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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.

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  1.  
    I posted a few months ago on internal wall insulation for my victorian refurbishment project. Following that rather animated (!) discussion I realised how much I didn't know, so to speak, and I am now working with an energy efficiency consultancy. This has proven to a useful investment so far and has given me more confidence that I making appropriate decisions. I still have a huge list of stuff of choose of course and one of these is MVHR.

    Having decided I needed it and would aim for 'as airtight as possible' I have obtained quotes from two companies. One specialist Eco type company and the other more of general ventilation specialist. The price differences are massive. The more expensive quote is twice the price. I am wondering if the higher costs really do translate to a better performing system? On the one hand I am hearing about overly noisy systems with plastic ducting that fails after 10 years, and on the other hand I am hearing that 'its just a fan in a box don't pay over the odds'. Does anyone have any real-life experiences they could share?
  2.  
    Some prices and data:

    Vent Axia FH
    Total £1868
    Airflow 300 M3h/r @100Pa
    Efficiency 90%
    Includes Radial ducting kit (plastic), 5 supply valves, 5 extraction valves, 3 rolls radial ducting, 2 distribution boxes
    Design included (£100 deposit)
    Silencers not included (£26 each)
    Delivery included
    Commissioning not included


    Duco-Box Energy 325 1Z
    Total £2556
    Airflow 350 M3h/r @100Pa
    Efficiency 91%
    Includes Radial ducting kit (plastic), 5 supply valves, 5 extraction valves, 3 rolls radial ducting, 2 distribution boxes
    Design included (£100 deposit)
    Silencers not included (£26 each)
    Delivery included
    Commissioning not included


    Zehnder Comfoair Q350
    Total £4600 - 5100
    Airflow 350 M3h/r @100Pa
    Efficiency 96%
    Includes rigid metal ducting
    Design included
    Silencers included
    Delivery included
    Commissioning is £800
    •  
      CommentAuthordjh
    • CommentTimeJan 23rd 2020
     
    I suggest getting an itemised breakdown of the costs as a first step, so you can see what the different components are and where the price difference arises. Also make sure to get the full part numbers for the MVHR units, the names you have used are ambiguous. What about the intake & exhaust air ducts and terminals?

    I'd also check the sizing calculations for duct runs: capacities and air speeds. Make sure they match the MVHR unit.

    Then you need to look at the features of each unit and decide what you need and what you can live without. I would suggest a pre-heater and automatic summer bypass are good places to start, for example. You may want control of a post heater, or some specific remote control attributes. What type of filters does it take and what do you need to do with them how often? How about cleaning the heat exchanger? Are they constant power or constant volume?

    Where do the efficiency figures come from and what are they measuring? Who did the tests? The Zehnder unit is the only PH-certified one of the three and that 'efficiency' figure doesn't match anything in the PH database. PH-certified units meet stricter requirements about efficiency and noise etc.

    FWIW, I commissioned my own system with my architect. I have plastic semi-rigid radial ducting and I don't expect it will fail in ten years and it's the opposite of noisy.
  3.  
    Thanks. The efficiency figures came from the quote for the first two and for the Zehnder it is what I could find on their website.

    Interesting to know that you are happy with the plastic ducting. Some are adamant that metal ducting is the only way.
    •  
      CommentAuthordjh
    • CommentTimeJan 23rd 2020
     
    Posted By: modernvictorianInteresting to know that you are happy with the plastic ducting. Some are adamant that metal ducting is the only way.

    As far as I'm concerned, the semi-rigid is better in every way. There's no way I would use metal ducting. But that's just my opinion and maybe others have different views.
  4.  
    I find it baffling that the market is prepared to pay such preposterous prices for mhrv units - essentially two fans in a box - just compare those prices against the prices of more complex appliances such washing machines, or heat pumps...! Though the market is small in the UK, it's large in other places, so that's no excuse.

    Not got there yet, but I am looking at using a number of smaller mhrvs such as HR100R or equivalent (£250 each). These would be distributed around the house, each serving one wet room and one dry. The main saving for us would be to avoid threading a network of ducts through the Victorian house. The claimed heat recovery efficiency is 20% poorer than the units you mentioned, but over an appliance's lifetime that doesn't justify the exorbitant price of fitting the larger units afaics.
    •  
      CommentAuthordjh
    • CommentTimeJan 24th 2020
     
    Posted By: WillInAberdeenessentially two fans in a box

    They aren't just two fans in a box though, are they? Not that I'm justifying the prices - the market does that somehow.
  5.  
    Posted By: modernvictorianDoes anyone have any real-life experiences they could share?


    I designed, installed and commissioned my own MVHR system. I chose a serial rather than radial system design for my Genvex compact unit. This compact unit is a combined MVHR, EASHP and DHW tank so not directly comparable to yours. I used rigid plastic ducting with an attenuator on the supply circuit and the system is silent and is run on the low setting all the time as I have found humidity levels acceptable without using boost. My unit is PHI certified. A consideration that not everybody realises is that the supply air will be at lower than room temperature by a couple of degrees. This can be offset by having a unit with an EASHP, hot water or resistance electric post heater. Another consideration is that building regs. ventilation rates are higher than PH rates and we run our system at PH rates.
    • CommentAuthorGreenPaddy
    • CommentTimeJan 24th 2020
     
    Something to bear in mind, re. PeterStack comment above...

    - once external temps drop to low single figures, condensate will start to form in the heat exchanger. It needs an occassional blast on high setting to clear this out, otherwise the exchange efficiency drops. Ideally "boost" once a day (shower/cooking times) in colder weather at least. I've had a rigid plastic system for over 10 years, and design an install them in all the houses I build, so have seen this quite a lot. When on boost, you'll initially hear water running to drain, for a short period. If you extract the heat exchanger, you'll find it full of water, like a litre's worth.

    I agree that the MVHR box appears to be very over priced, and have gone up in price dramatically over the last 10 years (like doubled in price). I like the principles of MVHR, and the concept of control of air and heat recovery, but I would like to find a magic alternative.
  6.  
    Vent Axia/Semi rigid here. As above, semi rigid seems better in every way - same principle as manifold plumbing where there are few joints and those there are can be accessed.

    No pre/post heater and have never noticed air being cool. We're in SE though - might be different if you live somewhere colder.

    Unit price is unbelievable - materials cost of the units looks so low. If they became a mass market appliance they'd cost about £200....
    • CommentAuthorborpin
    • CommentTimeJan 24th 2020
     
    I've got a Helios unit and semi ridgid pipe which I think is the way to go.

    I've posted elsewhere re the issues with getting a new fan and the cost (£1K).

    Yes they are expensive but they are working in a reasonably harsh environment (high humidity). It is also a market economy!

    On heater, I do note it kicking in to prevent the heat exchanger freezing up!
  7.  
    OK, I'm now leaning towards the cheaper options so its good to know that the Vent Axia/semi rigid option is working for you Simon. I imagine the installation would be a lot simpler in my victorian house compared withe the large metal ducts. I am thinking that as long as the ducting is installed well and performs well then the unit could be changed in the future if there were any issues or if better technology became available.

    I still have a slight concern about the longevity of the semi-rigid ducting because of comments like this:

    "MVHR ductwork is embedded into the fabric of the building and would be very disruptive to
    have to replace, so it is worth investing in a high quality system from the outset. We specialize
    in, and normally only offer, galvanised steel spiral wound rigid ducting for our systems,
    designed to last the lifetime of the building."

    And this article describing the downsides of the cheaper units (note written by a supplier):

    https://www.heatspaceandlight.com/how-the-cost-of-an-mvhr-relates-to-quality-and-performance/
    • CommentAuthorbhommels
    • CommentTimeJan 24th 2020
     
    Looking from a long term running cost standpoint: for a MVHR in a house with limited airtightness (2..3 ach), the power consumption (W / m3 h) becomes the factor that determines whether it is economical / will save energy or not. The smoother and bigger the pipes are, the less hard the fans have to work and the lower the power consumption will be for a given air exchange rate.

    So if you forget about all the other benefits MVHR brings, purely from a running cost standpoint it would make sense to use rigid pipes (if they weren't so expensive). Though as I am currently fitting rigid ducts for MVHR, I can see the benefit of semirigid ducting.
  8.  
    Posted By: bhommelswould make sense to use rigid pipes (if they weren't so expensive)


    I understood every turn/junction in a rigid pipe adds resistance (needed to calculate this for the large ridid external ducting). Semi rigid thus much more efficient for a given diameter.

    Cant see any reason semi-rigid wouldn't' last life of building as well.
    • CommentAuthorbhommels
    • CommentTimeJan 24th 2020 edited
     
    The fact that every bend & turn adds resistance is true for both rigid and semirigid, and does not tell you which one has lower overall resistance. My bet is on the rigid pipe, by some margin. And have you ever seen a straight section of semirigid pipe BTW? Not saying semirigid is bad per se, it might be good enough. I expect a lot of cursing (by me) as & when I get to installing the more complicated junctions in the rigid pipe system I bought the bits for.

    The trouble is that both probably last long, so whatever the losses are, they will be like that for the lifetime of the system.
    • CommentAuthorrevor
    • CommentTimeJan 24th 2020
     
    I built my own system a Titon unit and 200 x 60 rectangular and 150 and 125 round Domus rigid ducting. Only flexible is connection from the unit to the rigid duct. That is quiet except on boost. One thing I was not happy with, was the plastic air valves I originally acquired, so bought air valves from lindab. These are of a metal construction and fit into a galvanised socket bonded to the ceiling. It is then a simple matter to remove and replace the valve with a single turn without disturbing the connection to the ducting. This is of great assistance when you want to lock off the position of the valve once balanced. Don't forget that in the kitchen to fit a fire air valve that shuts off the intake when it gets too hot.

    http://www.lindab.com/uk/pro/pages/search.aspx?cat=pro&k=air valves
    •  
      CommentAuthordjh
    • CommentTimeJan 24th 2020
     
    Are rigid pipes used for radial arrangements? I'd thought they tended to be used in branched networks. Then the most important problem becomes noise: room-to-room crosstalk. With the need for extra silencers.

    As long as you use semi-rigid ducts that are smooth inside then I expect they are the equal of, or better than, spiral wound metal ducts of similar cross-section. Airflow is complicated but I suspect that actual bends introduce a lot more turbulence and thus drag than slow curves in the length of a duct.

    The main reasons I prefer semi-rigid are:
    (1) the lack of worry about crosstalk. One silencer in the main supply duct to the distribution box is all I needed. And there is no crosstalk.
    (2) Ease of installation. It helps that I used metal web joists, so it was very easy to pull the ducts through. The most difficult part was arranging the ducts near the distribution boxes.

    I would definitely recommend getting the system designed by a professional.
    • CommentAuthorgravelld
    • CommentTimeJan 27th 2020
     
    @djh - what sort of qualifications should you look for in a professional? Is this a mechanical engineering type discipline? Most retailers and manufacturers have their own design teams, some offering free designs.

    To the OP - have you considered a decent dc-MEV? You might find the costs a step down from the Rolls-Royce PH certified MVHR units, and depending on your permeability levels it might make a sensible trade-off. Look at Aereco who seem a decent outfit.
    •  
      CommentAuthordjh
    • CommentTimeJan 28th 2020
     
    Posted By: gravelld@djh - what sort of qualifications should you look for in a professional?

    No idea, sorry.
  9.  
    I've taken a look at dc-MEV recommended above but I'm still leaning towards a 'not so Rolls Royce' MVHR unit. The costs (for the cheaper quote) are reasonable compared to other systems.

    The system will be designed by the supplier (after a deposit is paid), I also have no idea of their qualifications for designing this successfully. The more expensive suppliers think they will do a better job on this, which could be true, but in a fairly standard house layout such as mine surely there are only a limited number of options for laying out the ducting etc.
  10.  
    Posted By: modernvictorianbut in a fairly standard house layout such as mine surely there are only a limited number of options for laying out the ducting etc.


    Indeed. What you’re really looking forgotten to do is the airflow calcs Layout is simple most of the time - extract from wet rooms, put into living rooms. Vents go furthest corner from the door to ensure good flow throughout room.

    You’ll probably need a few extra extracts to balance - I have extra extract from utility/laundry room and both an inlet and extract from kitchen diner.

    They can design exactly where ducts will go but there will be some adjustment on site as there will be areas wheee bend radii don’t work, structure is slightly larger or less neat than design, other utilities get in the way or joists/steels are in the way.
    • CommentAuthorSilky
    • CommentTimeFeb 4th 2020
     
    in answer to the original question, I would guess it is always worth paying more for higher efficieny in the heat exchanger, because the efficiency is cumulative over time.

    i.e. imagine you begin with 100% heat, no other system inputs or outputs and presume a perfect 1 ach.

    Over 24 hours a 96% heat recovery would give 39% of the original heat

    100
    96
    92.16
    88.47
    84.93
    81.54
    78.28
    75.14
    72.14
    69.25
    66.48
    63.82
    61.27
    58.82
    56.47
    54.21
    52.04
    49.96
    47.96
    46.04
    44.2
    42.43
    40.73
    39.11

    a 90% would give only 9% of the original heat

    100
    90
    81
    72.9
    65.61
    59.05
    53.14
    47.83
    43.05
    38.74
    34.87
    31.38
    28.24
    25.42
    22.88
    20.59
    18.53
    16.68
    15.01
    13.51
    12.16
    10.94
    9.85
    8.86

    With 2 ach those same figures would be achieved in 12 hours, and so on.

    Therefore my assumption has been that it isn't correct to say that a 96% is only 6% better than a 90% heat exchanger. Please correct me if I've missed something?
  11.  
    Think it's even worse than that Silky, because when some energy has been lost, the house air will become colder, and the heating will kick in. If the mhrv loses 10% of the energy in the air every hour, the heating will top it up, so after 24 hours it's lost 240% of the original heat.

    But when I looked at it last, it still wasn't worth paying £1000s more for a more efficient unit, it didn't pay back in the likely lifetime of the appliance (or its owner!).

    Your numbers illustrate how important it is not to over-ventilate. There's regular stories of people installing big expensive mhrvs to meet some arbitrary number of AC/h, then running them at 20% of capacity, or (worse) over-ventilating to excessively low humidity. The more you ventilate, the more heat is lost. A 90% efficient mhrv running at 5 times too high flow rate, loses more heat than a 70% efficient mhrv running at the required flow rate.
  12.  
    Posted By: Silkyin answer to the original question, I would guess it is always worth paying more for higher efficieny in the heat exchanger, because the efficiency is cumulative over time.

    i.e. imagine you begin with 100% heat, no other system inputs or outputs and presume a perfect 1 ach.

    Over 24 hours a 96% heat recovery would give 39% of the original heat

    a 90% would give only 9% of the original heat


    Posted By: WillInAberdeenBut when I looked at it last, it still wasn't worth paying £1000s more for a more efficient unit, it didn't pay back in the likely lifetime of the appliance (or its owner!).


    And the heat loss due to a less efficient heat exchanger should also be put against the heat loss of no MVHR and sufficient 'natural' ventilation to keep the mould at bay.
  13.  
    Posted By: Peter_in_HungaryAnd the heat loss due to a less efficient heat exchanger should also be put against the heat loss of no MVHR and sufficient 'natural' ventilation to keep the mould at bay.

    But also against the comfort factor of MVHR. Much better internal air quality than opening windows (extractor fans) in my view.
    • CommentAuthorborpin
    • CommentTimeFeb 7th 2020 edited
     
    Posted By: WillInAberdeenYour numbers illustrate how important it is not to over-ventilate. There's regular stories of people installing big expensive mhrvs to meet some arbitrary number of AC/h, then running them at 20% of capacity, or (worse) over-ventilating to excessively low humidity. The more you ventilate, the more heat is lost. A 90% efficient mhrv running at 5 times too high flow rate, loses more heat than a 70% efficient mhrv running at the required flow rate.
    Couple of thoughts.

    Running a bigger MHRV unit more slowly can mean it is quieter.
    However, running it too slowly may not clear the moist air sufficiently from the fans and it then suffers reliability problems.
    The only way to get the 'right level' of ventilation, is to measure the humidity and CO2 levels and ventilate accordingly.

    YMMV.
    • CommentAuthormark_s
    • CommentTimeFeb 13th 2020
     
    Just a thought - but how does the semi-rigid metal ducting differ from stainless flue pipe which is readilly available at low prices?
    •  
      CommentAuthordjh
    • CommentTimeFeb 13th 2020
     
    Semi-rigid duct is usually plastic. Metal duct is usually rigid. Flexible metal ducts are normally bad news for smooth airflow. Did you have some specific product in mind?
    • CommentAuthorcjard
    • CommentTime3 days ago edited
     
    Posted By: djh
    Posted By: WillInAberdeenessentially two fans in a box

    They aren't just two fans in a box though, are they? Not that I'm justifying the prices - the market does that somehow.


    No indeed; there is also a bit of polystyrene, a section of folded plastic and sometimes a kid's electronics project that ups and downs the fan speed based on the feedback from a humidity sensor
    • CommentAuthorcjard
    • CommentTime3 days ago edited
     
    Posted By: modernvictorianSome prices and data:

    Vent Axia FH
    ....


    Buy the Vent Axia. Run it at half what the design says, then tweak it according to a humidity/co2 monitor

    Have a look for second hand units on eBay etc- they do come up


    I loosely kept to the designs and specs when fitting mine; some air flows are different but the general notion of "bigger room , more flow" is kept to. Big rooms (over 15sqm) have an extract and a supply. Rooms furthest from the manifold (not quite centrally located) have two pipes to one vent, otherwise it's one. I also swapped one of the supply to be an extract to even out the flow rates a bit. All in all, physics/the universe doesn't care much for this meddling and everything will even out - it's not the case that if you fail to get it to within 1% of spec then it won't work - the biggest step for an amateur installer is ensuring to get the supply and extract the right way round :)

    All semi rigid ducting - easy to fit (but mine was a new build) and can't see why it wouldn't last - a) it's made of the same stuff that your water supply pipe is made of, b) it's buried inside something that stops UV light getting to it, like your water supply pipe is,, c) it encounters no mechanical stress, and d) it's made of the same stuff that will be polluting our oceans and landfill sites for the next billion years.. I find the notion that someone would try to make out that twinwall plastic pipe won't last the life of the building laughable to say the least

    Even when my McGraw wasn't balanced it still had its profound effect (and tbh balancing it made no discernible difference but it was probably reasonably well the way there to start with), namely that the house doesn't smell of anything - sure, you let the steamer boil dry and the burnt veg smell lingers for a few days but the thing I notice now more than anything else when I come home from being anywhere that doesn't have an night is how much my house doesn't have a smell -@ no more forty bedrooms" the mm said, and the boss agrees. She says she now wouldn't consider living anywhere without one, and if I built another house I'd fit one just for that reason alone

    BPC Ventilation by any chance?
   
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