Unless a fireplace with a 6-foot opening is made fully 28 inches deep, in
order that large logs will lie well inside, the advantage of the wide
opening is lost, for the logs will have to be split. A shallow opening
throws out more heat than a deep one of the same width, but can take only
sticks of smaller diameter; thus it becomes a question of preference
between the greater depth which permits of large logs that burn longer
and require less frequent replenishing and the shallower which takes
lighter sticks and throws more heat.
In small fireplaces a depth of 12 inches will permit good draft if the
throat is constructed as explained above, but a minimum depth of 18
inches is advised, to lessen the danger of brands falling out on the
floor. Wire guards should be placed in front of all fireplaces. In
general, the wider the opening the greater should be the depth.