Gas appliances are divided into four venting categories based on vent operating pressure and whether they are condensing or non-condensing. Category I is negative pressure, non-condensing. Category II is negative pressure, condensing. Category III is positive pressure, non-condensing. Category IV is positive pressure, condensing. These categories are used to determine the type of vent that should be used for the appliance.

Appliances that have a combustion efficiency of greater than 83 percent under American National Standard Institute (ANSI) test conditions are rated as condensing and will fall under Category II or IV.

Category I is what we normally think of when we think chimney. Negative pressure sucks the products of combustion from the appliance breech and deposits them outdoors.

Category II products are no longer manufactured. Negative pressure vents with combustion gases at or below the dew point. Heavy gases at the dew point are not buoyant enough to vent with a negative pressure flue.

Category III products are direct sidewall vented without additional apparatus. Positive pressure requires joints in flue material to be sealed. Because these 80 percent appliance flue gases are close to the dew point, and the vent material is single wall, corrosion resistant materials must be used. Drains are typically incorporated to remove flue condensation before it enters the heat exchanger. Category III has been used to solve installation problems where no appropriate flue is available.

For Category IV positive pressure condensing appliances, the joints in the flue material again must be sealed. This category applies to 90 percent-plus annual fuel utilization efficiency (AFUE) furnaces. Appliances are designed to dispose of flue condensate as well as condensate formed within the secondary heat exchanger. You may be able to sidewall vent at reduced distances to openings in the building than the National Fire Protection Association (NFPA) suggests. This applies to sealed combustion with combustion air intake in the same pressure plane.

Category I Vent Materials

Category I vent materials are found with older appliances. Single-wall galvanized pipe (26 gauge) is only used as a connector on 70 percent AFUE and lower gas appliances and all oil appliances. Masonry will use vitreous clay liner. In some cases, Transrite has been used, which may contain asbestos. This does not meet any venting requirements.

“B” vent is double-wall. It is galvanized steel on the outside, with an aluminum inner pipe. B vent is rated only for gas appliances. It is used as a vent connector for all 78 percent and 80 percent AFUE appliances. It may not be run outside of the building and may be used as a liner in an existing flue chase.

Flexible liner may be aluminum for gas appliances or stainless steel for gas and oil appliances. It is used to retrofit existing flue passages to meet code or a specific application.

“L” vent is double-wall with both the inner and outer pipes made of stainless steel. It is used with oil appliances.

All fuel, double-wall insulated vent is used with oil and solid fuel appliances.

Category III Flue Materials

For Category III venting, aluminum is no longer recommended because of corrosion problems. Stainless steel is specified by manufacturers to reduce corrosion issues.

High temperature plastic was the most common material specified by manufacturers. However, high temperature plastic has been involved in some recalls. Manufacturers have a specific installation protocol to ensure quality. Joints must be sealed using a high temperature sealant. Hangers must be spaced properly. Pitch back to the appliance must be ¼ inch per foot.

Category IV Flue Materials

For Category IV venting, PVC is used because of its low cost. Joints must be properly sealed. Vent must be provided with adequate hanger support. Pitch back to the appliance must be ¼ inch per foot.

CPVC is used for higher operating temperatures than PVC. Be sure to follow manufacturer instructions to ensure quality sealed joints.

Publication date: 1/19/2015

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