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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorCranbrook
    • CommentTimeJun 23rd 2020
     
    Evening all..

    As some may have noticed from some posts of mine a few weeks back, im currently in the process of extending my dormer bungalow. The extension consists of a ground floor masonry section (filled cavity wall), and above that it is timber frame, that has external EPS insulation (to be thin coat silicone rendered), with a warm flat roof.... Anyway, due to the good anticipated air tightness and u values that the extension should achieve, I feel like i'd be missing a trick not to put in some form of MVHR - but my room layout is making life difficult for me so i'm looking for a magnitude of advice it seems...

    The upper floor of the extension will consist of a master bedroom and open plan bathroom in one, but there is a step down to the actual bathroom section of the room. The bathroom section I intend to install wet UFH. So my first dilemma is that do I treat this bedroom/bathroom as an extract or not?

    The ground floor of the extension will be split 75:25, 75 being the garage, and 25 being a utility room. I am possibly going to have wet UFH in the utility room and this room could be home to the MVHR unit.. Can I still extract from the same room if I decide to put the unit in here? I would dry clothes in this utility room so i'm guessing this would be good to extract from?
    •  
      CommentAuthordjh
    • CommentTimeJun 23rd 2020
     
    Posted By: CranbrookThe bathroom section I intend to install wet UFH. So my first dilemma is that do I treat this bedroom/bathroom as an extract or not?

    You need to balance the supply flow rate with the extract rate, at least at an overall total level; and think about the air movement that is implied by the position of the supply and extract terminals. So if it was me, I would put an extract terminal in the bathroom section and a supply terminal across the other side of the bedroom.

    I am possibly going to have wet UFH in the utility room and this room could be home to the MVHR unit.. Can I still extract from the same room if I decide to put the unit in here? I would dry clothes in this utility room so i'm guessing this would be good to extract from?

    There's no problem with having an extract terminal in the utility room as well as the MVHR unit. In fact that's what I've done (except that I haven't fitted an actual terminal, just partially taped over the end of the duct to get the correct extract rate, since mine is in a plant room that is only otherwise used for storage).

    Do you plan on adding MVHR terminals in the existing part of the house too? I would recommend doing that as well if possible, to improve the air quality. It also means you would have somewhere else to fit a supply terminal matching the extract in the utility.
    • CommentAuthorCranbrook
    • CommentTimeJun 23rd 2020
     
    Posted By: djh
    Posted By: CranbrookThe bathroom section I intend to install wet UFH. So my first dilemma is that do I treat this bedroom/bathroom as an extract or not?

    You need to balance the supply flow rate with the extract rate, at least at an overall total level; and think about the air movement that is implied by the position of the supply and extract terminals. So if it was me, I would put an extract terminal in the bathroom section and a supply terminal across the other side of the bedroom.

    I am possibly going to have wet UFH in the utility room and this room could be home to the MVHR unit.. Can I still extract from the same room if I decide to put the unit in here? I would dry clothes in this utility room so i'm guessing this would be good to extract from?

    There's no problem with having an extract terminal in the utility room as well as the MVHR unit. In fact that's what I've done (except that I haven't fitted an actual terminal, just partially taped over the end of the duct to get the correct extract rate, since mine is in a plant room that is only otherwise used for storage).

    Do you plan on adding MVHR terminals in the existing part of the house too? I would recommend doing that as well if possible, to improve the air quality. It also means you would have somewhere else to fit a supply terminal matching the extract in the utility.


    It may be worth adding at this isn't that my longer term plan is to add a new separate extension to the house, which will be a single storey extension with a pitched roof that will become the new kitchen which will also probably have wet ufh..

    But in answer to your question, yes I would want to extract from a dedicated shower room in the main part of the house, and I can also extract from the existing kitchen (and eventually swapping that for the new kitchen). I have a separate living room and also 3 other bedrooms that would/could be supplied. My potential issue with the existing part of the house is that I currently have an uninsulated cavity on some external walls, which I'm guessing isn't great with mvhr
    •  
      CommentAuthordjh
    • CommentTimeJun 24th 2020
     
    MVHR doesn't care how much insulation you have, but it does make more sense (in a dollars-saved way) the more airtight your house is. The improvement in air quality doesn't really care either about air tightness.

    One big issue to think about is where you will route ducts. It can sometimes be quite difficult in an existing building.
    • CommentAuthorCranbrook
    • CommentTimeJun 24th 2020
     
    Posted By: djhMVHR doesn't care how much insulation you have, but it does make more sense (in a dollars-saved way) the more airtight your house is. The improvement in air quality doesn't really care either about air tightness.

    One big issue to think about is where you will route ducts. It can sometimes be quite difficult in an existing building.


    I have eaves space with it being a dormer bungalow and also high ceilings in the extension where I can utilise bulk heads so the duct runs should be manageable.. question is how do I determine what size ducting to use?
    • CommentAuthortony
    • CommentTimeJun 24th 2020
     
    Depends on flow, I use lower flows than most and think 150mm main duct with 100mm branches

    Best to use steel spiral ducts or round plastic ones if you can’t face installing steel, fittings are push fit but I screw and tape mine
    • CommentAuthorbarney
    • CommentTimeJun 24th 2020
     
    Be careful about linking rooms with ductwork - a duct is incredibly effective at transmitting sound so use either attenuators or run every room back to a header (preferably acoustically lined) near the MVHR unit.

    Don't be fooled into thinking that sound won't travel against the direction of airflow

    Basically though, work out what the relative room terminal needs to deliver (ideally in litres/second), pick a limiting pressure drop (say 0.5Pascals/m) and pick a limiting velocity (keep this low as velocity generated noise can be significant (say 1.0 metre/second).

    Google "Ductulator" and plug the numbers for each section into the many online duct sizing calculators to read off a duct size.

    As Tony suggested, use spiral wound metal or PVC ducting - try to avoid cheap "flexible ducting" - it truly is awful for anything other than awkward final connections, and even then I would avoid it. Keep it well supported, and insulate it if run outside the heated space.

    Be a bit creative about where you look for MVHR - some of the domestic units are fearsomely priced and are really rubbish quality - you'll get a far better quality if you look at small commercial heat recovery air handling units (as an example, those intended for single classroom ventilation or similar)

    Regards

    Barney
    • CommentAuthorbhommels
    • CommentTimeJun 24th 2020 edited
     
    Agree with all the advice above. In addition: think of a whole-house air flow plan, so that all spaces see some airflow, including landings, staircases etc.
    And I would definitely put bulkheads/terminals/branches in to anticipate having MVHR in the extension.
    I have just finished the MVHR retrofit in my small house, and installing it was a pig!

    FYI I did not do the calcs myself but went with the GBS design service instead. This works fine, but since the drawings I supplied to their designer were not super accurate, I diverted quite a bit from their design as I tried to fold the more complex joints in as little space as possible. Note that MVHR units have a condensate drain that needs to discharge somewhere without freezing, so if you can route it inside the thermal envelope, you probably should.
  1.  
    For retrofitting our old house, we are going for several small units rather than one big one, because:

    - several small units are much cheaper than one big one,
    - each unit can be positioned in different places around the house, so manifolds and duct runs are not required, we don't really have a good route available
    - they can be added one-by-one to fit around other work we are doing and we will add more or less capacity depending how we get on.

    Newbuild regs in England push you to install quite big capacity units with big ducts, several people have mentioned installing a big one then using only a low% of its capacity. For retrofitting our existing place we calculated the capacity we will actually use.
    • CommentAuthorgyrogear
    • CommentTimeJun 24th 2020 edited
     
    Posted By: bhommelsAgree with all the advice above. In addition: think of a whole-house air flow plan, so that all spaces see some airflow, including landings, staircases etc.


    +1

    WHF = whole-house fan... turns the whole place into an ''extended plenum'' and insufficienlty exploited in my opinion/experience !

    DANE = Ducts are A Necessary Evil !

    gg
    •  
      CommentAuthordjh
    • CommentTimeJun 24th 2020
     
    As you can see, Cranbrook, yours is not a straightforward question. Clearly we don't all agree on here where to start :bigsmile:

    You pays your money and you takes your choice.
    • CommentAuthortony
    • CommentTimeJun 24th 2020
     
    I chose to pay big money and have a Genvex, I have installed a budget MVHR for others pm me if you want to know the brand
    • CommentAuthorCranbrook
    • CommentTimeJun 24th 2020
     
    Thanks for all the information, and yes ive also considered going down the route of multiple smaller units which could be helpful when I eventually extend again and relocate the existing kitchen..

    Some issues I have thought about today:

    My dedicated shower room has no heating/radiator as it is literally a shower tray with stud walls (boarded and tiles as well ha) built around it and a glass screen in front of the normal internal door... is this still then worthwhile as using for extract?

    I have a separate room for the toilet and basin (in the extension) that I will put a towel rail in, but as im guessing there won't be too much moisture here, am I correct in thinking that this isnt suitable to extract from?
    • CommentAuthortony
    • CommentTimeJun 24th 2020
     
    Please use a tray with upstands and wetwall or similar panels = fail safe
    • CommentAuthorbhommels
    • CommentTimeJun 24th 2020
     
    Posted By: CranbrookThanks for all the information, and yes ive also considered going down the route of multiple smaller units which could be helpful when I eventually extend again and relocate the existing kitchen..

    Some issues I have thought about today:

    My dedicated shower room has no heating/radiator as it is literally a shower tray with stud walls (boarded and tiles as well ha) built around it and a glass screen in front of the normal internal door... is this still then worthwhile as using for extract?

    I have a separate room for the toilet and basin (in the extension) that I will put a towel rail in, but as im guessing there won't be too much moisture here, am I correct in thinking that this isnt suitable to extract from?


    Both should be used as locations for extract points in the whole house airflow plan. You don't want the moisture or smells to be wafting randomly through your very airtight house.
    •  
      CommentAuthordjh
    • CommentTimeJun 24th 2020
     
    Posted By: CranbrookMy dedicated shower room has no heating/radiator as it is literally a shower tray with stud walls (boarded and tiles as well ha) built around it and a glass screen in front of the normal internal door... is this still then worthwhile as using for extract?

    We have an extract terminal above the shower tray in our ensuite. It's about 2.2 m above the floor, the shower panel is 2 m and the shower is walk-in, with no door on the shower itself. The ensuite room is something like 2 m x 2 m and 3 m high. I sometimes find the air extraction gives me a cold back in the shower, though that's a feature rather rather than a bug at the moment. But it does keep the room pretty much free of condensation.

    I have a separate room for the toilet and basin (in the extension) that I will put a towel rail in, but as im guessing there won't be too much moisture here, am I correct in thinking that this isnt suitable to extract from?

    It depends what other plans you have to extract smells from the WC area?

    Building regs requires adequate ventilation in both these areas. MVHR extract from both will help meet the regs, otherwise you will need fans and/or opening windows.
    • CommentAuthorMike1
    • CommentTime7 days ago edited
     
    When choosing the MVHR unit, the Passivhaus Institute have a listing of those that they have independently assessed at https://database.passivehouse.com/en/components/list/ventilation_small which can help identify units that might suit (based on capacity, efficiency, noise, etc) though these are unlikely to be cheap. There are cheap units available, but the manufacturer's quoted figures are not necessarily as robust or as easily compared.

    As others have mentioned, the aim is to be running the unit on a low (and therefore quiet) setting in normal use, so that there is capacity to boost the flow when taking showers or you have a house full (there should be 3 or 4 settings).

    Other considerations are whether you might want a unit able to provide comfort cooling in summer (not many can do that), the summer bypass control method (based on temperature only / temperature and humidity / external + internal temperature & humidity), and the installation of additional pre-filters between air intake and the unit (e.g. course bag filter to remove general crud / NoX filter in urban areas).

    The best software I've found for DIY design (should you wish to attempt that) is the Aldes concepteur. It's available from a French website - https://services.aldes.com/logiciels/logiciel/conceptor-ventilation (see the green 'Telecharger le logiciel' button) but you can select to run it in English, and it's free & pretty comprehensive. The main downside is that it doesn't have an option to select English building regs (only French, German, Spanish, Italian Belgian, Netherlands or 'export'); not sure how close the English regs are to any of those. I'll have to find a comparison as I have a UK system to design, probably next year.
    • CommentAuthorCranbrook
    • CommentTime7 days ago
     
    Posted By: tonyPlease use a tray with upstands and wetwall or similar panels = fail safe


    Im a bathroom fitter by trade and the amount of times I get phonecalls to repair the leaks from poorly installed wet areas is crazy, so many 'tradesmen' ignore the importance of tanking and good sealing! But I appreciate the heads up all the same


    I have a separate room for the toilet and basin (in the extension) that I will put a towel rail in, but as im guessing there won't be too much moisture here, am I correct in thinking that this isnt suitable to extract from?

    It depends what other plans you have to extract smells from the WC area?

    Building regs requires adequate ventilation in both these areas. MVHR extract from both will help meet the regs, otherwise you will need fans and/or opening windows.


    Does each room have to have its own individual run back to the unit or can I tee off?
    • CommentAuthorMike1
    • CommentTime7 days ago edited
     
    Posted By: CranbrookDoes each room have to have its own individual run back to the unit or can I tee off?

    There are two basic topologies; one where everything runs to / from the unit in rigid duct, with branches to separate rooms (probably with cross-talk attenuators), and another where the rigid duct runs to a distribution box (or a box per floor), with semi-flexible duct to individual rooms.
    • CommentAuthorEd Davies
    • CommentTime7 days ago
     
    Why does the rigid/semi-flexible choice interact with the topology choice? Because rigid transmits noise from room to room more?
    • CommentAuthorCranbrook
    • CommentTime7 days ago
     
    Posted By: Mike1
    Posted By: CranbrookDoes each room have to have its own individual run back to the unit or can I tee off?

    There are two basic topologies; one where everything runs to / from the unit in rigid duct, with branches to separate rooms (probably with cross-talk attenuators), and another where the rigid duct runs to a distribution box (or a box per floor), with semi-flexible duct to individual rooms.


    How would I decide what is best? I can run rigid ducting if needed that isn't an issue. I wouldn'nt want too much noise transfer between rooms ideally. I have heard the stories about overheating and definitely would want to avoid that happening
    • CommentAuthortony
    • CommentTime7 days ago
     
    I don’t get noise transfer between rooms and have everything teed off more noise goes under doors
    • CommentAuthorbhommels
    • CommentTime7 days ago
     
    Posted By: tonyI don’t get noise transfer between rooms and have everything teed off more noise goes under doors

    Same here: rigid ducting with tee-offs for the room spigots. Every branch has an attenuator, and there are an additional large set around the MVHR unit.
    At normal levels the noise is inaudible, at boost level there is a bit of a hiss & rumble as you would expect, but no room to room crosstalk.
    If you are not sure about all the finicky details, I would recommend using a design service.
    •  
      CommentAuthordjh
    • CommentTime7 days ago
     
    Posted By: Ed DaviesWhy does the rigid/semi-flexible choice interact with the topology choice? Because rigid transmits noise from room to room more?

    Because semi-rigid duct is designed to connect to distribution boxes/plenums via proprietary connectors and comes in a very restricted range of sizes. Rigid steel duct comes in a gretaer variety of sizes and has such components as branches and in-line attenuators that are necessary to implement a branched topology.

    FWIW, my system uses semi-rigid duct and distribution boxes and I think it is a better choice. Easier to self install and commission, and no observed problems so far.
    •  
      CommentAuthordjh
    • CommentTime7 days ago
     
    Posted By: CranbrookI have heard the stories about overheating

    What stories are these? I presume in this context they relate specifically to MVHR and haven't heard any such myself.

    I would recommend getting an MVHR with an automatic summer bypass function.
    • CommentAuthortony
    • CommentTime7 days ago edited
     
    +1 ideal but there are ways round it and I manually change mine over twice a year
    • CommentAuthorbhommels
    • CommentTime7 days ago edited
     
    Posted By: djh

    I would recommend getting an MVHR with an automatic summer bypass function.

    +1
    And be careful that it has an actual physical bypass channel with a valve. My MVHR boasts an summer bypass that is only just switching off the supply fan. It then relies on some other bypass path (window) to be opened by the occupants.
    On the other hand, in a real heatwave you might want to switch off the summer bypass so the incoming air is cooled by the outgoing air.
    •  
      CommentAuthordjh
    • CommentTime7 days ago
     
    Posted By: djhI would recommend getting an MVHR with an automatic summer bypass function.

    Posted By: tony+1 ideal but there are ways round it and I manually change mine over twice a year

    Posted By: bhommelsin a real heatwave you might want to switch off the summer bypass so the incoming air is cooled by the outgoing air.

    That's why I specifically said an automatic bypass valve. For example today our bypass valve is open overnight and then closes itself in the mid-to-late morning and will open again in the early evening. So operation is far more often than twice a year.

    Note that PH-certified units are not allowed to implement either frost protection or summer 'bypass' by controlling fan speed. That and things like airtightness, noise levels and constant volume fans etc are why they cost more.
    • CommentAuthorbhommels
    • CommentTime7 days ago edited
     
    Posted By: djh
    Note that PH-certified units are not allowed to implement either frost protection or summer 'bypass' by controlling fan speed. That and things like airtightness, noise levels and constant volume fans etc are why they cost more.

    Mine is PH certified: Paul Focus 200. Not a shabby unit at all, but the smallest in the range and lacks some features of its bigger brothers :-(
    •  
      CommentAuthordjh
    • CommentTime7 days ago edited
     
    I wrote: "Note that PH-certified units are not allowed to implement either frost protection or summer 'bypass' by controlling fan speed. That and things like airtightness, noise levels and constant volume fans etc are why they cost more."

    And bhommels replied: "Mine is PH certified: Paul Focus 200. Not a shabby unit at all, but the smallest in the range and lacks some features of its bigger brothers :-("

    I stand corrected. All the more important to check the specs for individual MVHR units, IMHO. The PHI requirement spec is at:

    https://passivehouse.com/downloads/03_Reqs_and_testing_procedures_ventilation_en.pdf

    and it doesn't say much about summer bypass except that it should be tested at 100% flow rate. It doesn't say anything about exceptions to the rules for flow imbalance so I'd expect them to be obeyed, and the current certificate makes no mention of the 'bypass' feature, so I'd say Paul are skating on exceptionally thin ice (and that really surprises me). I wonder if the feature was tested?

    BTW, Paul have a pretty good introductory guide at https://www.paulheatrecovery.co.uk/mvhr-guide/
   
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