To measure for a chimney liner kit you need to determine the physical dimensions of your chimney as well as the specific requirements for the type of appliance you are hooking the liner to. A chimney liner kit that will serve a fireplace needs to have the largest diameter pipe that will fit inside your flue, whereas the liner size required for other fuel burning appliances will be determined by the manufacturer’s specifications and local safety codes.

The basic measurements you need before you select a chimney liner kit are the height of your chimney and the inside dimensions of your existing flue. From the top of your chimney, measure to the top of the fireplace smoke chamber or the connection point for your woodstove or furnace. You can use a long flexible tape measure or tie a bolt or other weight to a string and lower it down your flue to the point where the bottom of the liner will be connected and then measure the string to determine the chimney height. Measure the inside dimensions of the clay tile liner (if your chimney is lined with tiles) at the top of the flue.

For a fireplace, you should have a liner with a cross sectional area no less than one tenth of the area of the fireplace opening. This ten to one ratio is a rule of thumb that can vary depending upon the height of the chimney and other factors. If the liner diameter is too small, your fireplace will not draw well, resulting in smoky fires and smoke-filled rooms. If the size of liner that you need for your fireplace is too large to fit inside your existing flue, you may need to repair your clay tile liner rather than inserting a steel liner inside it. An alternative would be to modify the opening of your fireplace to make its effective area smaller, perhaps by installing doors with adjustable airways or by installing a wood burning insert.

To measure for a chimney liner kit that will serve a wood stove, furnace, hot water heater or other fuel burning appliance, you should follow the manufacturer’s specifications. The inside diameter of the vent opening on the appliance will determine the ideal diameter of your chimney liner. It is a good idea, and a code requirement in some places, to insulate the outside of a chimney liner, so you will need to size your liner to allow space for insulation between the outside of the liner and the inside of the flue. An insulated chimney liner will keep the exhaust gases hotter all the way out of the house and minimize condensation of creosote and acidic water inside the chimney.

Getting good measurements for a chimney liner kit before you buy will save a lot of time and frustration. A properly sized and installed liner will make your house safer and more energy efficient while extending the life of your chimney.