The annual fuel utilization efficiency (AFUE) of a furnace or boiler is the measurement of the appliance’s annual operating efficiency. For water heaters, its energy factor (EF) represents the efficiency.
In the United States, 70 percent of homes use furnaces and 11 percent use boilers. Space and water heating equipment account for more than 70 percent of gas consumption and 90 percent of the oil consumption in residential homes.

The annual fuel utilization efficiency (AFUE) of a furnace or boiler is the measurement of the appliance's annual operating efficiency. For water heaters, its energy factor (EF) represents the efficiency. All of these efficiencies are measured under laboratory conditions using a Department of Energy (DOE) test procedure. Since the original federal minimum efficiency standards were enacted into law in 1987, updated minimum efficiency requirements for water heaters went into effect in 2004, and DOE has been working to update the current minimum efficiency standards for furnaces and boilers. All heating appliance manufacturers have a significant number of higher efficiency models. More homeowners are seeing the value in upgrading their homes' systems with new high-efficiency products.

The vent-system installation of highly efficient appliances requires a closer look. Be sure vent sizing and materials and installation of the vent are appropriate in order to ensure safety and optimal system performance.


Last year, the Gas Appliance Manufacturers Association (GAMA) introduced a flash video on its Website, Titled "Venting Done Right," the video seeks to increase awareness about proper selection and installation of venting systems and materials used on gas appliances. The video stresses the importance of evaluating the existing venting system to be sure it is suitable to use with any new appliance.

Along with this new video, installers have several sources available to them to ensure proper venting. In addition to the manufacturer's installation requirements, the National Fuel Gas Code (NFGC) is the primary industry consensus code for information on gas-fired appliance venting in the United States. The NFGC is developed through a consensus standards development process under the auspices of the American Gas Association and the National Fire Protection Association. This code combines various industry standards into a single guide. The 2002 NFGC was revised and approved on Aug. 17, 2005, and released as the 2006 edition. (For further information or a summary of changes, visit

In Canada, the National Standards of Canada, Natural Gas, and Propane Installation Code (NSCNGPIC) contains venting requirements for Canadian installations. The NSCNGPIC code development body consists of members from the Interprovincial Gas Advisory Council, Provincial Inspection Authorities, and other code officials.

These guidelines must be adhered to for many reasons, the primary one being safety. The products of combustion can include carbon monoxide (CO), aldehydes, water vapor and nitrogen oxides, along with other elements. In cases of improper venting, a gas appliance can breathe in its own combustion products. When this occurs, increased CO levels are likely to result.

When improper venting materials are used or problems with the installation occur, the results could be potentially hazardous or fatal. Other reasons for following proper venting guides are to prevent premature failure of both the vent and appliance from condensation occurring inside the vent or in areas of the appliance where condensate is detrimental to the equipment.

Besides the NFGC, the manufacturer's installation instructions offer clear guidelines for proper vent system installation. Basic information can be found on the equipment's rating plate, which includes the manufacturer, model number, input, and vent category.


In most cases, the vent system is categorized by the product's safety standards. The category assigned to an appliance indicates the conditions inside the equipment's vent. As a general rule, as the combustion efficiency approaches or exceeds approximately 83 percent, the condensation wet time within the vent increases. It is the length of time that the vent remains wet that is of primary concern. These condensing appliances may or may not require specialized vent materials.

Category I and Category II appliances rely mainly on the pressure difference between the inside and outside of the structure and on the thermal buoyancy of the flue products for proper venting.

Category III and Category IV appliances rely on mechanical forces to exhaust the products of combustion from the vent, so when they are not in operation, residual flue gases may cool and condense. Acids in the flue gas may concentrate and destroy conventional vent materials.

For all vent systems other than Category I, it is important to use only those materials specified by the appliance manufacturer. Manufacturers offering appliances for vent categories other than Category I often specify corrosion-resistant materials such as PVC, CPVC, ABS plastic pipe, or AL29-4C stainless steel. Also, be aware that some local codes may prohibit the use of a particular type of material in some applications. Check with your local code official for any restriction on appliance venting materials.

In addition to the four vent categories, some manufacturers offer specialized venting systems. Direct-vent systems using a two-pipe system are typical in this specialized category; one pipe vents the products of combustion to the outside while the second is used as an intake pipe drawing combustion air from outdoors. Direct vent appliances can be any venting category; however, Category I and IV are the most common direct-vent categories. These specialized vents offer installation flexibility, since they allow sidewall vent terminations.

Another specialized vent system can be found on power-vented water heaters. These water heaters incorporate vent blowers that not only exhaust the flue products to the outdoors, but also dilute them with air, which permits plastic pipe to be used for the vent (e.g., PVC, CPVC, and ABS). When any specialized venting is encountered, it is important to follow the manufacturer's installation instructions.

Breakdown of Appliance Categories

Here are the properties of each appliance category:

  • Category I appliances operate with a nonpositive vent static pressure and with a flue gas temperature that avoids condensation in the vent.

  • Category II appliances also operate with a nonpositive vent static pressure, but its vent gas temperature may cause excessive condensation in the vent.

  • Category III appliances operate with a positive vent static pressure and with a flue gas temperature above the dew point, which avoids condensate production in the vent.

  • Category IV appliances operate with a positive vent static pressure as well, but with a flue gas temperature below the dew point, which causes condensation in the vent.

    Publication date: 07/24/2006