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This year’s AHR Expo recognized, for its prestigious Innovation Award, Infinicon’s FLUE Mate combustion analyzer as the winner of the Tools & Instruments category. Combustion analyzers are becoming more and more prevalent in service vans across the world due to rising concerns about the safety and efficiency of gas furnaces. As the measurement tools continue to improve, it is important to understand the basics and how a combustion analyzer works.

Theoretical Combustion

Theoretical, or “stoichiometric,” combustion between an ignited hydrocarbon (such as methane in natural gas, and petroleum in home heating oil) in the presence of oxygen, produces only three things: heat energy, carbon dioxide, and water.

FROM DATA TO ACTION: Technicians should familiarize themselves with what constitutes a typical or bad reading for specific applications. (Courtesy of Lianna Schwalenberg)

Because air contains mostly nitrogen, a nonflammable gas, and only about 20.9% oxygen, nature makes it extremely difficult to mix the exact amount of gas pressure and oxygen. Because of this, the combustion process in the field is always said to be “incomplete.” As a result, there is always a small release of partially oxidized compounds, such as carbon monoxide, along with the carbon dioxide and water in this process.

Too much gas, too little gas, too much air, and too little air will all cause carbon monoxide. Carbon monoxide not only makes people sick and in excess can lead to death, but it is also a sign there is something wrong with the furnace – usually one or a combination of the following: bad fuel pressure, bad combustion air, and poor condition of the heat exchanger. A very basic, beginner’s guide to combustion analysis will generally focus on three big measurements – oxygen, manifold pressure, and carbon monoxide. A more intermediate approach will also look at excess air, carbon dioxide, stack temperature, and carbon monoxide air-free. Knowing what these measurements mean is key to performing a proper combustion analysis.

One visual cue of poor combustion is the presence of soot, a black powdery substance, and byproduct of an incomplete combustion of a hydrocarbon. Not only is soot a nuisance to clean, but it can also cause problems if it clogs burners and/or prevents the flow of combustion air. Sometimes this is referred to as a “fuel-rich” combustion because there is too much fuel for the amount of oxygen present. While replacement of the unit or a thorough cleaning can be an immediate fix, if the original source of the combustion air problem is not addressed, the same problem will happen again. One thing to check for, especially in situations where a furnace or hot water heater is in a closet, is if the closet door has louvers. There must be some way that air enters the space.

Combustion air is generally divided into three categories: primary, secondary, and excess. Primary air is actually controlled by the gas pressure due to the venturi effect pulling air in as gas forces its way through very small orifices, immediately mixing with this primary air and controlling the amount of gas that can be burned. Secondary air is what surrounds the burners and it allows the given amount of gas to fully burn, completing the combustion process. Excess air, therefore, is the leftover air after complete combustion. A certain amount of excess air acts as a reassurance that the furnace is going through complete combustion with very little harmful byproducts. Too much excess air is also referred to as “air-rich” combustion. While too little air causes soot, too much air causes wasted heat as the excess oxygen will absorb the heat and send it out the exhaust stack.

Often when sending in a plugged heat exchanger for a warranty claim, the manufacturer will ask for the unit’s excess air measurement as this is a sure sign of the secondary being plugged.

Watching the O₂ and excess air readings as the blower turns on can be a quick and easy way to tell if the heat exchanger is cracked.

Because good combustion is the result of the right mixture of air and gas, usually the easiest thing to check and adjust is the gas pressure, measured in inches of water column (WC), with a manometer (some combustion analyzers have the ability to read gas pressure). Though gas pressure should always be dialed in when the furnace is commissioned, sometimes it does not hurt to check. The recommended process of adjusting gas manifold pressure is to clock the gas meter, especially with high-efficiency furnaces, to check the average heat content of the gas supplied. If the furnace is not tuned in with the actual BTU input, the furnace will not run as efficiently as it could be.

Sometimes, the gas valve in the furnace is not commissioned correctly, and a common visual cue might be the furnace tripping high limits.

Because combustion converts carbon into carbon dioxide based directly on the amount of combustion air, most digital combustion analyzers do not literally measure CO₂ – rather, it is calculated based on measured values. This is usually set to display if there is just enough O₂ (in other words, 0% O₂ in the flue). CO₂ is at its theoretical maximum of about 11.8%.

Stack temperature, the temperature of the flue gas, is a key indicator of performance as it measures how much heat is lost in the flue. High-efficiency furnaces have cooler stack temperatures because they put that extra heat energy into the home.

Carbon monoxide is an odorless, colorless gas where exposure to as little as 10 ppm can cause headaches, fatigue, and death in less than two hours at 800 ppm. Many customers have carbon monoxide detectors in their homes. However, because the typical CO sensor will not alarm at levels below 30 ppm, low-level CO detectors coupled with regular maintenance is the best protection against carbon monoxide poisoning.

Combustion analyzers typically give two readings for carbon monoxide: CO and CO air-free. Because excess air dilutes the CO in the flue stack, the concentration of CO may not look too bad. Therefore, the analyzer uses a formula to read out a CO measurement “air-free.” Therefore, technicians will find that the CO(0) is always higher than the CO measurement and significantly higher if the EA is high. There are standards for both, so often when you are communicating results with manufacturers, they might ask for the CO(0) measurement as well.