Gas basics, Part 6: Complete Combustion

Written on: June 19, 2013 by Timmie McElwain

We finished up April’s article addressing Combustion. So far, we have covered the “Meaning of Combustion,” “The Requirements for Combustion,” and the “Chemistry of Combustion”.
Still, after many years of working in the heating industry, and in particular on the gas side, I find that the study of combustion never ends. I am always learning something new. Years ago, when I started, we used “wet kits”—the old bottles which some are still using today. Those of us who did gas testing used both oxygen (blue stuff) and carbon dioxide (red stuff). Those, along with a draft gauge, thermometer and some kind of colorimetric ampoule for testing for carbon monoxide (CO) were basic tools.
I would sometimes find myself in the basement alongside the oil man doing similar testing, he on the oil boiler, myself on the gas. I used to notice that he only used the carbon dioxide bottle for testing. I was curious, so I asked, “Why no oxygen bottle?”
The oil man, being much older and wiser than the young pup of a gas man, stated that as long as a smoke test was done, the oxygen was not needed. I trusted he was older and more knowing than I, so I moved on. Those old timers also used to tell me that oil did not make carbon monoxide and that a “zero smoke” indicated no carbon monoxide, hence they very often did not test for CO.
I wonder, maybe, if that was why they had to clean the boilers so often. I have since learned that all fossil fuels can make CO. It was also a fact that oxygen readings should also have been taken. The old CO2 reading lined up on the fire finder, giving 80% efficiency, was many times off somewhat. So you do not misunderstand me, power burner oil systems have always been more efficient than atmospheric gas systems. The reason being the need for less “excess air” with the blower motor on the oil system compared to the gas system needing much more excess air. Today, as we move ever higher with our efficiencies on both oil and gas, we need even more to ensure “complete combustion.” Let’s talk about that as it relates to gas.

Complete Combustion

In April’s article, we showed an ideal model of complete burning of methane with oxygen. In gas appliances, air, rather than pure oxygen, is used to burn the gas. Air contains about 20 percent oxygen and 80 percent nitrogen.

FIGURE 6–Click on image for larger view.

Figure 6 (p.8) shows the complete combustion of methane in air in terms of molecules and atoms. The same number of molecules (and atoms) of methane, oxygen, carbon dioxide and water vapor appear as was the case in Figure 5 (April issue). Figure 6 differs from Figure 5 in that the nitrogen in the air is also shown.
Four nitrogen molecules are present in air for each oxygen molecule. The nitrogen does not take part in the burning process, so it appears in the products unchanged. The burning process shown in Figure 6 can be expressed in terms of cubic feet of gases (Figure 7).

FIGURE 7–Combustion products from burning one cubic foot of natural gas. Click on image for larger view.

For each cubic foot of methane, ten cubic feet of air are needed for complete combustion. Eleven cubic feet of products are thus formed. These products consist of one cubic foot of carbon dioxide, two cubic feet of water vapor and eight cubic feet of nitrogen. All of these products must be vented or discharged from an appliance.
In actual practice, more air is supplied to the combustion process than the ideal amount shown in Figure 7. This additional air is called excess air. The gases vented from an appliance for each cubic foot of methane burned, then, will be 11 cubic feet plus whatever excess air passes through the appliance. Carbon dioxide and water vapor formed in burning plus the nitrogen in the reactants that entered with the combustion air together are called combustion products. The combined combustion products and excess air are called flue products.

Water and Condensation

Water is produced as a vapor in the burning of gas. If the flue products remain hot enough, water is discharged as vapor to the outside through a vent system. If the flue products should become cool, this water vapor starts to condense out as a liquid. The temperature at which liquid water forms from vapor is known as the dew point (See Figure 8).

FIGURE 8–CLICK ON IMAGE FOR LARGER VIEW. The dew point temperature at which condensation of water vapor would occur is slightly different for different vent gases. The dew point for LP products of combustion tends to be lower than when natural gas is burned. Also, the dew point temperature changes with the amount of excess combustion air used and the amount of dilution air introduced at the draft hood. In the chart at right are some representative dew point temeratures developed by an operating, natural gas, conventional (draft hood) furnace. Note as the excess/dilution air increases, the dew point temperature is lowered. This means the products ofcombustion would have to be cooled much lower before condensation would occur. Typically, for conventional furnaces, temperatures between 126° and 128°F are likely to cause condensation in the vent gases.

Condensing water vapor from combustion products has an advantage. More heat can be recovered, since heat is given up when the change takes place. Efficiency or useful heat transfer is improved. However, this gain is offset to some degree by some problems which may occur because of the presence of the water (condensate). For example, a storage water heater may drip water on the floor beneath it in the winter. The problem usually occurs when very cold water enters the tank, chilling combustion products below the dewpoint.
A more serious problem is corrosion, which may occur inside of heat exchangers, flueways and vent pipes where the water condenses. The temperature of the flue products in the center of these passageways may be well above the dewpoint, but close to the cold walls, the temperature may drop off sharply and condensation may take place on the walls. A range oven door may become momentarily fogged when the oven is first turned on. The gases contact the cool glass and water condenses. As the oven heats, the glass heats up and the fogging disappears.
With many of the new condensing type heating system,s the dew point is actually used to some advantage. Many new high efficiency pieces of equipment may actually condense in the flue. There are others which condense in the equipment. These actually use the condensing as a means to extract heat from the flue gases. In turn, this causes the temperature to drop and therefore condensing takes place. The latent heat which is removed from these flue gases is absorbed into the medium being heated. In some instances, the heat is absorbed into return water, as in the case of boilers. With warm air furnaces, a secondary heat exchanger is used. It is very much like a condensing coil used on air conditioning equipment. The secondary heat exchanger is located just above the blower compartment. Return air, blowing across the secondary heat exchanger, extracts heat to be used to warm the dwelling.
Figure 9 illustrates the total amount of air required for combustion of one cubic foot of gas, about 1,000 Btus, depending on the heat value of the gas. It is important to realize that typically, this air may come from within the room in which the equipment is located. If there is insufficient air, then a provision for outside air will have to be made. The procedure for determining this air will be covered in the section on Air for Combustion in a later article.

Draft Hood

A gas-fired furnace should be equipped with a draft hood attached to the flue outlet of the appliance. The draft hood used on the appliance should be certified by the American Gas Association. Only gas conversion furnaces equipped with power-type burners and conversion burner installations in large steel boilers with inputs in excess of 400,000 Btuh are not required to have draft hoods.


A draft hood is a device used to ensure the maintenance of constant low draft conditions in the combustion chamber. By this action, it contributes to the stability of the air supply for the combustion process. A draft hood will also prevent excessive chimney draft and downdrafts that tend to extinguish the gas burner flame. Because of this last function, a draft hood is often referred to as a draft diverter.
Draft hoods may be either internally or externally mounted, depending upon the design of the furnace. Never use an external draft hood with a furnace already equipped with an internal draft hood. Either vertical or horizontal discharge from the draft hood is possible.

Combustion Process

FIGURE 9–The total amount of air required for one cubic foot of gas. CLICK ON IMAGE FOR LARGER VIEW

The combustion process involves the three things necessary for combustion Heat, Fuel and Air. With all things adjusted correctly a certain volume of products are produced, as illustrated in Figure 10. These products of combustion are part of what causes the transfer of heat in the equipment. A certain volume is necessary for proper venting of the remaining products of combustion. The basic “theoretical combustion” is the minimum necessary for proper combustion. The excess air is necessary to sustain proper combustion. An important factor is that excessive “excess air” reduces efficiency. There is, therefore, an ideal amount of excess air needed for proper combustion. If there are excessive amounts of air, the carbon dioxide is reduced and efficiency is also reduced.


There is an ideal amount of CO2 and O2 that is necessary for proper combustion. A ratio of 8.5% to 9.5% CO2 is usually good. The oxygen should typically be above 4% for safe operation, keeping carbon monoxide at the lowest percent possible (under 100 PPM). For heating and water heating equipment, the ANSI Standard for allowable level of air free CO in the flue is 400 ppm (parts per million). For gas ovens, it is 800 ppm.
These are reasons why it is important to insure proper air for combustion and also proper venting. We should strive for the lowest CO reading possible in a flue sample. It is also important to understand that as draft is created in the vent or chimney, this aids in bringing the proper amount of air into the combustion process. The term “natural draft” refers to the draft created when the burner is running. The draft is created by the temperature difference in the flue and the height of the flue. The inclusion of “dilution air” introduced at the draft hood or barometric contributes to controlling the draft in the chimney from being excessive. Normal draft is measured in inches water column [#” WC]; it is typically a negative reading, such as -.02″, -.03″, -.04″ WC. Anything exceeding -.04″ is excessive draft and can cause equipment problems.