The Anatomy of a Fireplace: Flues, Chimneys and More
From: DK - Do It Yourself Home Improvement
There are several things you need to consider when choosing a new fire or refurbishing an old one. You may need to install a new flue, or have an existing chimney lined or relined — the main types of flue are shown here. If your current flue does not need renovating or replacing, it will be easier to choose a fire suitable for it. Unless you have a direct vent fireplace or balanced flue, you need to consider ventilation; fires require an air supply to burn well. A lack of wall space can make it desirable to cover up a fireplace, either leaving the recess or making it flush with the wall.
Vent-free fireplaces have an oxygen detection safety-pilot (ODS) that automatically shuts off the gas supply if the oxygen level in the room falls to 18 percent. In 1995, the Vent-Free Gas Products Alliance of the Gas Appliance Manufacturers Association did a study on indoor air quality as it relates to domestic vent-free gas heating products and determined that vent-free gas heating products meet or exceed the most current and nationally recognized standards and guidelines for indoor air quality. Install a carbon monoxide detector whenever operating any gas product in the home.
As fires burn, they give out combustion gases, which are carried out of the home by a flue. The main types of flues are discussed below. As the gases are carried out of the house, they must be replaced by fresh air. In older houses, natural drafts were relied upon to provide this constant airflow. In modern, well-insulated homes, it is unlikely that a natural airflow exists, so ventilation must be provided. Some flues (balanced flues) have a built-in air intake. Ventilation requirements should be specified for particular fires. Fires and flues need to be checked annually by a professional to ensure that they are working safely and efficiently, according to the National Fire Protection Agency.
Fireplaces Without Chimneys
There are several types of flues that do not need a chimney. Two common designs are shown below and flueless fires are also available. The type of flue you need will normally be specified by the fire manufacturer. Some fires only radiate heat, but the fires shown here also warm air from the room by circulating it inside of the body of the fireplace, separately from the combustion process.
This kind of flue is built into the fire and runs straight through the wall behind. It takes fresh air in and lets combustion gases out. A balanced flue is used with glass-fronted fires so there is no contact between the fire and the air in the room.
The combustion gases are cleaned by a catalytic converter and released into the room, making flueless fires efficient because no heat is lost via the flue. Flueless fires can only be installed in larger rooms. The minimum room size varies, so carefully check the manufacturer's specifications.
Power Direct Flue
This type of flue has a fan to suck combustion gases through the flue and expel them outside. Power flues can be extended, so it may be possible to install a fireplace away from an external wall. An electric supply is required for the fan.
Flues in Chimneys
Modern chimneys are built fully lined with a corrosion-resistant, rectangular, precast concrete flue suitable for any fire. If you have an older chimney, it may have an unlined flue. If there are cracks in the mortar you should have it lined. Seek advice from a professional — they may recommend installing a concrete or metal flue liner compatible with your fuel type.
Unlined Flue: Basic chimney constructed of brick or stone.
Lined Flue: A chimney with a concrete or metal liner in it.
Chimney pots provide a decorative finish and raise the top of the chimney above the roof line, where the airflow draws combustion gases out. A chimney cap is often attached to the top to improve ventilation and keep out rain and animals. If any damp problems are associated with a chimney, check that the flashing and pointing are intact, and that a cap has been installed. Different cap designs are used with different fire types, so make sure you have the right cap. If you are opening up an old fireplace, you should enlist professional help to check that the top of the chimney has not been blocked and is fitted with the right type of cap, and that the chimney is in good repair. They may also install or replace a flue liner.
Standard Chimney Cap
If your chimney is well-situated and has no significant downdraft problems, a basic chimney cap will prevent entry of rain, hail, animals and birds.
This type of chimney cap can be adjusted to fit most chimney pots. It is designed to prevent downdrafts from blowing smoke and fumes back into the house, and to prevent entry of rain, hail, animals and birds.
Revolving Chimney Cap
Chimney caps that revolve are designed to encourage airflow in chimneys that don't draw air well and are prone to downdrafts.
Copyright 2009 Dorling Kindersley Limited
Text copyright 2009 Julian Cassell and Peter Parham