the Field

1. Always calibrate your analyzer annually – using uncalibrated equipment is asking for trouble
2. Test on every call, not just when you think there might be a problem
3. Look for trends over time – rising CO levels can indicate developing problems
4. Know that flue gas readings and ambient readings are completely different measurements
5. Pay attention to the relationship between O₂, CO₂, and CO readings during combustion analysis
6. Remember that excess air impacts combustion efficiency and emissions (too little air = increased CO production)
7. Document everything – it’s not just good practice, it’s legal protection

Understanding Combustion Analysis

For novice technicians, it’s important to understand that combustion analysis is more than just checking CO levels. It’s a comprehensive evaluation of how efficiently and safely a combustion system operates. During combustion analysis, we measure:

• O₂ (Oxygen) – Tells us about excess air conditions
• CO₂ (Carbon Dioxide) – Indicates combustion completeness
• CO (Carbon Monoxide) – Safety indicator and efficiency measure
• Stack Temperature – Shows heat transfer efficiency
• Draft Pressure – Ensures proper venting

Remember the basic concept: combustion requires the right balance of fuel, oxygen, and heat. When these elements are in proper proportion, combustion is efficient and clean. When this balance is disrupted, we get incomplete combustion—and that leads to CO production.

As a technician working with combustion appliances, you’re responsible for ensuring this balance is optimized for both efficiency and safety. Think of combustion analysis as your diagnostic tool for the heart of the heating system.

Safety First

Remember, if you ever find CO levels above 400 ppm in the flue gas, or any CO in the living space:

1. Shut down the equipment immediately
2. Ventilate the area
3. Notify the customer of the hazard
4. Document your findings
5. Don’t restart until the problem is fixed

Your personal safety matters too! Always ensure your own safety when performing any HVAC work. Carry a personal CO monitor whenever working around combustion equipment. ASHRAE recommends a maximum exposure limit of 9 ppm in living environments, and this applies to you as well while you’re working.

Taking it to the Next Level

Want to really step up your game? Seitron offers complete system solutions that can include:

• Portable analyzers for service calls
• Fixed monitors for ongoing protection
• Data logging capabilities for building management systems

Professional combustion analysis goes beyond basic safety checks—it can help you optimize system efficiency, reduce fuel consumption, and extend equipment life. By understanding and correctly interpreting combustion readings, you provide greater value to your customers while ensuring their safety.

As you gain experience with combustion analysis, you’ll develop an intuitive understanding of the relationships between different readings and what they tell you about a system’s operation. This expertise will set you apart as a technician who truly understands the science behind heating systems, especially as heating season approaches.

The Bottom Line 

As HVAC techs, we’re on the front lines of keeping people safe from CO poisoning. Every service call is an opportunity to prevent a tragedy. Take the time to do proper testing, invest in quality equipment, and never cut corners when it comes to combustion safety.

Remember: Your customers trust you with their lives, even if they don't realize it. Make sure you're worthy of that trust by mastering CO testing and safety protocols.

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