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In an effort to reduce energy consumption new homes are now being built to incredibly high insulation and airtight standards with the German Passiv Haus and the UK equivalent, AECB Carbonlite, being the very highest. Not too many homes are quite built to this standard yet, although many aspire to be and in fact the term 'passiv haus' has become a much misused description for any low energy home that goes some way beyond basic building standards. When it comes to installing a wood burning stove in these airtight homes special consideration must be given regarding how the combustion air will be supplied and the combustion gases removed without compromising the building envelope.
According to the current Building Regulations Document J (and national equivalents) every post-2008 property, even those with only the basic regulation insulation and air-tight standards, must have some form of additional ventilation dedicated to any stove that's installed in it to ensure its safe and efficient operation. Such ventilators would obviously compromise the integrity of the building envelope and the building's energy efficiency. As a consequence the stove industry now provides an acceptable alternative with many stoves (but not all) capable of taking all of their combustion air supply from outside of the building. This is sometimes known as outside air, external air or more correctly direct external air supply (DEAS) and is now a must-have feature when it comes to specifying the stove for such buildings.
With the introduction of the revised BS8303 Installation of Domestic Heating standard in 2018 the DEAS option became officially recognised as a safe alternative to the dedicated air vent – but with certain provisos. A series of mandatory smoke spillage tests must be carried out during commissioning. These include a mechanical extraction depressurisation test with all extractor fans, as well as any MVHR system, set to maximum extraction to confirm that there is no possibility of combustion gas leakage from the stove, particularly from around the door. Therefore ensuring that the stove has an air-tight door seal is also a prerequisite when choosing a wood burner for a low energy home. The best DEAS stoves for use where there is MVHR are generally regarded as those that have had their room seals tested to the critical German DIBt standard. However, there are many excellent DEAS stoves (for example Fondis) that would meet the DIBt standard but which simply haven't undergone the expensive DIBt test and The Stove Yard will be happy to advise you on these.
One thing is for certain it is unlikely that a highly insulated passive or AECB Carbonlite house is likely to require a high output stove and you should therefore be considering a stove with a heat output range that starts at around 2 to 3.0kW depending on room size. This output range would also provide you with a belt and braces maximum heat output of around 5 to 6kW for those extremely cold days or when you want an abundance of quick heat but do not want to use the primary central heating. This aspect of a stove's operation makes them an ideal companion for heat pumps which can be slow to react during cold snaps. Again, The Stove Yard will be happy to advise on the required heat output and also provide you with a good choice of stoves for your consideration.
When considering the chimney or flue system (also see the 'Creating a chimney for a new build' section) for a highly insulated new build, you will again need to ensure that the building envelope is not compromised with a break in the insulation when the twin wall flue system goes through the wall or roof. Twin wall systems generally require an air gap of at least 50mm around the flue pipe which, in highly insulated buildings, could create a cold bridge and potentially cause issues with condensation and mould when the stove is not in operation. Fortunately there are a number of twin wall systems which offer specially designed component solutions such as airtight 'protect boxes' and pipe which feature an extra 25mm of insulation (bringing it up to 50mm). At The Stove Yard our experts can design and supply competitively priced flue systems specifically for your highly insulated building project – please ask for details.
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