MVHR units by Helen Richards
Picture: Helen Richards
Helen Richards award winning architect and founder of ‘Powered Living’, www.poweredliving.co.nz, is currently undertaking a Masters degree in Sustainability from, ‘The Centre for Alternative Technology’, Wales, United Kingdom. Helen has kindly given one of her papers to be used on the Solarei Blog, Sun Path, in particular regards to Passivhaus, MVHR units and CO2 build up.
How difficult is it to achieve MVHR unit success in energy efficient homes in the UK?
An example of successful PassivHaus retrofit with MVHR; Tevesstrasse project, Germany. Before (left) and after (right). Images: PassivHaus Institut.
With the UK Building Stock currently accounting for around 40% of national CO2 emissions (MacKenzie, 2010) of which 30% and rising, is attributable to the domestic housing stock (DECC, 2010) there is urgent need to understand the complex nature of how our buildings consume energy (Oreszczyn & Lowe, 2010) (Zero Carbon Hub, 2013). Domestic space heating contributes up to 60% of this 30% energy emissions load (Hamilton et al., 2013) and it is therefore a continued regulatory goal to decrease this demand (Oreszczyn & Lowe, 2010). Ventilation heat loss through the external building envelope in a typical unimproved UK home can be approximately equal to heat loss through conduction (Banfill et al., 2011) and reducing natural air infiltration (measured as external surface permeability in m3/hr/m2) or increasing air tightness (measured as a volume; air changes/hour) in both new and retrofitted buildings is a priority. As homes become increasingly airtight MVHR (Mechanical Ventilation Heat Recovery) units become more heavily relied upon for providing healthy air exchange. These units are becoming more popular than conventional air exchange MEV (Mechanical Extract Ventilation) units as they are designed to recover heat from exhaust air with the intended result a reduction in the home heating load. However, research in the field shows definite concern in regard to the real success of MVHR technology in UK dwellings and this Blog aims to illustrate and discuss a variety of these findings.
‘A breath of fresh air’ www.building.co.uk MVHR system.
In recent research, White et al (2015) clarified that while MVHR is standard practice in parts of the USA and Europe, it has only in recent years been introduced to the UK (Balvers, 2012). The German PassivHaus approach which includes MVHR as an integral part of design methodology, is also in early stages in the UK with around 200 of a total 37,000 worldwide certified buildings and is fundamentally under-researched for the UK climate (McGill et al., 2014).
Banfill et al (2011) modelled air permeability in relation to MVHR performance success for the UK E-on retrofit house and concluded that, in spite of increases in both auxiliary power and building air change rate when the MVHR was in operation1 it was overall more successful with lower air infiltration rates (Banfill et al., 2011). For best practise MVHR and permeability rate (3m2/hr/m2) they found an annual saving of 10kWh/m2 in comparison with 10m2/hr/m2 (current UK Building Regulations).
Both papers noted the extensive work required for retrofitting to achieve lower air permeability rates mainly associated with siting ductwork. The thermal modelling of El Fouih et al (2012) found that MVHR in small best practise energy efficient homes in moderate, for example British, climates can save 10kWh/m2 over MEV units. This accurately reflects the findings of Banfill et al and would equate to 1MWh annual energy reduction (El Fouih et al., 2012).
1 White et al (2015) back this up with further monitored E-on house research.
MVHR also needs to be clearly understood in terms of success for managing Indoor Air Quality (IAQ). McGill et al (2014) monitored recordings for 24 hours in different seasons at three adjacent UK social housing Passivhaus homes with MVHRs and found elevated CO2 levels over 1000ppm recorded in all Living rooms in both winter and summer – even with windows open (McGill et al., 2014). Derbez et al (2014) completed similar but more extensive recordings on seven distinct new energy efficient homes also demonstrating high levels of airtightness and found half the houses showed unacceptable peak levels of CO2 of around 2000ppm. Internal air temperatures also reached the late twenties in summer (Derbez et al., 2014). Both papers conclude that MVHR units are providing inadequate ventilation.
The Zero Carbon Hub ventilation task group reviewed the performance of a range of built projects in the UK and were disheartened to conclude significant weakness in current MVHR practice for design, installation, commissioning, operation and maintenance, resulting in unrealised potential for poor IAQ and energy underperformance. Common issues included poor ductwork and unit installation, poor understanding of operational controls, poor positioning and balancing of supply and extract vents and filters not being cleaned/changed (Zero Carbon Hub, 2013) http://www.zerocarbonhub.org/resources/reports/mechanical-ventilation-heat-recovery-new-homes-final-report. These conclusions reflect findings in previous papers (McGill et al., 2014) (Derbez et al., 2014) (White et al., 2015). On a positive note, the task group found that 14 Wimbish UK PassivHaus homes were reaping rewards from good design and commissioning (Zero Carbon Hub, 2013) and it is not hard to source other well illustrated and successful examples mainly from private PassivHaus homes (Cotterell & Dadeby, 2012) (Bere, 2013) and it is stated that MVHR systems are ‘simple’ and rely on ‘only a few working parts’ (Cotterell & Dadeby, 2012 p.128) but also that unit efficiency is critical and “strongly influenced by the layout and design of the ducting” (Cotterell & Dadeby, 2012, p. 204).
There are undeniable issues with MVHR performance illustrated and cross referenced across a variety of research papers for energy efficient homes in the UK. It is apparent that both further research and competency is required in all areas of the industry from design, installation, commissioning and operation through to maintenance in order for MVHR success; energy benefit and healthy Indoor Air Quality to be realised. The research evidence indicates that MVHR technology is complex with large potential to go wrong (Zero Carbon Hub, 2013) (White et al., 2015) (Balvers, 2012) with the end effect of poor IAQ (McGill et al., 2014) (Derbez, et al., 2014) but there is evidence to show it can be successfull (Zero Carbon Hub, 2013) (Cotterell & Dadeby, 2012) and energy savings can certainly be achieved (Banfill et al., 2011) (El Fouih et al., 2012) but this is more likely in the minority of applications where those installing and operating the system have both the required level of knowledge and commitment. More research, education and application is needed before MVHR can be applied successfully to homes in the UK.
Balvers, J. (2012) ‘Mechanical ventilation in recently built Dutch homes: technical shortcomings, possibilities for improvement, perceived indoor environment and healthy effects’, Architectural Science Review.
Banfill, P., Simpson, Sophie, Gillott, Mark and White, Jennifer (2011) Mechanical ventilation and heat recovery for low carbon retrofitting in dwellings. Available at: http://www.researchgate.net/profile/Phillip_Banfill/publication/267946090_Mechanical_ventilation_and_heat_recovery_for_low_carbon_retrofitting_in_dwellings/links/54b942830cf253b50e291889.pdf.
Bere, Justin (2013) An Introduction to Passive House. RIBA Publishing.
Cotterell, J. and Dadeby, A. (2012) Passivhaus Handbook : A practical guide to constructing and retrofitting buildings for ultra-low-energy performance. UIT Cambridge Ltd.
DECC (2010) Department for energy and Climate Change, 2008 Greenhouse Gas emissions Final Figures by End User.
Derbez, M., Berthineau, B., Cochet, V., Lethrosne, M., Pignon, C., Riberon, J. and Kirchner, S. (2014) ‘Indoor air quality and comfort in seven newly built, energy-efficient houses in France’, Building and Environment, 72, pp. 173–187. doi: 10.1016/j.buildenv.2013.10.017.
Derbez, M., Berthineau, B., Cochet, V., Pignon, C., Ribéron, J., Wyart, G., Mandin, C. and Kirchner, S. (2014) ‘A 3-year follow-up of indoor air quality and comfort in two energy-efficient houses’, Building and Environment, 82, pp. 288–299. doi: 10.1016/j.buildenv.2014.08.028.
El Fouih, Y., Stabat, P., Rivière, P., Hoang, P. and Archambault, V. (2012) ‘Adequacy of air-to-air heat recovery ventilation system applied in low energy buildings’, Energy and Buildings, 54, pp. 29–39. doi: 10.1016/j.enbuild.2012.08.008.
Hamilton, I. G., Steadman, P. J., Bruhns, H., Summerfield, A. J. and Lowe, R. (2013) ‘Energy efficiency in the British housing stock: Energy demand and the Homes Energy Efficiency Database’, Energy Policy, 60, pp. 462–480. doi: 10.1016/j.enpol.2013.04.004.
Haynes, D., Farr, Andrew and Hill, Frances (2013) ‘The interaction between domestic occupants and Mechanical Ventilation and Heat Recovery (MVHR) system performance’. Available at: http://nceub.org.uk/mc2013/pdf/mc2013-p03_haynes_deborah.pdf.
MacKenzie, F. P. (2010) Energy Efficiency in new and existing buildings Comparative costs and CO2 savings. BRE Trust.
McGill, G., Qin, M. and Oyedele, L. (2014) ‘A Case Study Investigation of Indoor Air Quality in UK Passivhaus Dwellings’, Energy Procedia. (6th International Conference on Sustainability in Energy and Buildings, SEB-14), 62, pp. 190–199. doi: 10.1016/j.egypro.2014.12.380.
Oreszczyn, T. and Lowe, R. (2010) ‘Challenges for energy and buildings research: objectives, methods and funding mechanisms’, Building Research & Information, 38(1), pp. 107–122. doi: 10.1080/09613210903265432.
White, J., Gillott, M. C., Wood, C. J., Loveday, D. L. and Vadodaria, K. (2015) ‘Performance Evaluation of a Mechanically Ventilated Heat Recovery (MVHR) System as Part of a Series of UK Residential Energy Retrofit Measures’, Energy and Buildings. doi: 10.1016/j.enbuild.2015.09.059.
Zero Carbon Hub (2013) Mechanical Ventilation with Heat Recovery in New Homes – Final Report. Available at: http://www.zerocarbonhub.org/resources/reports/mechanical-ventilation-heat-recovery-new-homes-final-report (Accessed: 14 November 2015).