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MVHR systems and wood burners make a good combination in a new low carbon, low energy home and generally speaking there shouldn't be any real issues when installing both providing that you select the correct room sealed appliance and the appropriate BS8303 spillage tests are carried out by a competent person during commissioning. In addition, selecting and maintaining an MVHR system set-up that pressurises the building rather than depressurises it will also help ensure the safe performance of the flue updraught.
MVHR systems work by removing stale warm indoor air and extracting the heat from it to warm a supply of replacement outside air which it then pumps into the building. MVHR by itself will not usually be sufficient to comfortably heat a home, even those MVHR systems with additional line heaters built in, and therefore many people use additional heat sources such as under floor heating and wood burners. Unfortunately under adverse conditions, for example poor maintenance or bad weather, MVHR has the potential to lower room air pressure which in turn could adversely affect the performance of a stove's flue in removing combustion gases, possibly spilling these into the living space.
Therefore, in order to to maintain safe indoor air quality the stove installation and stove must feature a direct external air supply (DEAS) independent of the room air and importantly, the stove should have an effective door-seal to make it and its flue virtually 100% room-sealed. This means that the stove's safe operation will not be affected by potential changes to the room air pressure caused by mechanical extraction. Simply put, when the stove door is closed none of its combustion air can then be taken from the room and conversely none of the combustion gases the stove produces can escape.
Note however when selecting your stove, that not all stoves with DEAS are necessarily 100% room-sealed. Some may still take air from the room. Any stove with a traditional primary air control valve and / or open secondary air inlet grille would not normally be considered safe for use with MVHR systems or indeed within any low energy home, even if it had DEAS. If you can afford it we would strongly recommend considering a stove which has been room-seal tested to the exacting German DIBt standard which is currently the best indication of the quality of a wood burning stove's air-tightness – for example one of the many DIBt tested models from Hwam.
DIBt is the only test that looks for potential leakage when the stove is under negative pressure and when subjected to thermal and mechanical stress. The test will specifically check that the stove‘s door seal maintains air-tightness, even after being opened and closed at least 6,000 times. Interestingly the new CE EN16510 standard for stoves, which is estimated eventually replace the current EN13240 / EN13229 will incorporate a similar optional room seal test for DEAS appliances. Alternatively you could also consider a stove from a reputable manufacturer, who has specifically made a declaration that their DEAS stove provides a 100% room seal and therefore would also be safe to use with MVHR, for example Fondis inset stoves.
When using a DEAS stove with MVHR, particular attention should also be paid to the seal on the external air ducting connections as well as joints on the flue pipe and other flue system components. During commissioning the stove should undergo a number of rigorous worst-case scenario BS8303 spillage tests including with the stove door open and the MVHR set at maximum extraction – ideally undertaken in conjunction with the MVHR engineer. The stove owner should also be aware of the need to minimise the time the stove door is left open during refuelling (a stove with a self-closing door is therefore ideal) and the need to regularly clean the MVHR air filters and service the MVHR unit.
Please note The installation of the CO detector (which is a mandatory component of all safe stove installations) should not be considered as a substitute for carrying out the correct BS8303 (2019) risk assessment spillage tests during the commissioning of the appliance.
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