Viewpoint August 2012 Mechanical Ventilation with Heat Recovery: Designing and implementing a robust and effective ventilation system Dr Sarah Price and Helen Brown Summary Mechanical Ventilation and Heat Recovery (MVHR) is likely to become increasingly common in UK developments. As building regulations demand better air tightness and lower energy consumption then sufficient ventilation and recovering heat from exhausted air will become more and more important. The success of MVHR in maintaining good indoor air quality, efficiently and without undue disruption to building occupants, rests on the integrity and robustness of the design, installation and commissioning of the system. Poorly designed or maintained systems are noisy, inefficient and will not perform as intended. However, if quality control and qualification procedures are well regulated, MVHR can be used as a robust and effective ventilation system. 1 Since MVHR is a relatively new technology in the UK, we have put together some guidelines for good design and installation of an MVHR system based on our experiences and research. 1 The Robustness and Effectiveness of MVHR in Airtight Dwellings, Plujim, 2010
MVHR is already a core part of very low energy building design such as Passivhaus. 2 Well-designed MVHR systems are essential in very air tight buildings to ensure good indoor air quality 3, a comfortable draught free environment and lower energy demands. Poor ventilation can result in condensation forming on internal walls which encourages mould growth, which can cause health problems for the occupants. 4 Figure 1 MVHR functional schematic Passivhaus Institute ipha MVHR systems should ideally recover heat with efficiency in the main unit greater than 90% and the total electrical consumption of the best systems is around 0.3W/m 3 h. Filters are incorporated into the main unit and should be of sufficient grade to ensure good air hygiene; minimum filter grades of F7 on the intake and G4 on the extract are recommended. 5 The system must be sized to deliver the whole building ventilation rates required by Building Regulations 6 and air flows should be controllable at three levels; basic (70%), standard (100%) and increased (130%). It is recommended for the user controls to have a boost function for easy access to increased ventilation. What is MVHR? Mechanical ventilation with heat recovery (MVHR) is a whole building ventilation system, driven by fan power. Systems comprise a network of ducts connected to a central heat exchanger (the main unit). Stale, moist air is extracted from wet rooms and foul air spaces, heat is recovered in the main unit and then pre-warmed fresh air is supplied to habitable rooms. Air transfers between rooms via the circulation spaces (stairs, corridors etc) through gaps beneath doors or proprietary grills. The system is balanced so that the total volume of air extracted matches the volume supplied. Well designed MVHR systems will entirely comply with the regulatory requirements for whole building ventilation so that there should be no requirement for single room extract points or trickle vents in windows. 2 For more information see the Passivhaus Trust website http://www.passivhaustrust.org.uk/ 3 The Ventilation System there is no alternative, Author: Dr. Vajda József, University of Pécs, Hungary http://www.passipedia.org/passipedia_en/planning/building_services/ventilation/the_ventilation_system_- _there_is_no_alternative 4 NHBC Technical Newsletter, April 2009, Standards Extra 5 Requirements and testing procedures for energetic and acoustical assessment of Passive House ventilation systems for Certification as Passive House suitable component, November 2009 6 The Building Regulations 2010, Approved Document Part F, Means of Ventilation 2 encraft.co.uk
Noise MVHR units are not particularly noisy, but they rely on fans running continuously to push the air around the system and the speed of the fans depends on the operation mode that the user has selected. Good design, coupled with a thorough commissioning process will help to ensure noise nuisance is eliminated. Building Regulations part E 7 states that rooms for residential purposes shall be designed and constructed in such a way that they provide reasonable resistance to sound from other parts for the same building and stipulates certain performance standards for sound insulation materials used in separating walls or floors. Some social landlords or local authorities may require different maximum levels of noise depending on the use of the living area. The Passivhaus requirements 8 for noise associated with MVHR systems are <35dBA in the installation/plant room, <25dBA in living areas and <30dBA in functional areas (kitchens etc). In a domestic property it is usually sufficient to install an MVHR in a cupboard in a hallway or next to the kitchen and ensure that the door is well fitting. If the MVHR is in a room or cupboard adjacent to a bedroom, including the loft, more care must be taken and it may be necessary to improve the sound insulation in the walls or ceiling. Schools have some restrictions for sounds passage in certain types of room, however there are no regulations in all other types of commercial and industrial buildings (apart from health and safety regulations where noise levels are much higher than those that are emitted by an MVHR unit). Other factors which affect the levels of noise generated by MVHR systems include the correct sizing of the main unit (undersized units are noisy), and the correct sizing and appropriate selection of the room air valves. Condensate drain Moisture condenses when warm air cools. In an MVHR system this happens inside the main unit during the heat exchange process. All systems should therefore be fitted with a drain to remove the condensation which will inevitably form. The condensate drain should always be connected to adequate drainage and have an appropriate fall angle. It should also be insulated if outside the thermal envelope to prevent freezing. Insulation If positioned outside the thermal envelope of the building, the whole MVHR unit will need to be insulated, ideally with the same level of insulation as the building. This can have a big impact on the space needed to install the MVHR unit but is essential to ensure efficiency in the heat exchange process. Frost protection If the temperature of the intake air is subzero, frost will form inside the heat exchanger as moisture condenses and then freezes. This should be avoided by some form of active frost protection which is typically provided by direct thermostatic electric heating. Earth pipes can be a viable low carbon alternative. Some MVHR systems claim the provision of frost protection through use of variable fan speeds to avoid or reduce the intake of air when temperatures fall below zero: However this approach is not recommended in very air tight buildings because with variable fan speeds the system cannot be relied on to maintain continuously good indoor air quality. 7 The Building Regulations 2010, Approved Document Part E, Resistance to the Passage of Sound. 8 Requirements and testing procedures for energetic and acoustical assessment of Passive House ventilation systems for Certification as Passive House suitable component, November 2009 3 encraft.co.uk
Access for maintenance/repairs This is very important since the filters in an MVHR unit need to be replaced between two and four times a year. Usually this means access of at least a metre to the front of the unit. In commercial buildings, plant rooms are typically more generous and this is isn t generally a problem. However in domestic properties this can be more difficult to design, particularly for social houses where maintenance and repairs are undertaken by external contractors organised by the housing association. Figure 2 MVHR plant room with external access Geoff Carter Architects One particularly neat solution to this is to build a plant room that can only be accessed from the outside (Figure 2). It is still within the thermal envelope so the MVHR unit doesn t need to be insulated, the room is built in block work so there is no noise pollution to the rest of the house and the tenants do not need to be in when the MVHR contractor comes to replace the filters since they do not need access to the house. It is because of the maintenance requirements that the loft space is not considered an ideal location for the main unit. If at all possible, space should be found in a ground floor location, however in retrofit projects, especially for small dwellings the loft space can sometimes be the only viable option. Duct design Ducts are embedded into the building fabric and early planning at concept stage is recommended in order to inform initial ideas about layouts of the building. Duct design should aim to minimise pressure losses in order to maintain efficient operation of the fans as doubling the pressure loss will double the energy consumption. Rigid circular duct is preferred to minimise friction loss but other types of rigid duct can also work sufficiently well. Use of flexible duct should be avoided. Duct lengths Supply and extract duct lengths need to be kept to a minimum wherever possible to reduce low friction losses. Any hot air ducts outside the thermal envelope should be kept to a minimum length to reduce heat losses. Try to keep the total combined length of the intake and exhaust duct <3m by positioning the main unit on an external wall. This is especially critical to the efficient operation of the MVHR system. Filters Filters in the ducts themselves are essential where there is a lot of dust or pollutants being produced in a room, for example, kitchens require oil filters in the extract duct. Filters are also fitted to the main MVHR unit to stop pollution, pollen and other particulates entering the house. These are usually an integral part of the MVHR main unit. Be aware that filters may need to be replaced for the first time soon after commissioning since a significant amount of dust can be produced during the finishing and cleaning of a new building or retrofit project. 4 encraft.co.uk
Positioning of supply and extract valves in rooms Valves are installed at the end of ducts to regulate the flow of air into or out of a room. Correctly sized and positioned valves ensure that rooms are properly ventilated. Most rooms will require supply air, however some, such as kitchens, bathrooms and other rooms that produce some kind of pollution or vapour will need air extracted. This means that air will flow in certain directions in a building depending on how the duct work is designed. In general, supply and extract valves should be positioned away from doors. In kitchens recirculating cooker hoods are recommended to be separated from the MVHR system. This is to avoid the excessive build up of oil particulates inside the MVHR system and the extract valves should ideally be at least 2m away from the cooker. Figure 3 Three dimensional duct design Green Building Store Silencers It is recommended to fit silencers between all rooms on the supply air duct so that sound does not travel through the duct work from one room to another. At least one silencer is also required on the extract duct. In areas of high density housing, to prevent the transmittance of noise to the outside, it is also recommended to fit one silencer on the exhaust duct. Silencers are wider in diameter than the ducts and therefore have an impact on the space requirements when planning the MVHR and building design. Condensate traps Condensate traps may be required on long vertical sections of duct work to remove any moisture flowing downwards. All ductwork must be properly vapour sealed to prevent condensation forming inside the duct. Rigid or flexible duct? This is an important decision since it can have impacts on the building construction and duct installation. Rigid ducting is recommended to ensure efficient operation and is less likely to distort in shape than flexible ducting, however it may be more difficult to install, especially in a retrofit scenario. In a new build, the type of duct work chosen may affect the sequence of building construction, for example it may be that rigid ducting has to be installed as the internal floors or ceilings are constructed. Both types of duct work need to be adequately supported; flexible ductwork so that it does not sag and rigid ductwork so that the seals do not break between sections. Ductwork joints may be permanently sealed if there will be no access post construction. 5 encraft.co.uk
Position of intake and exhaust on exterior of building The intake and exhaust must be positioned on the same side of a building, i.e. they should both be facing in the same direction. This is to avoid large pressure difference (for example, due to wind) between the two which may adversely affect the performance of the MVHR. Ideally they should be 2m apart on the horizontal plane to avoid cross-flow. If necessary, it is possible to place the intake an exhaust much closer on the vertical plane if space is limited. In this case the exhaust which will be emitting warmer air should be situated above the intake which will be pulling in cooler air. Through wall penetration is preferable to through roof because if moisture condenses on a through roof exhaust there is greater risk of water running back down the duct and into the main unit. Thermal envelope and air tightness layer penetrations At some points the MVHR duct work will have to penetrate the thermal envelope of a building and the air tightness layer. This is normally at least two penetrations whether the MVHR is installed externally or internally to the building. One further penetration for the condensate drain may be required. Special rubber grommets and air tightness tape should be used to properly seal the ducts to the air tightness layer. There should be no gaps between the insulation and the duct work or this may result in thermal bypass and negate the effect of the insulation around the duct work. Insulation should continue on through wall sections. Air transfer between rooms Air is extracted from some rooms and supplied to others, in between are the circulation spaces (stairs and corridors) which do not require either extract or supply but through which air transfers in order to balance the system. Air transfer can be through gaps beneath doors or through proprietary vents/grills where sound attenuation is required. It is important to ensure air transfer apertures are of sufficient size to allow air to flow between rooms at low speeds. Air should travel at a speed no greater than 1m/s and preferably around 0.7m/s. The speed of air flow is a function of the aperture area and the balance of flow rates delivered by the ventilation system in rooms on either side of the transfer zone. Usually a 20mm undercut beneath a door (above final floor coverings) will suffice in domestic projects. Planned air tightness must be sufficient to make MVHR worthwhile If you analyse energy performance of MVHR for a dwelling in SAP it is recommended to achieve an air tightness of less than 3 air changes per hour at a pressure of 50Pa. When air tightness is worse than this the energy required to run the MVHR fans will be greater than the energy saved by the heat exchanger. However, if the decision is to be made on the energy balance alone, we recommend that you do not plan to install MVHR unless you also plan to achieve Passivhaus standards of air tightness (<0.6 ach at 50Pa). This is because achieved air tightness is often worse than planned and also because the favourable energy balance is only marginal at 3ach so the extra expense of MVHR is probably not 6 encraft.co.uk
warranted unless it is absolutely required to provide the fresh air requirements, as it is in very air tight buildings. Of course a decision to install MVHR is not only about the energy balance because other factors such as comfort, pollutants and humidity control should also be taken into consideration. The air tightness must be confirmed with at least one air tightness test. One or more tests may be required during construction so that any leaks can be dealt with, and ideally another should be undertaken post construction to confirm the final air tightness achieved. Commissioning Comprehensive commissioning of the system is vitally important to ensure efficient operation. Part of the commissioning process is to check that the system has been installed as designed. This is especially important because variations to the duct design could lead to greater friction losses and inefficiencies or noise nuisance. The commissioner could be the installer or the product supplier or an independent, acting in the client s interest. Their main task is to set and balance fan speeds in the main unit and to fix valve positions in each of the rooms, ensuring that the required flow rates are delivered to each room at each of the three main fan settings (basic, standard and increased). Ideally, fans on both the intake/exhaust side as well as the extract/supply side should be balanced. Flow rates should be directly measured in each of the rooms and directly measured across ducts using fit for purpose anemometers. The commissioner should also ensure user control settings are set up to enable ease of use for future occupants. It is advisable to label control units and make sure the user knows how to operate them. Further commissioning work will be required if heating is to be delivered via the ventilation system to ensure the successful integration of the heating and ventilation functions. About the author About the author Helen Brown is a senior consultant at Encraft and a Certified Passivhaus Consultant. Dr Sarah Price is a Passivhaus specialist at Encraft, an independent engineering consultancy specialising in energy efficiency and building-scale renewables. 7 encraft.co.uk