Any gas appliance such as a water heater or furnace needs to transport the combustion gases safely out of the indoors environment. In order to make a vent, one of three types of pipe are typically used, Triple Wall, also called chimney pipe, Double Wall or Single Wall.
Triple Wall pipe, used mostly for chimneys, has an inner stainless steel liner and two outer liner walls made of either stainless or galvanized steel. It is almost never used for gas fired venting, being considered too expensive, and is reserved for coal, wood or oil burning appliances. However, it can act as a substitute for Type B (Double Wall) pipe if required.
Double Wall, or Type B, pipe is galvanized steel with an aluminum liner. It is available in round and oval shape configurations, and the most common diameters for water heater vents are 3 and 4 inch diameters, oval pipe diameters being measured across the longer axis. The double wall construction allows a layer of insulating air to cool down the exterior wall of the pipe, and that is why it is required by code in unoccupied spaces such as attics or between floor spaces. Single wall pipe can be used elsewhere, and is usually used between the water heater’s vent collar and the ceiling. Type B pipe must be a minimum of 1 inch distance from combustible material, according to most codes.
Single Wall, or Conduction pipe comes in copper and galvanized steel; the galvanized steel is less expensive and should be used unless copper is required by the local building code. Building codes will also dictate what gauge metal is required for specific diameters of pipe. Clearance space to combustible materials must be far more for single wall than double wall pipe because it gets so hot.
You can buy conduction pipe in preformed round 10 foot long sections, or so-called Buttonbead form. Buttonbead is flat sheet that has been formed with lengthwise edges that connect to each other. Since it comes in flat form it is easier to cut to the size required, using tin snips. (Preformed single wall has to be cut with a hacksaw, which can be awkward, especially with thinner gauges.) After cutting, you roll it up like a tube then snap the ends together.
In order to install a water heater vent through a roof, you will also need adjustable elbow sections for bending around obstacles, a starter plate support for where single wall conduction pipe transitions to double wall pipe at the ceiling, a vent cap, and a roof jack for flashing where the vent penetrates the roof.
Vents can also be routed through an exterior wall, but this configuration is more costly and time consuming to install. For exterior wall venting, you will need two-piece telescoping spacer called a thimble that goes through the exterior wall, a wall supported starter plate, support clamp assemblies for attaching the vent pipe to the wall, a roof jack, and a vent cap. In both cases, minimum clearances to combustible materials such as drywall, insulation, roofing and siding must be respected.