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  • HPH256 : What makes a well designed MVHR system? – with Tom Heywood

    HPH256 : What makes a well designed MVHR system? – with Tom Heywood

    Tom Heywood from Green Building Store explains why we need mechanical ventilation with heat recovery in high performance homes and why it needs to be well designed.

    Interview with Tom Heywood

    Tom studied architectural technology at university and was disappointed not to have been taught anything there about efficient building. So when it came to doing his dissertation he wanted to do something different, and decided to write it on Passivhaus principles.

    Tom is now the MVHR design manager at the Green Building Store.

    Airtight buildings need mechanical ventilation

    Natural ventilation tends to be unreliable. The supply levels might depend on how windy it is, with areas of the building in the path of the wind receiving more than elsewhere. There isn’t a way of delivering a particular amount according to the requirements of a specific area.

    With the increased ability to make buildings more airtight, comes the reduction of natural ventilation through trickle vents and leaky building fabric. At this point mechanical ventilation is required to provide enough fresh air to keep CO2 levels down and prevent dangerous build-ups of volatile organic compounds and other indoor pollutants.

    With a properly commissioned MVHR and an airtight envelope, you can deliver the right amount of fresh air to different spaces, ie more for a double bedroom than a single. The MVHR unit will also extract as much heat as it can from the exhaust air which will in turn drive down the heating load.

    Tom suggests that, as a rough guide, when you get to less than five air changes per hour, that’s when you should start introducing some form of mechanical ventilation, such as Mechanical Extract Ventilation (which is just extracting, not supplying or recovering any heat).

    Once you get down to three air changes per hour an MVHR starts to make sense, because then there’s not as much cooling going on with natural ventilation coming in.

    The MVHR supplies clean, fresh air

    An MVHR unit has two fans – a supply and an extract, and a heat exchanger. Stale, moist air (such as is produced in bathrooms, kitchens and utility rooms) is extracted and goes back to the heat exchanger, which is where the fans are.

    The heat is transferred to the incoming fresh air that is then supplied to the habitable rooms, such as the bedrooms and living areas. The air is also filtered, ensuring a clean, fresh air supply to the rooms.

    Concentrate on getting the design right

    “People often focus mainly on the specification of components, which is right, but at the same time you can spend as much money as you want on the specification of components; if it’s not designed right, they’re not going to work anyway. You’re going to have to turn it off because it will be too loud and it will be inefficient.“

    Rather than someone who provides a schematic of some ducting on a piece of paper, you want someone who is going to design it around the structure to make sure when you get to site that it will work efficiently and that the air supply will actually make it all the way along the ducting route to the required rooms.

    Tom says key information to ask from a designer at the design stage is for sound data and pressure loss data. Pressure loss is the resistance that the duct is providing to the airflow. If pressure loss is too high the fans will have to work harder to push the air through. And when the fans are working harder, it will increase energy consumption and the noise will be louder. Likewise, if you never change the filters, the fans will have to work harder to drive the air through.

    Tom says it’s always good to oversize the unit so that it’s not running too hard.

    He stresses how important it is to get the system designed early. Even as soon as the architects and engineers are doing their drawings you should be getting your plans submitted to a designer to get their opinion on unit location, duct sizing, where they need to run and what sized joists are needed to accommodate them.

    There are branch and radial systems

    Tom likes the branch systems (of rigid ducting) because it’s easier to get a low pressure loss with careful sizing of ducts.

    “A lot of people think that the radial system saves space, but if it’s done properly, instead of having one one-hundred mill. duct to, say, a bedroom, you’d have two ninety mill. ducts. Because those ducts are carrying the same amount of air but they’re smaller, so there needs to be more of them to compensate for that.”

    A radial system, also called an octopus system, has a main manifold at the start which is a box with lots of small spigots on it. The semi-rigid plastic radial ducting plugs into that and goes off to the rooms. Each room would have its own branch (or two or three), all the way from the manifold at the start to the air valve in the room. It is a good system for preventing any cross-talk sound travelling from room to room because the sound can’t make it all the way along the ducts.

    Tom believes radial systems do have their uses, especially in retrofits where the rigid ducts of a branch system would just be too big to get in.

    Whatever system is used, Tom stresses that the component parts should be on site when the joists are being put in place, so they can be installed as the joists are being erected.

    Commissioning a system takes a lot of toing and froing!

    Tom explains the method of commissioning a branch system. The unit is told what it is running at, eg 300m³ per hour on fan speed 2. If the supply to a master bedroom was to be 35m³ per hour then they would go into that bedroom and adjust the damper to allow more or less accordingly until it was at the required amount. An anemometer over the air valve measures how much air is coming out.

    But as that valve is closed down it will drive the air into another air valve somewhere else. So the next room would need to be checked to see if it has been impacted and needs adjusting. Any changes to one mean going back and checking all the others again, as it’s all on one system.

    Carefully consider external vent placement

    Ideally you want the intake and exhaust ducts on the north elevation. That way they’ll be shaded from the sun in the peak summer heat. The system will work more efficiently because you’re bringing in cooler, shaded air.

    You should also avoid placing them right next to a window or facing onto a neighbour’s garden, because they could be blowing quite hard and at audible levels.

    Too close to the ground and they could drag up leaves, debris and animal hairs into the intake duct, which will block up the filters more quickly. Ideally they should be 1.5m above ground level.

    If incoming smoke from neighbouring properties is a problem, then you can get active carbon filters which filter out the smell and smoke even better than the F7 pollen filters would.

    The controls should rarely need using

    The idea of the controls is to have them as simple as possible and generally left running on fan speed 2. They may only really need changing if you’re going to be away from the house for a long time, in which case you could drop it down to fan speed 1. And if you have a large gathering and are going to be doing lots of cooking, then hit the ‘party button’ which increases it to fan speed 3 for 45 minutes, before automatically dropping back down to fan speed 2.

    Generally there would be one main controller which would usually be in the kitchen for convenience to boost it if required. Alternatively you could have the main controller somewhere else with just a remote boost switch in the kitchen. Tom doesn’t recommend having it in a bathroom as boosting the system while showering early in the morning and late at night could become audible in the bedrooms and disturb the occupants. The fan is designed anyway to remove enough moisture whilst showering without the need to boost.

    Only a small amount of maintenance is required

    There shouldn’t really be too much maintenance required, other than changing filters every three to six months (depending on indoor and outdoor air quality, and whether you have children and pets).

    After five years it might be worth unscrewing the air valves in the bathroom and giving them a clean as they could collect dust and towel lint over time.

    And depending on the MVHR unit specified, the extract fan may need very careful cleaning, preferably by someone who has done it before.

    If the unit is turned off for more than two or three days there could be a risk of mould growth in the ducts. There shouldn’t however be any danger of that happening if the system is left on and each branch to each room has air flowing through it.

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    Transcript

    Download a transcript of the interview with Tom Heywood.

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  • HPH212 : What challenges does an ‘L’ shape Passivhaus present? – with John Fenwick

    HPH212 : What challenges does an ‘L’ shape Passivhaus present? – with John Fenwick

    John Fenwick discusses the challenges of building his award-winning Passivhaus with an atypical form.

     

    Interview with John Fenwick

    It was around eight years ago that John and Jeanette Fenwick sold their house in order to retire to the West Highlands of Scotland. The idea for a self build only came about when they couldn’t find anything on the market that they wanted to move into. The large plot they bought looks out onto Ben Wyvis and was originally crofting land that was overgrown with birch, and quite boggy in places.

    Their intention had been to build a kit home but weren’t sure how they would get a builder for it or how to take it to planning. It wasn’t until they attended a Housing Expo in Inverness in 2010 that they came across Passivhaus buildings. After speaking with the consultant there, and later the architect that had designed the Passivhaus Terrace for the Expo, they decided that this would be the route forward.

     

    A modern interpretation of a traditional style

    Key to their plans for Tigh na Croit was a single storey dwelling that would reflect the traditional steading style of building in the area. The ‘L’ shape of the design came naturally with one length of the house incorporating daytime accommodation and the other length housing the bedrooms.

     

    Getting the ventilation strategy right was the biggest challenge

    John explains that ideally an MVHR unit would be at the centre of a house so that the system is balanced, with everywhere being treated equally when it comes to ventilation, heat recovery and heat distribution. In Tigh na Croit, the ventilation equipment is at the corner of the ‘L’. John has installed sensors so is aware of the temperature differential between the recovered heat that is distributed closest to the ventilation equipment (30-35 degrees), and those rooms at either end of the building (23-24 degrees). To get through the harsh winters the living room at one end of the ‘L’ has a Passivhaus certified stove, and the main bedroom at the other end of the house has a heated mirror-like far infrared panel. Together with a water-based post heater supplied by an air-source heat pump, they provide the main heating to the house. Additionally the air-source heating circuit powers three small towel rails in the bathrooms.

     

    Tigh na Croit uses a Passive Wall system of construction

    OSB panels are pre-filled with insulation, pre-membraned and put together like a timber frame to create the airtight envelope. The finish gives clean lines inside the house and with three double-height spaces within the floor area they were able to achieve an attractive effect using the closed panels for the walls and for the roof.

     

    Frequent site visits would have been advantageous

    Building in the Highlands anyway was a big challenge with there not being a great many builders to choose from, and the fixed-price contract deterred a few from tendering. One local company agreed to take on the contract and sub-contracted out many of the trades.

    As they were living 300 miles away, they employed the architect on a traditional contract where he would visit the site once a month.

    “We thought once a month would be sufficient, but clearly, when you’re building a house using principles perhaps not familiar to the builder, I think being on site for the build would have been hugely advantageous. The build wasn’t without its issues, let us say.”

    Being able to communicate with the builder and know which days he had been on site was very difficult to monitor from such a distance away. With the builder invoicing monthly, and John only able to visit every six weeks or so, he would sometimes arrive wondering what work had actually been carried out since his last visit.

    The contractor’s lack of attention to detail and knowledge of what was required to achieve the Passivhaus standard meant there were some frustrating delays.

    “So, the first air test was actually a waste of time apart from telling us really that the builder hadn’t understood that attention to detail as far as taping and stuff like that, was absolutely paramount.”

    They found a fist-sized hole where air was getting in and in areas where airtight foam had been specified, the builder had used standard building foam because he hadn’t been able to get the correct one at the local builders’ merchants.

     

    The harsh climate is a challenge even for a Passivhaus

    John and his wife did debate whether or not to install underfloor heating, and believe they made the right decision not to go ahead with it. Being a Passivhaus would have a beneficial effect in that the underfloor heating wouldn’t be worked as hard as in a standard home, but the problems arise when the house is suddenly flooded with sunshine and can become difficult to cool down, as underfloor heating takes a few days for the effects to wear off.

    Even being a Passivhaus there are times when the heating needs to have a bit of a boost. This is particularly the case if the house hasn’t been occupied for a period of time. Generally though they manage to maintain a temperature of around 18 degrees which, without drafts, they find comfortably warm.

    Perhaps the one thing they would change if they did it again would be their bedroom window. It’s a very large picture window that is triple-glazed, but despite the quoted u-values, isn’t as insulated as a wall would be so does feel cool and will inevitably lead to a temperature decrease in the room. Finding blinds to fit it has also been one of the biggest challenges they’ve faced!

    Copyright HLM Architects

    John thinks the house naturally settles at a temperature of around 15 degrees warmer than outside.

    “So, if it’s minus eight outside and you’ve got a mechanical ventilation running, the air that’s coming in is at minus eight whatever happens. You can put a defroster on the system to make sure that the whole system doesn’t freeze up, but you’ve still got pretty cold air actually going into the mechanical unit. Five degrees is often the air going in, and you’ve only got so much heat from the extracted air mixing with that. So, the colder it is outside, the more challenging it is for the house full stop.”

    John has experimented with switching the MVHR unit off overnight in the winter and found that it doesn’t make a great deal of difference.

    “You have to be aware that if the temperature goes up a few degrees outside, you do notice the difference inside. So, the house reacts to the temperature outside, there’s no doubt about it. And where we are, the challenges are pretty extreme really.”

     

     

    Find out more

    Visit the website of HLM Architects.

     

    Transcript

    Download a transcript of the interview with John Fenwick.

     

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  • HPH147 : A Passivhaus standard Q & A – with Elrond Burrell

    HPH147 : A Passivhaus standard Q & A – with Elrond Burrell

    Architect Elrond Burrell provides answers to a few common questions that people have about the Passivhaus standard.

    elrond-2-v-2

     

    Interview with Elrond Burrell

    As part of Ben Adam-Smith’s video work, podcasts and meeting with other self builders, he is often asked questions about the Passivhaus standard that he doesn’t know how to answer. He recently took the opportunity to put those questions to Elrond Burrell.

    Born in New Zealand, Elrond has been working in the UK for the last 18 years. As well as being a qualified Architect and Associate at Architype, he writes a plain speaking and successful blog about Passivhaus, which has a global following. Before he returns to New Zealand we caught up with him to tap into his Passivhaus expertise, and also find out what his plans are for the future.

     

    Is it necessary to have a heating system in a small Passivhaus?

    This will depend on the specific heating demand of the project. It is possible on a very small project to get away with just a portable electric heater, bathroom heated towel rail or similar.

    A risk of doing this is that if the house is sold or has different occupants they might not be fully aware of this and find that they do get a little cold on rare days of the year. Elrond suggests a more robust approach is to just put in a very small heating system.

    Another thing to consider is that heating is only one part of the equation in terms of power consumption, and actually in a Passivhaus building, providing the hot water can quite often be a bigger heating load than providing the space heating.

    “And so you tend to design a system that can provide all the hot water needs for the year, and have enough capacity in that to provide the small amount of heating that you might need in the cold days of the year.”

     

    Why might you not want the heat source to be incorporated into the ventilation system?

    Firstly, air only has the capacity to carry a certain amount of heat.  Water and solid surfaces can carry and radiate much more heat than air can. If you need more heat than the air can provide then you can’t do it all through the ventilation system.

    Secondly, you need fresh air all of the time! In order to guarantee good quality, fresh, filtered air all year round, then you need the ventilation system running all the time. You don’t necessarily want the heating system running all the time though. There may also be times when you want more heating than the ventilation can provide. Although there is some logic in combining them, they are two quite separate systems in a way.

     

    To avoid overheating, can you integrate a cooling system into the house?

    The Passivhaus standard allows for 15kW annually of heating per m², and likewise allows for the same of cooling if required. In a relatively mild climate like the UK you shouldn’t need cooling if you’re designing the building well. In much warmer climates you would expect that in a very efficient Passivhaus you may have a small amount of cooling, the same way you have a small amount of heating. Like the heating, it can be provided by the ventilation system or separately through a radiant or other cooling system.

    In a Passivhaus project you wouldn’t put something in as a ‘just in case’ measure, because using the Passive House Planning Package (PHPP) software you can very accurately design and predict how the building will perform.

    “So you shouldn’t be putting in stuff just for the eventuality that it might be needed and you should be designing it to get it so you don’t have that need.”

     

    Does it make sense to have mechanical blinds to give you a sense of control?

    More commonly used in Europe and the US than they are in the UK, Ben says that some of the people he comes across like to have the mechanical blinds to give them an extra bit of control. Elrond suggests that it is important for people to feel in control of their environment and that they can modify and change things if they wish to. They shouldn’t feel that they’re living in a house which they’re not allowed to touch or interact with, so if people feel strongly about having the blinds then they should go with it.

    Something to be wary of is if they’re built within the window it can be an expensive operation to fix or replace them if something goes wrong.

    “So it’s not something specific to Passivhaus but it’s more the approach a lot of people that come to Passivhaus want to take is really simplification and trying to avoid moving parts and things that might go wrong at a later stage.”

    The power consumption and how they get detailed and built into the building will also need to be accounted for in the Passivhaus modelling.

     

    How adaptive can the house be in terms of comfort?

    Regarding temperature, this will come partly come down to the design of the building. The upstairs rooms are likely to be a little warmer so personal preference about whether to sleep in a cooler or warmer room may be a contributing factor to the design. And if heat is being delivered by the ventilation system then wherever the air is being delivered is going to be warmer than where it’s being extracted from. If you’re heating through a radiant rather than ventilation system, then these can be located in areas where you would want to feel warmer.

    Another thing about adaptive comfort, which is in the Passivhaus Institute guidance, is ensuring that all occupied rooms have got windows that can be opened.

     

    Future Projects

    When Elrond returns to live in New Zealand in the near future he will be continuing with his blog at elrondburrell.com. He has also set up a new company, VIA architecture, and will be offering a ‘PHeasibility’ service.

    It is important to Elrond to embody his own personal values in the work he does. “So not just what looks like sustainability but what really counts in the details and in the putting it all together.” VIA architecture is based on the idea “that particularly using the Passivhaus standard we should be able to use architecture to transform the quality of peoples’ lives and so it’s really a process of using architecture to get to a good outcome. Whether it’s a really healthy, comfortable home, or a school or a business or what-have-you. And so architecture is kind of the process to get there rather than the end product in itself.”

    The PHeasibility service involves Elrond taking early designs and running them through the PHPP in a basic way, before a commitment to employ a Passivhaus designer has been taken. It will give an indication for self builders or architects without Passivhaus experience, as to what might be involved, taking into account constraints and design, for the building to achieve the Passivhaus standard. It will look at the key aspects which determine if the fabric, orientation, windows etc are going to be of the required standard. They will then be better informed as to whether to pursue the certification.

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    Find out more about PHeasibility

    Visit the PHeasibility website

    Special offer: subscribers to Elrond’s blog can get Pheasibility for £495 instead of the standard £745 if they book it in September or October. To subscribe to the blog (direct link to subscribe: http://elrondburrell.com/hph_subscribe) and then send an email to PHeasibility@elrondburrell.com

     

    Find out more about VIA Architecture

    Visit the VIA Architecture website

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    Relevant Articles

    5 things to know about Passivhaus

    Passivhaus Overheating: Design it out

    Passivhaus: Solar Orientation Doesn’t Matter. Or Does It?

     

    Transcript

    Download a transcript of the interview with Elrond Burrell.

     

    The Hub update

    The Hub has now been going for a year and we’d like to thank the founding members who have put their faith in us. We’re always looking at ways to improve what we are offering and in the coming months we’ll be trying to make the modules even more step by step and easy to follow.

    Our second case study is now under way, the Buckinghamshire Passivhaus, headed up by architect Janet Cotterill from CTT Sustainable Architecture. The project first started out as a retrofit so find out why it didn’t continue that way, and went on to be a new build.

     

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