Building Science 201: Combustion

July 01, 2013
July/August 2013
A version of this article appears in the July/August 2013 issue of Home Energy Magazine.
Click here to read more articles about Indoor Air Quality

Over the last decade, weatherization and home performance analysts have become more involved with appraising and testing combustion appliances. This is a positive trend, because these professionals are likely to understand the interaction between the operation of the combustion appliance and competing forces, such as exhaust fans, better than most heating system specialists (see “Current Trends”). As we all know, the parts of a house act as a system, each part affecting the others. The more we understand, the better we can serve the interests of our customers.

The Basics

The requirements of complete combustion are straightforward. Fuel, oxygen, and heat are required in the proper proportions for combustion to begin and to continue (see Figure 1). Too little of any one of these big three results in no combustion or, worse yet, incomplete combustion. Combustion appliances are designed to provide the right quantity of each; if there is a malfunction, it could be hazardous.

Rick Karg

Here’s what the Energy Information Administration (EIA) claimed. In 1993, heating and cooling energy amounted to almost 58% of home energy use. By 2009, the fraction had fallen to 48%. The EIA further claimed that the residential sector consumed essentially the same amount of energy in 2009 as it did in 1993. See Figure 1.

What is the victory? Energy consumption inside America’s homes barely changed in those 16 years, even though both population and average house size increased.

Figure 1. The combustion triangle.

Figure 2. Category I natural-draft furnace. (Source: Heating and Gas, Natural Resources, Canada. Adapted from Minnesota Energy Code 7672.0900 and Canadian General Standards Board 51.71.)

Figure 3. Category I fan-assisted furnace, induced combustion, natural draft. (Source: Heating and Gas, Natural Resources, Canada. Adapted from Minnesota Energy Code 7672.0900 and Canadian General Standards Board 51.71.)

Figure 4. Category IV forced-draft, condensing furnace. (Source: Heating and Gas, Natural Resources, Canada. Adapted from Minnesota Energy Code 7672.0900 and Canadian General Standards Board 51.71.)

For some fuels, the proportions required for the big three are quite precise. For example, natural gas requires a narrow range of 4–14% gas-to-air mixture to ignite; outside this range, combustion won’t occur. For propane gas, the range is narrower: 2–10% gas-to-air mixture for ignition and continued combustion.

Generally, 1 cubic foot of air is required for each 100 Btu of energy released from combustion, regardless of the fuel. Because the Btu density of fuels varies, the amount of air for combustion varies accordingly. For example, 1 cubic foot of natural gas (1,000 Btu) requires 10 cubic feet of air; 1 cubic foot of propane (2,500 Btu) requires 25 cubic feet of air; 1 cubic foot of butane (3,200 Btu) requires 32 cubic feet of air, and 1 gallon of #2 fuel oil (140,000 Btu) requires 1,400 cubic feet of air. Of course, it is always important that the air required for combustion be supplied in an effective manner.

If a combustion appliance were designed to mix the perfect amount of fuel and air (which contains oxygen), combustion efficiency would be maximized, but it would not be safe. This is because there are many events that might reduce the amount of air getting to the combustion process, such as a change in air pressure resulting from the operation of exhaust fans or a decrease in draft resulting from an increase in outdoor temperature. Such changes might cause the appliance to operate with insufficient air (oxygen), producing CO as a by-product. In order to minimize such hazards, manufacturers design appliances to provide excess air to the combustion process. This increases safety, but lowers efficiency, because the additional air cools the combustion process.

We use combustion analyzers to measure the efficiency and safety of an appliance. For a steady-state efficiency test, just two values are needed: the percentage of oxygen in the flue gas and the net-stack temperature. The lower each value is, the higher the efficiency. For maximum efficiency, the oxygen percentage in the flue gas would be zero, meaning that all the oxygen is used up in the combustion process. This would result in perfect combustion, but it would be hazardous, because there would be no excess air for a margin of safety. Typical oxygen percentages are 4–9% for gas- and oil-fired systems.

The net-stack temperature is the difference between the temperature of the flue gas air and the combustion supply air. Low net-stack temperature indicates that more thermal energy is heating the dwelling or the domestic hot water and less is being lost through the vent system to the outdoors. Typical net-stack temperatures are 300–600°F for noncondensing units and around 120°F for condensing units.

Combustion analyzers also measure CO. Significant amounts of CO might be produced even though the steady-state efficiency of an appliance is within an acceptable range. It is always wise to check the CO level in the flue gas when you are measuring steady-state efficiency.

Recommendations for Field Analysts
  • Learn as much as you can about the appliance codes used in your service territory.
  • Know the code book terminology for combustion appliances. See “Use of Proper Terminology.”
  • Understand the characteristics of each of the four vent categories for gas-fired appliances and each of the three draft types (natural, induced, and forced). If you have difficulty identifying the vent category and draft type for an appliance, refer to the installation manual, if it is available.
  • Specify backdraft-resistant equipment, such as Category III and IV forced-draft, direct-vent units when replacing combustion appliances. These units have higher efficiencies and generally are safer.
  • Replace Category I natural-draft gas furnaces you find in the field. The average service life of a gas furnace is 18 years. These units were taken off the U.S. market in 1990, so the youngest unit you will find in the field is now hazardously aged; it is time for replacement.
  • Perform combustion safety testing pre- and postweatherization.

CO is often a by-product of combustion gone wrong, but there are only two underlying malfunctions that cause the production of CO. The first is probably the best understood—it is too little oxygen for a given amount of fuel, or too much fuel for a given amount of oxygen. Either condition makes the chemistry of combustion impossible to complete, producing CO. One common cause of insufficient oxygen is aluminum foil covering the holes in the floor of a gas oven, restricting the oxygen flowing past the oven burner, and producing CO.

The second malfunction that causes the production of CO results when a flame is cooled to below its burn temperature by touching a metal, ceramic, or other surface. The same cooling effect can occur when air blows across a flame. Of course, if the entire flame is cooled to below its burn temperature, the flame extinguishes. This happens when you blow out the candles on your birthday cake. However, cooling a flame can produce hazardous amounts of CO when air from a furnace fan blows through a defect in the heat exchanger and across a gas burner. This problem can also cause frame rollout and can damage electrical wiring on a furnace.

With all the variables in our dwellings, it is surprising that combustion appliances work as reliably as they do; this is certainly positive testimony for appliance manufacturers’ design and construction. However, as most of us know, there are times when the combustion process falters because the appliance has not been serviced properly. Combustion appliances need regular service, just as our cars do. As a general rule, oil-fired appliances should be serviced annually and gas-fired appliances should be serviced every three years.

Current Trends

There are a number of positive trends related to combustion appliances. Many are moving us in the direction of higher efficiencies and increased safety. Here are some examples.

  • Increased popularity of Category IV, direct-vent, forced-draft, gas-fired furnaces. Vented through the wall, these systems are safer and significantly more efficient than the systems they are replacing.
  • Increased popularity of Category III, gas-fired, forced-draft water heaters that are less vulnerable to backdrafting and spillage. Vented through the wall, these systems are safer and more efficient than the Category I, natural-draft systems they are replacing.
  • The move away from chimneys to through-the-wall vented appliances (Category III and IV, forced-draft appliances).
  • Installation of combustion appliances with higher efficiencies.
  • Increased prevalence of combustion safety testing in retrofits, both pre- and postweatherization.

Identifying Combustion Appliances

Understanding and properly identifying combustion appliances can be challenging, but it is very important (see “Recommendations for Field Analysts”). I have found that it is helpful to classify combustion appliances using two schemes. The first is the vent category, and the second is the draft type. These classifications are taken from the code books, so nothing is being invented here.

There are four vent categories. They are conventionally designated with Roman numerals I, II, III, and IV. The vent category is shown on the appliance nameplate, usually located inside the burner compartment on a gas furnace or boiler, or near the burner on the outside of a gas water heater. Two factors determine the category to which an appliance belongs. These factors are the pressure and the flue gas temperature in the appliance vent connector. The four vent categories are described below.

Category I: negative pressure and high temperature in the vent connector. The characteristics of this category:

  • AFUE is usually 65–83%.
  • Category includes noncondensing equipment.
  • Appliance usually has an atmospheric burner.
  • A nonairtight vent connector is allowed.
  • Includes Category I fan-assisted gas furnaces (manufactured since 1990).
  • Appliance has natural or induced draft (discussed below).
  • Combustion supply air is usually taken from the combustion appliance zone (CAZ).

Category II: negative pressure and low temperature in the vent connector. The characteristics of this category:

  • Category includes condensing equipment.
  • There is very little equipment in this category.

Category III: positive pressure and high temperature in the vent connector. The characteristics of this category:

  • AFUE is usually 83–87%.
  • Category includes noncondensing equipment.
  • Appliance requires an airtight vent connector.
  • Mechanically assisted draft is defined as forced draft.
  • Combustion supply air is usually taken from the CAZ.

Category IV: positive pressure and low temperature in the vent connector. The characteristics of this category:

  • AFUE is usually greater than 90%.
  • Category includes condensing equipment.
  • Appliance requires an airtight vent connector.
  • Mechanically assisted draft is defined as forced draft.
  • Combustion supply air is usually taken directly from outdoors. Recent versions are direct-vent, also called sealed-combustion, units. In these units, the exhaust pipe is airtight and a separate, airtight pipe from the outdoors supplies combustion air to the appliance.
Use of Proper Terminology

I started training service technicians at gas utilities when gas furnaces, boilers, and water heaters were simpler than they are now. The need for more efficient appliances has resulted in more complex equipment. Examples include condensing appliances, through-the-wall venting, and direct-vent gas furnaces. The increasing complexity of combustion appliances, and the need to protect our customers from hazards, make it very important that analysts identify appliances correctly.

For the sake of our customers’ safety, it is important that a standardized terminology be used by service technicians and energy analysts alike. It requires time to learn, but it is worth the effort. This terminology already exists in the code books (NFPA 31, NFPA 54, and NFPA 211), but it is not always used. This can lead to poor or inaccurate identification of appliances in the field. Such misidentification can be hazardous for occupants.

If you are in a room with other analysts, try this terminology question: What is an induced-draft gas furnace? My guess is that you will get a number of different answers, but none will agree with the definition in the National Fuel Gas Code, NFPA 54. This is a serious problem.

Rather than buying the code books, download this free online resource to get you started with the right code terminology.

These categories apply officially to gas-fired appliances, but it is instructive to use the same categories for appliances that are not gas fired. Most oil-fired systems in the field would be defined as Category I, for example.

Now let’s look at draft types. There are three types of draft, as described below.

Natural draft. Conventionally, combustion equipment relied upon natural draft. With natural-draft appliances, negative draft is created by the natural buoyancy of hot air in the vent system. As a result, the flue gas temperature, the outdoor temperature, the height of the venting system, and the internal dimension of the vent all affect the draft. Natural-draft appliances are the most vulnerable to spillage and backdrafting, because the strength of the draft depends primarily on variable natural forces. Conventional gas-fired water heaters are natural draft and Category I. Most oil-fired systems are natural draft. (See Figure 2.)

Induced draft. In induced-draft combustion appliances, negative draft is created by a fan at or near the point of vent termination (usually at an exterior wall) that pulls the flue gases from the appliance. The controls of such a fan must prove that the fan is operating and pulling before the appliance burner is allowed to fire. Once the burner stops firing, the fan shuts off. Some water heaters and heating systems use induced draft and are classified as Category I or II. Note that it is a common mistake to call a Category I fan-assisted furnace an induced-draft furnace. Actually, a Category I fan-assisted furnace has a natural draft. (See Figure 3.)

Forced draft. In forced-draft combustion appliances, positive draft is created by a fan within or just above the appliance that pushes the flue gases out through an airtight appliance vent. The vent must be airtight so that flue gases will not leak into the house. Category IV condensing furnaces (sometimes called 90 plus) use this type of draft, as do many Category III gas water heaters. (See Figure 4.)

There is too much misinformation published about combustion appliance terminology and identification, some by well-known organizations. All organizations that are setting combustion appliance protocols for efficiency and safety testing should use the definitions in the codes as their guide. Being able to identify a combustion appliance by vent category and draft type can affect many decisions out in the field, including how to

  • perform combustion safety testing;
  • measure for CO;
  • perform a draft test on an appliance;
  • perform a steady-state efficiency test;
  • set the appliance up for a blower door test;
  • appraise appliance code compliance; and
  • specify appliance repair or replacement.

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The More We Know

Now that we are part of a growing profession, rather than a loosely knit band of roving mavericks (some of you can remember how things were 20 years ago!), increased responsibilities fall on our shoulders. One of these responsibilities is continuing education; another is effective communication with our colleagues and customers. The more we know about what we do, the better off we will be.

Rick Karg is president of Residential Energy Dynamics, LLC, an innovative software firm providing free online energy and building diagnostic tools.

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