A chimney liner (chimney flue) in a masonry chimney is defined as "A clay, ceramic, or metal conduit installed inside of a chimney, intended to contain the combustion products, direct them to the outside atmosphere, and protect the chimney walls from heat and corrosion."
Chimney Liners (chimney flues) serve three main functions:
1. The Chimney liner (chimney flue) protects the house from heat transfer to combustibles. In the NBS tests, unlined chimneys allowed heat to move through the chimney so rapidly that the adjacent woodwork caught fire in only 3 1/2 hours.
2. Chimney Liner (chimney flue) protect the masonry from the corrosive byproducts of combustion. In the tests it was determined that if the flue gases were allowed to penetrate to the brick and mortar, the result would be a reduction in the usable life of the chimney. The flue gases are acidic in nature and literally eat away at the mortar joints from inside the chimney. As the mortar joints erode, heat transfers more rapidly to the nearby combustibles and dangerous gases such as carbon monoxide can leak into the living areas of the home.
3. Chimney Liner (chimney flue) provides a correctly sized flue for optimum efficiency of appliances. Modern wood stoves and gas or oil furnaces require a correctly sized flue to perform properly. The chimney is responsible for not only allowing the products of combustion a passage out of the house, but the draft generated by the chimney also supplies the combustion air to the appliance. An incorrectly sized chimney liner can lead to excessive creosote buildup in woodburning stoves, and the production of carbon monoxide with conventional fuels.
The tests by National Bureau or Standards revealed that unlined chimneys were so unsafe that researchers characterized building a chimney without a liner as "little less than criminal".
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