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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthordeniance
    • CommentTimeFeb 9th 2020
     
    Hi all, me again!!

    Now that my insulation work in attic is nearly complete I need to think about mvhr!
    I know you can only have it in a really tight house but I thought sod it, I’ve got the unit, may as well give it a bash!

    So... I was wondering if you could cast your eyes over the following drawings please!

    The original plan was to fit the unit and the supply extract as far away from wood burner on the chimney as possible, hence it’s position in the attic, it can really only go there.

    Then I was going to supply bedroom one and two and then drop down the wall and supply living room.
    Then I was going to extract from bathroom and drop through bathroom to extract kitchen.
    Also drop the condensate pipe into bathroom.

    Easy peasy.....but we’ve now had another baby and we are thinking of changing bathroom to bedroom 3 and building a 3m x 3m flat roof cube extension stuck on the back of the kitchen , so if I change the old bathroom extract to a supply that means that upstairs only has supplies and no extract, and downstairs will only have extract in kitchen possibly linked over to new bathroom? Is this a problem with only supplying fresh air to upstairs? Will I still get the circulation ? Or am I over thinking it?

    I’m planning on the vents being as far away from room doors so air travels across each room and I’m also hoping that I can use the ducting that is fully made of eps type stuff, just big eps tubes!

    Pictures in the next post below
    Cheers guys!
      D74A16B1-B209-4FA0-98DA-ECFB22E98471.jpeg
    • CommentAuthordeniance
    • CommentTimeFeb 9th 2020
     
    This is upstairs and attic level
      0C3736B5-64C0-472E-9CAA-353DCE678CB7.jpeg
    • CommentAuthordeniance
    • CommentTimeFeb 9th 2020
     
    This is downstairs level
      E0E01BB2-4611-4B98-AD8F-51B0E8F0B4C9.jpeg
    •  
      CommentAuthordjh
    • CommentTimeFeb 9th 2020
     
    I don't think supply-only upstairs will be a problem, but you must balance the total supply rate with the total extract rate.
    • CommentAuthordeniance
    • CommentTimeFeb 9th 2020
     
    Posted By: djhI don't think supply-only upstairs will be a problem, but you must balance the total supply rate with the total extract rate.


    That’s all done in the calibration at vent terminals isn’t it?
    •  
      CommentAuthordjh
    • CommentTimeFeb 9th 2020
     
    Posted By: denianceThat’s all done in the calibration at vent terminals isn’t it?

    Well no. You have to get the basic flow rates right first by proper design of duct lengths and diameters and flow restrictors at source. The vent terminal adjustments are only for fine adjustment if needed.
    • CommentAuthordeniance
    • CommentTimeFeb 10th 2020
     
    Posted By: djh
    Posted By: denianceThat’s all done in the calibration at vent terminals isn’t it?

    Well no. You have to get the basic flow rates right first by proper design of duct lengths and diameters and flow restrictors at source. The vent terminal adjustments are only for fine adjustment if needed.


    God that sounds complicated, is there any websites that I could read up on to try and figure it out? Are you saying that I need an equal amount of supply vents and extract vents? Sorry if this sounds stupid I’ve just got a really small brain!

    The ducts will all be 150mm
    • CommentAuthorRobL
    • CommentTimeFeb 10th 2020
     
    I think our MVHR (vent-axia sentinel kinetic) which I diy-installed is somewhat similar. It's a 4 bed house, no carpets ie big gaps under all doors. I fitted 2 inlets upstairs and 1 downstairs, then an exhaust upstairs (bathroom) and one downstairs (kitchen). All of the ducts are big - most are 150mm diameter, the longest one in the loft is 200mm diameter. Big ducts are important to keep things quiet, and big gaps are important if some rooms don't have MVHR ducts.

    All of the inlets and exhausts have adjusters on them - I left them all fully open - we don't want air constrictions making a noise and being inefficient unless we have to.

    To "commission", I set the fan at 50%, and measured every inlet and outlet airspeed with a cone over each and an anenometer. I added them all up, with a view to adjusting the MVHR unit itself - it can tweek fan speed up or down a bit. It was all un-neccesary, as all the inlets added up to the same as the exhausts.

    For upstairs, looking at your drawing, I suggest making the long extract run (in the loft?) to the bathroom be larger diameter (200mm say), as it looks like the bottleneck to me (comparing to 2* 150mm dia inlets). A 200mm run has almost 2* the area of a 150mm diameter run btw.
    • CommentAuthordeniance
    • CommentTimeFeb 10th 2020
     
    Posted By: RobLI think our MVHR (vent-axia sentinel kinetic) which I diy-installed is somewhat similar. It's a 4 bed house, no carpets ie big gaps under all doors. I fitted 2 inlets upstairs and 1 downstairs, then an exhaust upstairs (bathroom) and one downstairs (kitchen). All of the ducts are big - most are 150mm diameter, the longest one in the loft is 200mm diameter. Big ducts are important to keep things quiet, and big gaps are important if some rooms don't have MVHR ducts.

    All of the inlets and exhausts have adjusters on them - I left them all fully open - we don't want air constrictions making a noise and being inefficient unless we have to.

    To "commission", I set the fan at 50%, and measured every inlet and outlet airspeed with a cone over each and an anenometer. I added them all up, with a view to adjusting the MVHR unit itself - it can tweek fan speed up or down a bit. It was all un-neccesary, as all the inlets added up to the same as the exhausts.

    For upstairs, looking at your drawing, I suggest making the long extract run (in the loft?) to the bathroom be larger diameter (200mm say), as it looks like the bottleneck to me (comparing to 2* 150mm dia inlets). A 200mm run has almost 2* the area of a 150mm diameter run btw.



    My unit can adjust each fan I think? Sure it does!

    The unit has 4 x 150 mm outlets

    I’ll do some proper measurements soon as I read on here you need to work out room volumes I think?

    At a guess I’d say the extract run is 8-9 metres to the kitchen , and the supply run is probably 8 metres to the living room

    I’ll have a look around the inter web see if I can find some starting points for air rates and duct sizing etc!
    • CommentAuthorMike1
    • CommentTimeFeb 10th 2020
     
    FWIW, I was looking for some affordable duct sizing software a while back for one of my future projects.

    The cheapest I found was a US program - AccuDuct, $295, http://www.adteksoft.com/SoftwareRes.htm. Of course that uses US standards, units of measure, terminology, etc. Better than guessing, I imagine, if you can work through all those points, but not as good as getting someone to design it.

    I'd be interested to learn the outcome of your own search.
    • CommentAuthorGreenPaddy
    • CommentTimeFeb 11th 2020
     
    A few pointers (I design these systems for the houses I build)

    - you don't seem to have a supply to the front room?
    - you should do a room volume calc, and establish the flow rates for each room With a lot of supply rooms, and only 2 extract rooms, you will have quite high extract rates for those 2 rooms. No bad thing in itself, except the duct velocity will be a bit higher and possibly create some noise. Consider 2 extract points in the kitchen, either side of the recirculating extractor hood.
    - be careful running from bathroom to kitchen, as you can get cross talk (sounds travel very well along tubes). That is one of the advantages of the systems with the flexy single ducts for individual rooms.
    - get the room balance of air flows right, according to your calcs, starting with the outlet to the room farthest away from the MVHR unit, with that outlet fully open (this is the index leg). Then close down other room outlets to establish the flow rate ratios. Getting the exact numbers is less important than getting the ratios for each room, as altering the fan speed will drive the actual flow numbers. (same applies to the extract side).
    - when the room balance (ratios) is right, then you can set the fan speeds for "setback", "normal" and "boost", to give the volume air changes. Note, the two fans will likely be at different settings, as the duct runs will have different pressure drops. (eg supply @ 15%/30%/80% ; extract @ 12%/27%/71%)
    - I do the final overall balance using temperature, so the differential between intake to supply and extract to exhaust is the same (or close), which will confirm you've got total balance of air flows.
    - This air balance will change, as filters become blocked, unless you've splashed out for a constant volume system, so occasional checks on the temp differentials will allow a slight adjustment of fan speeds. No one ever does this, as you need to have an idea what you're up to, but slight filter blinding can make a big diff in air flow balance, and end up with quite cool air in the supply. This is one downside of MVHR, and why they ought really to all be constant volume and have temp indicators, the real indication of performance.

    It really isn't rocket science, but you need to be clear what you're trying to achieve - same as most of what we discuss on here.
    • CommentAuthordeniance
    • CommentTimeFeb 11th 2020
     
    Wow! Thanks for that green paddy! I'll take it on board!
    • CommentAuthordeniance
    • CommentTimeFeb 11th 2020
     
    If I work the room volumes out, how do I work out what the flows should be? Are there guidelines or a table somewhere?
    • CommentAuthorGreenPaddy
    • CommentTimeFeb 12th 2020
     
    3 different rates to calc, then use highest as default...

    1. Extract rate - sum of all wet rooms (kitchen 13 lite/s, each additional shower/bath/util 8 litre/s)

    2. Dwelling rate - sum of all bedrooms (13 litre/s for first bedroom, plus 4 litre/s for each additional)

    3. Minimum rate - 0.3 lite/s times total floor area, all floors.

    usually, minimum rate is highest. It varies, but usually this is way more than you actually need to keep the house fresh, so that becomes the "boost" setting, with normal and setback being lower of course. There's a bit of art in it, but as others have pointed out, the backstop of having a CO2 detector to power up to boost helps.

    By running at lower flowrates most of the time, you really do need to hit boost when showering or cooking, which helps the immediate clearance of moisture/odours, but also raises the 24 hours general air washing of the house. Also, and importantly, in winter, the heat exchanger can become clogged with moisture/condensate on the extract side, so needs a good blast to shove it through, and into the drain. Otherwise, the air flow routes get blocked, and out of balance results.

    I'd love to find a less "sensative" way of doing what an MVHR does, and for less money...still looking.
    • CommentAuthorandyman99
    • CommentTimeFeb 12th 2020
     
    I used the spreadsheet in this thread to get an idea of the installation. The suppliers did their own calculations but the spreadsheet was useful as a starting point

    http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14912
  1.  
    As GP said, there's different ways to calculate the flow. The numbers are always going to be very rough estimates, because it depends how much water and CO2 is being created at any point in time (how many people are at home, what washing/bathing/cooking is happening, pets, houseplants) and on the weather (how dry is the fresh air that is being brought in)

    In Scotland the building standards tech book suggests continuous extract of 6litres/s from the kitchen and 4 l/s from each bathroom or utility, and double that on boost. Then supply the same amount, split across the living and bed rooms.

    Many folks seem to buy an oversized mhrv to cover all possible eventualities, then end up running it at a small % of its true capacity. The building regs encourage this for new builds because the building standards officers have no way of knowing how the building will be occupied.

    As Paddy said, it's important to turn it up and down as required. If you just leave it on at a fixed rate, many times you have more ventilation than needed, which wastes heat and makes the air excessively dry.

    You have already got a mhrv though? The brookvent one you have can do 60+ litres/second, which should be plenty. I'd be interested how you find it, I was also looking at that one?
    • CommentAuthorcjard
    • CommentTime3 days ago edited
     
    > The original plan was to fit the unit and the supply extract as far away from wood burner on the chimney as possible, hence it’s position in the attic, it can really only go ther

    It doesn't matter that the unit be positioned away from the chimney- the unit can go wherever but heed these points:

    * position the intake-from-world vent upstream from your chimney relative to your prevailing wind direction
    * make the duct runs to the world as short as possible
    * the intake-from-world/exhaust-to-world duct runs should be fully insulated if they are inside the warm envelope of the house
    * the extract-from-house/supply-to-house duct runs should be inside the warm envelope of the house, or fully and well insulated if they aren't
    * 75mm semirigid ducting is a lot easier to fit than rigid. If you're doing a branch system you don't need to use massive ducts all the way to the ends
    * you could place an extract in the hallway upstairs if you're bothered that there is only a limited number of extracts. Having only supply ducts upstairs could see the downstairs smelling a bit "farty bedroomy" particularly if you have teenage boys one day :)
    * ensure your wood burner is a sealed type (for other eco reasons)
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