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.
Types of Flues
There are four types of oven flues:
- Open Flue: This type of flue enables the waste air inside the oven to rise up in a circular form, much like a vortex. An appliance containing an open flue usually draws air from its immediate surroundings. A wood fired pizza oven flue works in this way as air is generally provided to the fire from the front entrance, combustion gasses are then expelled from the oven vault via the top of the front entrance, and then travel up the flue and out a small chimney.
- Closed Flue: A closed flue is used for appliances or ovens situated in a closed room where it is desired that the primary source of oxygen not be from the same room. In this case an adjoining air duct transports air flow directly to and from the appliance, thus making it impossible for fumes to escape into the room. These flues are further divided into two types:
- Power Flue: These flues have fans that help in directing outside air into the appliance, while simultaneously directing exhaust fumes back to the air outside. The air and flue ducts in this case may be longer and come with the option of changing the direction of air flow. Additionally many times ducts are often arranged in a concentric pattern. A smaller duct will be placed inside a larger duct. Thus any leaks in the smaller exhaust duct will result in the exhaust fumes escaping into the larger inflow duct containing it. The potentially hazardous fumes are prevented from escaping into the room.
- Balanced Flue: The balanced flue has the flue outlet and air inlet situated very close to each other, in order to create equal pressures between the air that enters and exits. These make use of the draught created by the exhaust fumes, thus keeping the flue only as short as would facilitate its passage through a nearby wall or source where the appliance is situated.
- False Flue: False flues are designed to enhance the artistic value of the oven and serve no functional purpose.
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:
|Internal Dimensions||Hearth Surface Area||Dome Shape||Vault Height||Door Height||Door Width||Flue Width||Foundation||Pizzas (10")|
|30" diameter (76cm)||706inch2 (4,363cm2)||Igloo||16" (41 cm)||10" (25 cm)||16" (41 cm)||18" (46 cm)||53" x 66" (135cm x 145cm)||1-2|
|34" diameter (86cm)||907inch2 (5,808cm2)||Igloo||16" (41 cm)||10" (25 cm)||18" (46 cm)||20" (51 cm)||57" x 70" (145cm x 180cm)||2-3|
|42" diameter (106cm)||1,385inch2 (8,824cm2)||Igloo||18" (46 cm)||11" (28 cm)||18" (46 cm)||20" (51 cm)||65" x 78" (165cm x 200cm)||4-5|
|50" diameter (130cm)||1,963inch2 (13,273cm2)||Igloo||22" (56 cm)||14" (36 cm)||18" (46 cm)||20" (51 cm)||73" x 86" (185cm x 220cm)||7-9|
|32" x 36" (82cm x 92cm)||1,152inch2 (7,544cm2)||Barrel||16" (41 cm)||10" (25 cm)||16" (41 cm)||18" (46 cm)||55" x 72" (140cm x 180cm)||1-2|
|35" x 39" (90cm x 100cm)||1,365inch2 (9,000cm2)||Barrel||18" (46 cm)||11" (28 cm)||18" (46 cm)||20" (51 cm)||58" x 79" (150cm x 200cm)||3-4|
|47" x 59" (120cm x 150cm)||2,773inch2 (18,000cm2)||Barrel||22" (56 cm)||14" (36 cm)||18" (46 cm)||20" (51 cm)||70" x 99" (180cm x 250cm)||5-6|
|59" x 71" (150cm x 180cm)||4,189inch2 (27,000cm2)||Barrel||28" (71 cm)||18" (46 cm)||22" (56 cm)||26" (66 cm)||82" x 111" (210cm x 280cm)||10-12|
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.
Infront of the Door
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.
Behind the Door
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.