Sunday, April 15, 2012

Brick Oven Specs #2

unfinished oven showing chimney in front
Universally, brick ovens have their flues in front of the chamber. This allows the heating fire to move across the dome roof before exiting thus depositing BTUs in the roof and walls of the oven.  It also means that when a door is placed over the oven chamber, the heat is trapped inside.
Brick oven mouths (the opening into the brick oven) work best when they conform to a proportion of 1:1.6 or 11" high and 18" wide.  The mouth can be a bit wider but the limiting the height of the mouth prevents too much exchange of cold outside air during firing and baking.

no chimney (exhaust hood covers oven)
outdoor clay oven w/o flue
Chimney height is entirely optional.  Some ovens have no chimney or flue and the smoke exits directly out the front.  An outdoor oven might have a three foot chimney which prevents smoke from spilling across the face of the oven (except when the fire is excessively stoked).

Indoor oven with first flue arch
Indoor ovens need to be guaranteed not to spill smoke into the living space.  Tall chimneys create a natural upward draft.  A chimney of 12 feet or greater will draw the smoke and flue gases up the chimney sufficiently to prevent smoke in the room.  Other factors, such as the ridge of the roof and horizontal angle of the exiting chimney will alter this equation.  Most chimney installers have this equation, but it is approximately 3 feet of chimney height negates 1 foot of horizontal angle.  This means that if you angle the flue of the oven at a 45 degree angle for six feet you need 9 feet of height to negate the loss in draft (45 degrees is halfway to horizontal so the equation is based on a net-3 feet multiplied by 3= 9 feet).

Math, that high-school nemesis, rears its useful head for masons.

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