These are known as category 1 type gas appliances and they are much more prone to developing unwanted condensate in vents and connectors. Because of the many dangers related to acidic condensate, the sizing of gas vents is extremely critical for category 1 type gas appliances. These appliances, typically classified as type II, III, and IV, also require proper vent sizing, but they are much more forgiving than category 1 appliances. Condensation is expected to occur in some of these appliances, and safeguards are in place to handle it. The NFGC and the GAMA methods of sizing are quite similar, but there are some differences that will be explained later.
Many factors affect the required vent and connector sizes for gas appliances.
The NFGC manual provides a whole array of venting 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. The first step in designing a vent system entails selecting the vent and connector material types. The vent connector is the pipe that connects the appliance to the main vent pipe. The main 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.