Stoves and MVHR systems: Important New Safety Considerations

Today, a growing number of new and refurbished homes include the combination of whole-house Mechanical Ventilation Heat Recovery (MVHR) systems and the must-have home accessory of the moment, a wood burning stove as a major plank in their heating strategy. This mix simply makes great sense in terms of heating efficiency, environmental credentials and lifestyle. However, as their joint use increases, Hetas, the organisation charged with promoting the safe installation and use of stoves, is keen to raise awareness with installers and specifiers of the potential safety issues that must be addressed during the stove and MVHR commissioning process. MVHR and other ventilation systems can lower the air pressure in a building, and when a stove is also installed, this can have a knock-on effect on the flue draught pressure – if great enough, this could cause the spillage of harmful combustion products from the appliance.

Their forthcoming general guidance for the design, installation risk assessment and commissioning of stoves has identified dwellings with low air permeability and MVHR at the highest risk of carbon monoxide (CO) spillage from the appliance. As air permeability of the building structure decreases, then the risk to internal air quality increases and this situation is exacerbated by the introduction of MVHR extraction and/ or wood burners. Sadly, many people within both the stove and MVHR sectors are not fully aware of this safety issue arising from such a common-place combination. Therefore, in co-operation with the Stove Industry Alliance (SIA) which represents reputable UK manufacturers and importers, Hetas has specifically developed new best practice commissioning guidelines which, for the first time will incorporate worst case scenario flue draught depressurisation tests for MVHR / stove combinations. Successful spillage tests which follow the new Hetas protocols will be the only way for the system design to comply with building regulations (eg Document J in England and Wales and Technical Booklet L in NI).

Choosing the correct wood burner to work with MVHR is critical – 95% of the stoves available are just not up to the task and the correct choice is certainly not going to be your sub-£300 builders merchant special. You must ensure that your stove features a dedicated external air (direct air) supply and also, and this is the bit that really narrows your choice, that the effectiveness of the room seal on the stove door has been subjected to the rigorous passive house DIBt (German Institute for Buildings) test. The DIBt test is the only one in the world and certifies that the stove conforms to its strict criteria for safe burning without the use of any of the surrounding room air. As part of the test the stove‘s door seal is pressure tested for air-tightness after being opened and closed for a minimum of 6,000 times and, apart from guaranteeing the quality of the room seal, it can also stand as a testament to the overall build quality of the stove. The alternative to DIBt certification is to choose a model where the manufacturer has considered the potential of MVHR room depressurisation and carried out their own appropriate tests and can provide you with written verification that their stove is capable of being fitted safely in a low air permeability building incorporating such a system.

Installers can then be confident then that CO spillage issues can be all-but eradicated with a well designed external combustion air supply linked to a correctly installed and verified room sealed appliance – but only when the stove's safe operation has been clearly demonstrated throughout the appropriate spillage and depressurisation tests. All you have to do next is ensure that everyone understands the importance of maintaining a clear stove flue as well as clean MVHR air filters.

Geoff Royle, Technical Director The Stove Yard is a founding member of the Stove Industry Alliance (SIA) and Geoff represents The Stove Yard on the SIA and Hetas Technical Committees as well as RHE/28 British Standards Committee