Fireplace flue installation should only be done by a professional, experienced installer. If this is not possible, then it should be at the least overseen by a professional; they will know the latest regulations and building codes, best materials and safest most efficient designs.
If flues are not properly installed you risk drawing deadly Co2 and other fumes into your home, as well as chimney fires. The following outlines some basic principles but we recommend the actual installation not be done by the average homeowner.
A fireplace flue is the vertical passageways in chimney used for transporting exhaust gases from a fireplace to the outdoors. Wood fire combustion products contain carbon monoxide and other dangerous compounds, so proper ‘draft’, and admission of replacement air is critical. Flue materials, design, and installation are controlled by building codes and other regulation standards.
Clay Flue Liners
One easy way of incorporating a flue into a fireplace chimney is to construct it with pre-cast concrete chimney blocks. These are available in 16 x 16 x 8 inch and 16 x 20 x 8 inch sizes, in order to accommodate 8 x 8 inch flue liner.
Flue liners are tube-like sections made of fire clay, volcanic pumice, refractory quality concrete or ceramic, mortared into the chimney blocks with refractory mortar.
A similar method is followed for installing the flue liners in a traditional brick and mortar construction chimney, but an infill aggregate is used between the brick and the liner. Take care to check the flue liners for cracks and damage prior to installation.
Although the above is the simplest method, installing the liners into an older chimney can be tricky, due to nonstandard sizes or uneven condition of the interior surface. Flue liners were not widely used in chimney construction before 1965, and many homeowners want to update their chimney for safety and ease of maintenance. For these applications, you can consider stainless steel flue liners or cast in place flue liners.
Stainless Steel Liners
These come in both flexible and straight rigid sections. They work best with wood stoves or pellet stoves since their inside diameter is usually too small for an open fireplace. Installing these is a two man job, one person on the roof and one on the ground at the fireplace location. The liner is lower into the chimney on a rope.
Insulation can be installed around the liner. A clamp and close off plate is then fitted to the top of the chimney, to support the weight of the liner, and then bedded in mortar. At the bottom end of the liner, a register plate connects the stove or fireplace insert to the liner via an adapter pipe.
Cast In Place Liner
This process employs an inflated tubular bladder around which a mixture of refractory cement and insulating aggregate is pumped, forming an empty space, the flue, in the middle of the concrete. The result is a seamless insulated flue which is easily maintained. It has the additional benefit of structurally reinforcing older weakened chimneys, and can also be used to make multiple flues inside larger chimneys.
Heat Retention and Masonry Heaters
Flues are intended to release gases into the atmosphere, but they have the disadvantage of also releasing useful heat to init. In masonry heater fireplaces, flues are built into a heat preserving structure within which the flue gases pass over the surface of heat retaining bricks before atmospheric release, avoiding large heat losses to the chimney and outside air seen in traditional chimneys.
The heat from the flue gases is absorbed by the heat retaining bricks and then slowly dissipated into the house instead of the outside air via the chimney. In a properly insulated house, a single load fire burning for one and a half hours twice a day can be sufficient to keep an entire home warm for a 24 hour period, saving fuel is used. These flues can also incorporate second combustion chambers so combustibles in flue gases are burnt a second time, reducing soot and pollutants while increasing energy efficiency.
A fireplace chimney operates on the principle of hot air rising above cold air; the hot gas in a chimney rises due to the fact that it is less dense than the air outside the house. This rising gas creates a pressure differential known as draft, that draws combustion air into the fireplace and pushes out the exhaust gas through the chimney. The hotter the gas in relation to the outside air, the stronger the draft.
The fireplace flue should be the same size as the flue collar. Chimneys of the past tended to be too large for the fireplace they served, however, bigger is not better. Why? Because the flue gas will rise quicker, hence have less time to lose its heat in a smaller chimney flue.
There are other ways to get the air warmer as well. One method is to use a flexible metal chimney liner and insulate it’s outside with rockwool sleeves, chimwrap or leca.
Photo by David Morris, Creative Commons Attribution License