A flue is an opening, duct, or a pipe in a chimney that is used to expunge waste gases or leftover gaseous residue from a boiler, heater, or a traditional wood fired oven. It was traditionally considered akin to a chimney itself. However, in recent years it has come to exist as a separate entity from the chimney, that helps in the expulsion of gases to the chimney.
There are four types of oven flues:
When building a flue for a pizza oven, one should keep in mind that the size of the flue (the space around and above the surface area, also called the hood, box, or air vent) should be enough to allow all exhaust fumes out and into the chimney, for expulsion. This is especially important in indoor ovens, where even the slightest of mistakes can result in the waste gas leaking into the room.
Technically, the flue should be wider by at least an inch when compared to the width of the entrance. If you can make it even wider than that, it will only be to your advantage. A wider flue will not only give you greater working space, but also result in less heat loss into the box. Additionally, a wider flue blocks wind from flowing into the oven from outside, thus acting as an effective insulating agent. Without this protection, wind may affect the smooth flow of gases in and out of the oven, preventing an efficient fire. The table below provides guidance for flue width:
While measuring depth, keep in mind the following: measure from the inner side of the wall of the oven facing front and the oven entry point. And remember, that the higher you want the deco arch to be, the deeper your collection hood, or flue, should be constructed.
Generally the flue is placed in front of a door in a traditional wood-fired oven, such as those used in Italy. The flue placement means heated air must first travel out of the door opening, and then flow up through the chimney. This enables the oven to raise the temperature more readily and provides the option of sealing the vault entirely through the use of a door. While a door is in place, the temperature loss in the oven is slowed drastically. This enables the cooking of food such as roasts and bread, which prefer a stable temperature without a constant fire.
For ovens where the flue is placed behind the door, heat and smoke travels directly from the vault up and out of the chimney. The direct flow of air in via the door and out the top of the vault minimizes the effect of the fire. The establishment of a high temperature thus requires a substantial and constant fire. If the oven door is closed the vault is not entirely sealed as air may continue to move through the chimney. Without maintaining a fire while the door is closed, the temperature will gradually drop. It is possible to place a valve or cover on the chimney to lessen the effect. For cooking pizza this difference is not important as pizzas are generally cooked over a short period while the oven door is open. The primary factor in cooking pizza is establishing a high temperature of over 370°C (700°F).
One of the biggest concerns with setting up a flue to build it properly, since improper construction can result in more heat loss than was originally intended. Generally, this happens when the flue is left wide open after the fire is lit. Hot air leaves the oven through the chimney, whereas cold outside air is sucked inside. The ideal way to use a flue is to open it all the way until the fire is lit, and then gradually close it until it is wide enough only for the slow and efficient escape of smoke.
Always wear gloves while operating a flue, since they heat up from the fire of the oven.