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If you're planning a stove then you must also have a chimney or flue system. Wood burners can never be 'flueless' or operate with 'balanced flues' like gas fires. They must have a fully functioning flue which creates sufficient updraught to safely remove all of the combustion gases, including carbon monoxide, from the stove. How you flue the stove and exactly how and where the flue will eventually exit the building through the wall or roof can have a dramatic impact on component and installation costs as well as the overall appearance of the interior and exterior of the building. Most importantly, where the flue eventually terminates can also create particular installation issues which, if they are not fully considered at the outset, could affect the stove's performance as well as potentially create air quality problems for occupiers of the building and its neighbours. Importantly, pay particular attention to the proximity of the flue termination point any air intakes or extracts for MVHR and air source heat pumps.
Today you have three basic options when you create a chimney in a new build:
Known as a Class 1 chimney, these are made from block or brickwork and are positioned either inside the building so that you create a chimney breast or on an exterior wall so that you have no intrusions into the living space (the latter is great for modern inset stoves).
Traditional Class 1 block or brick built chimneys can be constructed around interlocking clay, concrete, ceramic or lightweight pumice chimney systems with square, rectangular or round cross-sections to ensure a consistent, insulated and better-performing flue draught. However, some of these systems can be very expensive so if you're never going to use the chimney with an open fire (they're so inefficient and will have such a negative impact on your SAP calculation, so why would you?) there's no real need to over specify the flue block lining if it's simply being used for a stove with flue liner.
For wood burning stoves a flue block system with a round cross-section is to be preferred, typically 175mm diameter (7"), because this will mirror the stove's flue outlet but also avoid the inefficient cold corners of square and rectangular flue blocks. Although not required by building regulations, most installers will still prefer the belt and braces approach of adding an additional stainless steel flexible liner inside these systems. This provides a much better gas-tight seal than otherwise would be the case when an adaptor is used between the flue block and the connecting flue pipe (the flue pipe that comes off the top of the stove). This will also be much better at avoiding potential leaks from any condensation and creosote build up inside the flue block system. In this way a complete flue system that more precisely mirrors the stove's flue outlet shape and size can be created and which will also add another layer of insulation for greater flue efficiency.
NB It is not acceptable to use a chimney liner alone in a new built chimney void which itself hasn't been flue block lined.
This is by far the most cost effective route to install a stove where there is no existing chimney, or it's too expensive to build a Class 1 chimney. A twin wall flue system is simple to construct and is made from a series of standard-sized stainless steel pipes which comprise a 25mm thick insulation layer sandwiched between two sleeves of stainless steel and usually only requires a safe distance to combustible materials between 60 and 75mm, depending on the make. These distances will have to be carefully considered when the twin wall passes near any ceiling joists and roof spars. Each of the twin wall sections has an air-tight labour-saving connecting system which also strengthens the joints between pipes to make them, not only gas-tight, but also completely rigid so that the number of support brackets required to keep the system safely in place is minimised. With this method you virtually have complete freedom to place the stove exactly where you want. It's ideal for saving space when combined with a free-standing convector stove.
You can keep most of the twin wall system inside the building eventually exiting it through the roof which will have the benefit of creating a warmer flue which will make the stove much quicker and easier to light and operate, especially when it's particularly cold outside. Alternatively, you can site the stove on an outside wall with the twin wall flue pipe piercing the wall at 45º just above the stove running the remaining part of the flue system up the outside wall to a point above the eaves. Building Regulations Document J provides calculations for the safe minimum heights of the termination points of flue systems. Don't be tempted to save on the cost of a component by reducing the stipulated safe height, as we've seen too many times, this can be problematic and will usually require retrofitting the missing component anyway.
Air tightness can be maintained at both the wall and roof apertures with special sealed sleeve components which will also ensure that potential water ingress is eliminated. To minimise the visual impact of exterior plain stainless steel twin wall pipework some systems can be specified in a long-lasting maintenance free powder coated colour. Black is usually a stock colour for most components and doesn't significantly add to the overall cost.
Either external or internal route is acceptable and both have their own advantages. When the flue goes through the roof you have more flexibility when it comes to exactly where you site the stove within your living space. This is a great for large multi-purpose open plan living spaces where the stove can also be used to demarcate the living spaces and the heat can be more evenly distributed. This is where double-sided and round stoves with 180º+ views of the stove fire box come into their own. Some stoves can be fitted with a practical swivel turntable so that the window and view of the fire can be aimed at different parts of the living space depending how it is being occupied, while heat is maintained around 360º of space.
Something that will also need to be considered is that in buildings with two or more storeys an internal flue system will need to run through the living spaces above the stove (and the roof space) and those spaces will need to be protected with the flue being 'boxed in' and ventilated to maintain the appropriate safe distances to combustible materials. Although they miss out out on the heat gain from the flue system that slightly raises the ambient temperature of the upper spaces it passes through, generally speaking most people prefer to avoid the visual disruption and loss of floor space internal flues cause to their upper floors.
As for external flues, although they may not be quite as efficient because of the extra bends and the impact the colder air temperature may have on combustion gas temperature they're still a very practical alternative to building a traditional chimney and of course there's no impact upstairs. You'll have to route the flue system through an external wall and therefore it will limit exactly where you can place your stove. You'll also probably need a higher looking flue at the edge of the roof to maintain the necessary flue height required by building regulations Document J (England and Wales) but this isn't usually much of a problem for most people. One thing to consider is that like-for-like an external twin wall system is likely to be more expensive than an internal system because of the extra bends and external soot trap that are needed.
Mock or faux chimney breasts can be created to conceal a prefabricated stainless steel twin wall insulated flue system and can be used for either free-standing stoves with a typical fireplace opening, with or without a fire surround, or for inset stoves for a clean contemporary look. In all cases these structures need to be extremely well ventilated at the top and bottom to avoid hot spots and potentially dangerous heat build up. Providing you create a suitable foundation or have an existing solid floor these can be made from blocks or bricks. They should never be constructed from standard grade plasterboard and timber stud work. There are also a number of specialist labour-saving systems and materials which have been specifically created to safely and cost-effectively produce these types of structures and The Stove Yard will be happy to help and advise you on the best of these.
With a mock chimney breast you have the option of routing the twin wall system through the outside wall so that the flue pipe doesn't interfere with the room space above, this will also allow you to provide a handy soot trap for easier sweeping. If you continue the flue through the room and roof space above you must box it in to maintain the recommended safe distances to any combustible materials. Importantly, you must ensure that the flue can also be accessed for cleaning, although sweeping through the stove is usually acceptable in these cases.
Please Note For more detailed information please refer to BS15287-2 Design, Installation & Commissioning of Chimneys (for room sealed appliances).
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