You might remember the days when a boiler tune-up mostly meant adjusting the flame. The flame was supposed to be solid blue with the tips slightly rounded. If the flame was soft, wavy and contained orange, there was too much gas and not enough oxygen. If the flame was hard, spiky and noisy, there was too much oxygen in relation to the amount of gas. That was pretty much the extent of combustion analysis.

Years have passed, electronics have arrived, and now we have a device called a combustion analyzer to check out the invisible gases that accompany a flame. Combustion analysis is important for both fuel efficiency and safety.

The information that a combustion analyzer provides can help keep both you and your customers out of trouble. Knowing oxygen (O) and carbon dioxide (CO2) percentages and the flue temperature allows you to adjust the boiler for greatest fuel efficiency. Measurement of carbon monoxide (CO) lets you assure combustion safety for the building occupants.

Reading a combustion analyzer

The combustion analyzer’s screen displays the measurements. The best way to interpret the numbers is to compare them with specifications given by the equipment manufacturer. Here are some general guidelines:

Oxygen naturally makes up about 21% of our atmosphere. Generally, we need 10 cubic feet of air to burn a cubic foot of natural gas. In a flue, as reported by the combustion analyzer, you would want to find approximately 6% to 8% oxygen on an atmospheric natural draft boiler. This is oxygen left over from the combustion process.

Carbon dioxide is typically a calculated number based on the detected amount of oxygen. Carbon dioxide is a normal byproduct of the combustion process. A 7% to 9% range on the analyzer is typically desirable on the natural draft product.

These guidelines are for natural and propane gas. Fuel oil guidelines are different. These numbers also can change based on the type of burner used on the equipment. Mechanically mixing air and gas allows for lower oxygen levels and typically better efficiency.

Carbon monoxide: Code often allows 400 parts per million (ppm). But to be safe you would not want to see any more than 150 ppm. A reading above 150 ppm would be a red flag to double-check the combustion. High levels of carbon monoxide suggest that there is not enough oxygen for the amount of fuel being burned. Or, conversely, there is too much fuel for the amount of oxygen. High carbon monoxide levels also indicate that there is not complete combustion and that fuel is being wasted.

Depending upon the appliance, you can generally adjust air, fuel or both. If you find high levels of carbon monoxide, either increase the oxygen or decrease the fuel until you find safe carbon monoxide measurements in the flue.

Using a combustion analyzer

A combustion analyzer fits in your hand like a multi-meter. Start outdoors. When you turn on the combustion analyzer, its pump draws in air for 60 seconds to clear the probe hose and the sensors. Turning the unit on and clearing it outdoors enables you to then walk into the mechanical room with the analyzer running and know whether the room is safe by checking for carbon monoxide. The combustion analyzer also can be used to test carbon monoxide levels in an occupied or living space.

To sample the flue products, first drill a hole in the flue about a foot above the boiler and at least a foot away from any elbow, but below any source of dilution air such as a draft hood bonnet. Be sure to plug the hole before you leave.

Set the dial to “flue test.” Attach the probe and hose to the analyzer. Let the boiler run for a couple of minutes. Insert the probe into the flue, getting the tip as close to the center of the flue as possible. If the pump has turned off, turn it back on, and within 30 seconds you will see readings on the screen. Make boiler adjustments to match manufacturer specifications.

Once the proper adjustments have been made and the combustion is within specifications, you can press the Save button. This records the real-time readings for printing or copying later.

The flue test also indicates stack temperature, which can be another indicator of proper combustion. The probe tip has a built-in temperature sensor. If, for example, the manufacturer says to expect a stack temperature in the 200º C range, and the analyzer is showing you 120º C, you know there is a combustion problem.

A simple combustion analyzer

Once upon a time combustion analyzers were expensive and difficult to use. Now they are easy to use and can be priced reasonably. But with a range from $500 to over $5000, it is easy to get confused. For HVAC, aim for the basics: carbon monoxide, oxygen, carbon dioxide, and stack temperature.

A combustion analyzer with additional features might give you two pressures (gas and draft), two temperatures and allow you to perform a furnace heat exchanger test. The analyzer also could give you the difference, known as delta T, for pressure and temperature.

When selecting a combustion analyzer, keep calibration in mind. Older designs must be sent in periodically for calibration. Newer designs use replacement cartridges rather than recalibration. The newest technology requires neither. Expect to periodically replace sensors.

Most of the heat we create comes from burning fuel. We put fuel and oxygen together in the process called combustion. The result is heat, which we gladly take credit for, and other products of combustion, which we would prefer to ignore. But flue gases must be taken seriously because of concerns about efficiency and safety, and a combustion analyzer is the tool to do it.

Carol Fey is a technical trainer who has been in the HVAC industry for over 25 years. You can find her books and DVD at To see her adventures while a heating mechanic in Antarctica, go to