Home  5  Books  5  GBEzine  5  News  5  HelpDesk  5  Register  5  GreenBuilding.co.uk
Not signed in (Sign In)

Categories



Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

Buy individually or both books together. Delivery is free!


powered by Surfing Waves




Vanilla 1.0.3 is a product of Lussumo. More Information: Documentation, Community Support.

Welcome to new Forum Visitors
Join the forum now and benefit from discussions with thousands of other green building fans and discounts on Green Building Press publications: Apply now.




  1.  
    I am retrofitting MVHR into an old property so there are obvious limitations with locating duct runs, manifolds etc. The design the supplier has provided includes a lot of 75mm double duct runs. I am wondering if this is an absolute requirement or if I can reduce a couple of the double duct runs to single runs (back bedroom and loft?). There were no figures for air changes, flow rate etc. provided which probably doesn't help with the decision.

    Plan attached.
      MVHR.JPG
    • CommentAuthorArtiglio
    • CommentTimeApr 15th 2021
     
    On the design i received the double runs were on all the extracts but only on the supplies that exceeded a certain length. Similar to yourself its in a refurb that had already been drawn up, i made a few stud walls deeper than originally envisaged and managed to route all but 1 outlet/inlet as designed, this has been reduced to a single run for about half its length ( ceiling plenum has two ducts , but these merge half way back to manifold box, was the best compromise i could manage for that run)
    Again like yourself still not received flow rates or any calcs, can’t say i’ve been overly impressed with the suppliers performance , compared to the assurances in the dealings before transferring the cash.
    • CommentAuthoran02ew
    • CommentTimeApr 15th 2021
     
    It depends on the extract rates for those rooms, power of the fan unit and bends within the ducts. You will have to wind the extract fan up to get the flow rates from a single pipe over a long distance and tighten the vents closer to the unit which may make them whistle a bit!
    •  
      CommentAuthordjh
    • CommentTimeApr 15th 2021
     
    Posted By: modernvictorianI am retrofitting MVHR into an old property so there are obvious limitations with locating duct runs, manifolds etc. The design the supplier has provided includes a lot of 75mm double duct runs. I am wondering if this is an absolute requirement or if I can reduce a couple of the double duct runs to single runs (back bedroom and loft?). There were no figures for air changes, flow rate etc. provided which probably doesn't help with the decision.

    Insist on being told the flow rates and the linear air speed in the ducts and at what settings on the MVHR unit. The flow rates need to meet building regs at least and the linear speed is what determines noise levels. PH has more stringent requirements but as I remember we only have one double duct, from the kitchen. We have the D-shaped ducts though (AE35), which will make some difference I suppose. If you can fit larger ducts in place of the 75 mm ones you may be able to avoid the double runs. But the first thing is to check that the design flow rates are not too high. Definitely worth minimising bends where possible as well.
    • CommentAuthoran02ew
    • CommentTimeApr 15th 2021
     
    One solution would no to oversize the fan unit. Then a lower fan speed would result in the same air flow but quieter. However their is a maximum amount of air you can expect to push through a single pipe.
    •  
      CommentAuthordjh
    • CommentTimeApr 16th 2021
     
    It's the speed of the air through the ducts that creates the turbulence and noise. The fan noise is reduced by the attenuator and sound absorption built in to the distributor boxes.
    • CommentAuthorRobL
    • CommentTimeApr 16th 2021
     
    I went for oversize ducts, to keep the noise down - it was Swmbo's only requirement - would it be noisy? We use 150mm diameter pipes (or equivalent rectangular) to the bedroom, and elsewhere. I think I can just about hear it sometimes at night maybe. Never hear downstairs ones, there's so much other noise generally. We have a CO2 sensor, so it adjusts up/down to give the right ventilation most of the time, which I would recommend. I think from memory 4m/s was the critical flow speed to stay under, although obviously the lower the better.
    •  
      CommentAuthordjh
    • CommentTimeApr 16th 2021
     
    RobL said: "I think from memory 4m/s was the critical flow speed to stay under"

    That sounds about right. PH standards are a bit tighter. There's a very good guide to MVHR at http://www.passivhaustrust.org.uk/UserFiles/File/Technical%20Papers/2018%20MVHR%20Good%20Practice%20Guide%20rev%201.1.pdf

    It says "Another limiting factor of air velocity in ducts is noise. High velocity air generates noise, and therefore air speed limits are recognised for ducts in different locations to minimise noise problems. Within a single dwelling the air speed in the primary ducts should not exceed 2~3 m/s and in the final runs to room valves the speed should be limited to 1~2 m/s."

    So for my radial semi-rigid system I believe the design limit was 2 m/s.
  2.  
    Some good suggestions thanks. I did manage to get hold of the system calculations although the units are different (m3/h). They suggested that I could use a single duct to lounge and back bedroom and still be within Building Regs values. This is probably what I will do. As I am retrofitting it is much more difficult to use larger ducting unfortunately. Fingers crossed the noise isn't too bad.
      mvhr snip.JPG
    •  
      CommentAuthordjh
    • CommentTimeApr 19th 2021
     
    They look like the standard calculations, which is good.

    1 Litre per second (L/s) = 3.6 cubic metres per hour (m3/h)

    You can check the flow speed through the ducts if you know the internal cross-sectional area of the ducts. It might be worth seeing what the speed in the duct to the lounge is with just a single duct. I suspect the other thing they were considering is the pressure loss in the longer runs, which will mean the fans have to work a little harder.
    • CommentAuthoran02ew
    • CommentTimeApr 19th 2021
     
    IMO you will struggle to get 36m3/hr through a single 75mm radial pipe especially if its a long convoluted route, plus at 126m3/hr total its a small fan unit which you will have to wind up considerably. what unit have BPC spec'd? you could up the flow rate to a nearby room, dinning? to balance out the lack of flow in the lounge.

    Also make sure you have 10mm minimum air gap under the doors.
Add your comments

    Username Password
  • Format comments as
 
   
The Ecobuilding Buzz
Site Map    |   Home    |   View Cart    |   Pressroom   |   Business   |   Links   
Logout    

© Green Building Press