The rebate is the space between the inside edge of a fireplace the wall. This space allows the back panel, slips or cast insert to fit behind the fireplace. The most common case is 1⁄ 10 of the fireplace opening area, but with a minimum size specified. Increasing numbers of households are opening up old fireplaces to either fit log burning, multi fuel stoves or reinstall open fires it is important that the fireplace opening or stove flue is correctly calculated to the chimney size. Amongst other provisions this British National Standard lays down the sizes for the traditional open fireplace with its trapezoidal fireback used in Britain and Ireland for inset open fires made to BS 4834. A “builder’s opening” or “original fireplace”, is at the foot of almost every chimney flue and is of a pre-determined size (you get what you are given). Standard fireplace inserts will fit into existing fireboxes but it’s always a good idea to check to make sure* Height – Up and Down Using measuring tape, record the height of your firebox from the opening, the rear and the lintel (more on this in the next paragraph). Do not Enlarge the Fireplace Opening From it’s Original Dimensions. Hearth sizes: There are a few standards sizes when it comes to fireplaces, these are dictated by building regulations. (400 mm) in front of the fireplace opening. Building codes typically require a minimum flue size, expressed in terms of cross-sectional area, in relation to the area of the fire-place opening. Fire openings are quite an exact science. Flues above 6m tall should generally be not less than 1/7th to 1/8th of the area of the fireplace opening, e.g. (this can be reduced depending on model of stove) Make sure your woodburner is at least 300mm from any combustible materials, such as a wood fireplace surround or mantel. All fireplaces have rebates, and they tend to be 25mm, 35mm 50mm or 75mm. It’s important to have a builder who knows how a fireplace works and makes sure the fireplace opening and the flue size are the right match to create excellent venting. (300 mm) on either side of the opening and 20 in. (500 mm) in front of the fireplace opening. https://www.englishfireplaces.co.uk/measuring-for-a-fireplace Do not attempt to make the opening any larger than it was originally. In general, the flue can be 1/10th to 1/12th as large as the opening.” Therefore, the fireplace generally requires enlarging to provide the 150mm clearance required around the sides and back of most woodburning stoves. Do not confuse a small open fire with the builder’s opening, small open fires are often built within the builder’s opening and excavation will reveal the larger recess. fireplace opening and 16 in. If the fireplace opening is greater than 6 ft. 2 (0.55 m. 2), building codes require hearth extensions of 12 in. Size of Room in Feet 10 x 14 12 x 16 12 x 20 Fireplace Opening in inches 24 to 32 32 to 36 36 to 40 ; Please read the entire tek spec 3 The fires are given nominal sizes based on the approximate width of the fire front, 400mm (or 16 inch) being almost universal. For open fires the general rule is 1:8 ratios. The mantle's height, and other dimensions, should be in proportion to the fireplace opening and the size of the room. For an open fireplace to function properly and for the chimney to draw, the ratio between the chimney and fireplace needs to be exactly right. Fireplace Opening. Altering their shape and size will affect the efficiency of the fire and possibly lead to a “blowback” of smoke etc. When a home is built with a masonry fireplace, there is a huge range of possible sizes. fireplace opening and flue size, in-creasing the chimney height in-creases the amount of draft.