If you’ve been following my writing for the last couple of years, you know that I like to blather on and on about combustion analysis. Now, I am by no means an expert on the subject, as I live in North Texas (which is not known for its intense heating season), but I love using my analyzer. It gives me actual data to look at. Each year, I dig a little deeper into what the numbers mean, as I am challenged to learn by problems I encounter in the field. 

This year, I’ve been sent back to the drawing board by an odd furnace problem. I’ve been exploring the actual chemical process of combustion itself. In years gone by, I deemed this part “unimportant” as it didn’t seem to apply much to me. Physics is cool because that’s what HVAC is. But chemistry? Come on, that’s for the super nerds.

Luckily, we have some great HVAC school content from Rachel Kaiser that dives into the chemistry of combustion. This content, along with some real-world examples, will hopefully stir up some interest for you and help you see just how fascinating and interconnected all the different parts of our job really are. Much of the basic information for this article comes from our longer gas furnaces article, which you can find here.

The Ideal Combustion Triangle

Three things are needed for flame propagation: oxygen, fuel, and heat. Together, these things make up the combustion triangle. If one of the three things is removed completely, the process ends. If all three are present, but the ratios are off, the combustion process can become incomplete, which presents health and safety hazards.

Oxygen for the combustion process in a furnace comes from the air, which can come from either the area immediately around the furnace (open combustion) or dedicated piping from outside (sealed combustion).

The heat needed to start the combustion process can come from a few different sources, including a spark, hot surface ignitor, pilot light, or a combination of those things. Each fuel has a specific ignition temperature. With gas fuels, the ignition source only needs to be applied at the start, or initialization, of the combustion process. Once the process is started, heat from the flame constantly ignites new fuel and air that enters the combustion area.

Fuel for combustion can come from a variety of materials—oil, liquefied petroleum (propane or LP), natural gas, and even wood. In the case of gas furnaces, the fuel will usually be natural gas, which is supplied by a utility company via underground gas lines, or LP delivered to an onsite storage tank. LP and natural gas may consist of propane, butane, or a mix of both, along with other additives. Fuel providers across the United States are not required to provide a specific ratio of the different types of fuels used. However, they are required to ensure a specific heat content of the gas provided.

Under ideal circumstances, combustion will take fuel and oxygen and convert those into carbon dioxide (CO2) and water vapor; this is called complete combustion, and it explains why condensation happens in furnaces.

Out In the Wild

However, complete combustion isn’t always possible. As Rachel mentions in the podcast linked above, different fuel sources need slightly different numbers of oxygen molecules for complete combustion. It is impossible to get the exact ratio of fuel to oxygen, so to compensate for this, gas furnaces provide an abundance of oxygen. This ensures a safer burn, even at the cost of a little loss in efficiency. 

Let’s look at a couple of before and after pictures of my analyzer so you can see how making field adjustments to the combustion process can have real effects. 

High Gas Pressure

When I arrived at this furnace, the gas pressure was 4.2 inWc. Because of the high gas pressure, the fuel-to-air mix was too rich, leading to incomplete combustion and high carbon monoxide (CO) readings.

After adjusting the gas pressure to 3.4 inWc, you can see that the carbon monoxide readings dropped significantly, and the excess air increased. This makes sense; we are lowering the amount of gas being fed to the combustion process, which, in turn, increases the oxygen.

Low Gas Pressure, High Airflow

In this example, the gas pressure was set a little low at 3.2 inWc. I adjusted it to 3.4 inWc (as stated on the valve), and we can see the results in the second set of pictures. At a lower gas pressure, we have less CO, a higher O2 reading, and a cooler flue pipe. This combination makes sense because when we have less fuel, we have less heat. Is this a problem? Well, it can be. We don’t want to turn the gas down too much, or we risk running too cool of a flue pipe, which can cause condensation. 

In this case, the furnace was also running high airflow. The nameplate says the furnace's heat rise is 40-70, and this furnace had a 45-degree heat rise. The speed tap had been adjusted at some point from medium to medium-high.

The results were interesting. The CO rose a little, which makes sense, but in this case, I considered it worth it to run a warmer flue pipe and get the readings more in the range of “normal.” 

Reflections

As you may have noticed from the examples above, there is not much you can actually adjust on a furnace in the field. Airflow and gas pressure are it. But it is amazing to see what making some small changes can do to the combustion process out in the field. All of it works together in a very dynamic and interesting way. If you have a low heat rise, do you adjust the gas pressure or lower the airflow? Conversely, if you have a high heat rise, should you increase airflow or lower gas pressure? How do you know you won’t cause other problems like high CO or condensate in the flue? It’s fascinating to dive into the world of combustion. How cool is it that we get to take care of people and be chemists at the same time?!

—Matt Bruner