Arkansas HVACR NewsMagazine November 2018

S tate, National, Chapter News Tech News

provides a whole array of vent tables that take all these factors into account so that both minimum and maximum allowable vent and connector sizes can be specified for any given situation. Vents oversized and undersized cause problems. Undersized vents cause too much restriction for the amount of gases being released, and thus a proper draft may not be established to keep the gases moving upwards. If the under sizing is severe enough, a backdraft can occur such that combustion gases could reach occupied spaces with disastrous results. Oversized vents cause numerous problems as well. The main problem with oversized vents is that the combustion gas can become too cool, as the velocity of the gas is slow in large vents. A low exhaust velocity allows the gases to cool, and condensate begins to form. A Category 1 appliance needs the combustion gases to stay above the dew point, typically 136 F for natural gas exhaust, in order for acidic condensation not to occur. Large masonry chimneys also make keeping the gases above this temperature very difficult, and that’s why flexible liners for chimneys are frequently needed for Category 1

appliances. See more on chimney venting issues here . The first step in designing a Category I vent system entails selecting the vent and vent connector material types. The vent connector is the pipe that connects the appliance to the vent. A vent connector can be either single-wall metal pipe, or Type B, which is double-wall metal pipe with an insulating effect. The vent can never be made of single-wall metal pipe. It must be either Type B pipe, tile lined masonry chimney, or a flexible metal liner within a chimney. Selection of single-wall metal or Type B vent connectors is mainly governed by cost and applicable restrictions. Single-wall metal vent connectors are lower in cost than Type B connectors, but they operate at much higher surface temperatures than do Type B connectors. Consequently, there are many restrictions on the use of single wall metal pipe connectors. For example, single-wall metal connectors cannot be used in attics due to the fire hazard, and they must have greater clearances on all structural components than Type B connectors. A good strategy for maximum safety and minimum it. If there is any doubt as to the presence, size, and quality of a tile