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A double flue is a set of two flues which are routed out of a structure together. These types of flues are sometimes inserted into existing chimneys, and in other cases, a chimney may be designed from the start with the intent of being a double flue. For an example of what this type of flue looks like, a perusal of photographs of large mansion houses may provide some examples, as many of these structures use double flue designs to minimize the number of holes in a roof, and as a result chimney pots are often seen in pairs which emerge from the same chimney.
There are a number of reasons to use a double flue. In cases where multiple fireplaces, woodstoves, or other heating devices are attached to the same chimney, that chimney may not be able to generate enough draw to handle all of these attachments. As a result, lighting a fire upstairs might cause smoke to belch out downstairs. In these cases, enclosing a double flue inside the chimney allows for two fireplaces to be on separate flues. The design also makes it possible to cap one flue to prevent drafts when a room is closed off without impairing the function of another device connected to that chimney.
A double flue can also be used when two different types of fuel-burning devices are attached to the same chimney. For instance, someone might use a double fuel to vent an oil heater and a gas stove through the same chimney. Since different fuels combust at different temperatures, leading to differences in the temperatures of the flue gases they produce, it may be necessary to use separate flues to get an appropriate draft, or for safety reasons.
New double flue construction is popular because it provides people with flexibility and some additional options. The separate flues can be used to vent a wide variety of fuel-burning devices, and if a flue becomes unnecessary, it can be capped to minimize drafts, and uncovered later in the event that someone wants to use it.
Some special code restrictions apply to a double flue design, for safety reasons. When designing this type of flue, it's important to use a heating specialist who is familiar with flue design and the relevant codes to confirm that the flue will work efficiently while complying with the law. In the case of a modification to an existing chimney to create a dual flue, a heating professional can help the modification go smoothly and as painlessly as possible.