To look good and function properly, a fireplace needs pleasing proportions and an appropriate relation of fireplace area, flue size, and chimney height. Though designs vary, many common patterns have emerged through experience with successful fireplaces. SIZING THE OPENING Determining the size for a fireplace opening is mainly an aesthetic decision that depends on the size of the room and the visual prominence the fireplace is to have. Another factor is the size of fire that's appropriate for the room and its intended uses. MULTIOPENING FIREPLACES Some fireplaces have more than one open face. Corner or L-shaped units have two adjacent faces. Other designs include the look-through type with two opposite faces and three-face types. BUILDING CODES SET MINIMUM FLUE SIZE Once the size of the fireplace opening is set, you can determine the appropriate flue size. Building codes typically require a minimum flue size. EFFECTIVE FLUE AREA The American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) recommends using round or oval flue liners whenever possible. When you use square or rectangular liners, you must calculate the effective flue area (EFA) using the inside dimensions of the liner minus any corner.