16 Nov 2009

Whither MVHR?

An interesting letter in last week’s Building from Tim Gough who has a CV as wide as a Passivhaus wall.

In it, he berates the Zero Carbon Hub for being no such thing. They are courting compromise in asking for the mandatory use of mechanical ventilation with heat recovery (MVHR) to be ditched. They say here that is a step too far for a temperate climate like the UK. Problem is that without MVHR, you can’t really build to PassivHaus standards because it’s so airtight that you would keel over every time someone farted.

Passivhaus standard uses just 15kWh/m2 for annual space heating requirements. Zero Carbon Hub are suggesting that this is relaxed to somewhere between 30 and 45kWh/m2 – i.e. two to three times as much, which is about as low as you can go safely without MVHR. And is really not much lower than where Part L will pitch us next year.

Maybe this debate is a bit obscure, but it’s nevertheless interesting. I rather tend to side with the Zero Carbon Hub on this. I think mandatory MVHR is just possibly a step too far in the UK. If you look closely at Tim’s text, you can begin to make out just what the problems are.

This is not a question of diminishing returns – to halve or quarter heating/cooling costs by means largely of a MVHR system costing a few thousand pounds can hardly be said to be that writes Tim.

Except that it ignores the cost of running the fans which drive MVHR system. OK, they are low. We now have fans operating at under 100w, but multiply that by the 8760 hours in a year and you have consumed enough power to heat and cool 58m2 of Passivhaus. The actual energy saving achieved by MVHR is therefore pretty minimal. Some commentators have even suggested that it’s not a net energy saver at all. They certainly aren’t if residents are so bold as to open their windows.

MVHR systems do not, contrary to what is claimed, adversely affect indoor air quality – even if the filters are not cleaned.

That is true. The reason Passivhaus insists on them is because of air quality, not energy saving. The problem is that they don’t always work as planned; they break down; people turn them off (ironically sometimes to save energy). That is where the problems may lie.

Housebuilders worry about reliability, but this is a proven technology that is no more complex than an extract fan.

And extract fans never break down?

What, I wonder, is the real agenda of the promotion of these poor standards?

In a word, caution. What ought to happen is that several hundred new homes should be built to PassivHaus standard, complete with MVHR systems installed, and then lived in by ordinary folk for something like five years. That would be a sensible test of whether the system is workable. If the MVHR systems proved to be reliable and popular and people learned to live in the houses the Passiv way (i.e. not opening windows), then MVHR could be safely rolled out as a mandatory building regulation requirement in new airtight homes. But until that’s done and dusted, making MVHR mandatory would be pretty rash. Stupid even.


  1. The critical bit here is that the 15 kWh/m² figure (unlike U values!) is not refering to any per-kelvin temperature difference between inside and outside the house. It is climate independent and adjusts itself automatically to the local climate.

    A Passivhaus meeting that standard in a temperate English climate (typical winter temperature: −1 °C) can have less stringent insulation (U values) and air-tightness requirements than a Passivhaus meeting the same standard in say southern Germany or northern Skandinavia (winter temperatures of −15 °C).

    The kWh/m² figure simply does not need adjustment to a particular climate. Claiming that it is not appropriate for the more "temperate climate" of England is either a misunderstanding (or misrepresentation) of basic mathematics by the Zero Carbon Hub.

  2. Markus,

    I think you are making the same error as Tim Gough. You are right to point out that the 15kWh/m2/a figure — the Passivhaus target — is climate independent, and that consequently an English Passivhaus need not be so exacting to build as a Swedish one.

    But that's not why Passivhaus may be inappropriate for the UK climate. It's the MANDATORY use of MVHR which makes it inappropriate; or at least makes it a risk of being inappropriate. Without a fair trial on a population of residents who have no particular leanings towards low energy living, there is no way of knowing.

    The risk is that it will be an expensive mistake, people will turn off the MVHR, leave the windows open, and all that extra cost involved with building to Passivhaus standard will not save a joule of energy.

  3. Err, there already has been a huge experiment like that you suggest. There are many thousand passive houses throughout Europe, some of which have been there for a decade or more.

  4. This is exactly the same attitude that existed in the 1940’s towards central heating.

    When relatives, who had experienced it abroad before the war, had it installed in their average size home, the plumber said it was only suitable in commercial buildings and that they would hate it. They loved it!

    Central heating, like MVHR, can breakdown; in fact there are a great many more things to go wrong. But would you live without central heating (unless you already have a Passivhaus)?

    Well, would you?

    All this prevaricating by the Carbon Hub and others is irresponsible. It would be stupid to ignore of the success of the 20,000 Passivhaus homes already occupied, and to delay its use in the UK.

  5. I think this touches on a major, if hidden divide. The powers that be and certain interest groups are very keen on calculating theoretical savings in energy that can be made by moving from where we are now to Passiv Haus or whatever. This is certainly useful in its way.

    However, what Mark is suggesting is looking at what actually happens with real people living in the real houses in the UK - something that no one much in government seems interested in. I suspect they fear that all sorts of anomalies might be thrown up that might complicate the thrust to 'drive up standards' and would therefore be dangerous.

    For myself I would love to see a really good study of actual energy consumption and usage of say 200 identical ordinary new houses (Barrat?) compared to their theoretical consumption. I would expect, er, 'considerable variation' There's got to be a PhD there for somebody.

  6. To date, almost all Passivhauses built in Europe have been elective. People have got tax breaks for building them, and they have generally had an interest in low energy living. Nowhere have they been rolled out as standard. To do this (which is the UK proposal) is definitely an experiment. We should first check that it will deliver anything useful.

    As for the strange comments about central heating - no way is this comparable. For a start, no building reg ever required people to fit central heating; it was consumer choice, pure and simple, which drove this innovation. And if CH breaks down, you fix it because you get cold. If MVHR breaks down, no one notices.....

  7. "If MVHR breaks down, no one notices....."
    How does that square with
    "without MVHR, you can’t really build to PassivHaus standards because it’s so airtight that you would keel over every time someone farted"?
    Of course you can open windows on a fine day, even in the 'heating season' while the MHVR's on (just like while central heating's on - you may or may not think to turn it off).
    Who's going to keep windows closed all year? No need to run MHRV all year, so nothing like 8760hrs, and surely it can be less than 100W for the fans?

  8. What about the ventilation system that has a similarity with the Passivhaus term. Passive ventilation. Not the opening windows style, but the methods that use wind and convection to cycle the air.


  9. Now we are getting to the nub of the problem. The problem is likely to be that users have little or no comprehension of what the role of MVHR is. If it breaks down, or the inlets/outlets fall out of adjustment, then you will get poor IAQ which may or may not manifest itself with bad smells. The response MAY be to repair the MVHR, but it may well not be apparent what the error is (unlike central heating malfunction). Instead, people open windows and turn up the heating. Or do something else.

    My point is that without long term observation, its largely guesswork as to what will happen.

    Passive stack insulation will not work in a Passivhaus because there is very little air movement. They are designed to be pretty much temperature consistent across all rooms, hence no stack effect. And of course PSV is a big heat drain - you can't put heat recovery onto the exhaust air. ITHO have a product (Demand Control Ventilation) which might suit but it's so far not tested on PassivHaus.

    And Tom, you are right, you don't have to run MVHR all year, but as you barely notice whether it's on or off, it's a tough call to remember to turn it off every time you open a window, but then turn it on again when you close it. It's a manual override on a system with no immediate feedback. As for power consumption, I am aware that the watts used get lower and lower and don't know what the lowest power use now is, but it's unlikely to get below 50w, so it will always carry an energy load that bears comparison with the amount the heat recovery saves.

  10. Mark, I don't know where you're getting your fan numbers?

    Let's take your 58 m2 house. Suppose it has a generous 2.5 m ceiling height so its volume is 145 m3, and suppose we use a reasonably generous 0.5 ACH. That gives us 73 m3/hr, which 20.28 l/s.

    The Energy Savings Trust guide to Energy Efficient Ventilation http://www.greenspec.co.uk/documents/refurb/Efficient_ventilationGPG268.pdf says that best practice ventilation systems must have a specific fan power on all settings of less than 1 W/l/s, which gives us 20 W of fan power. FIVE TIMES better than your claimed 100 W!

    Or look at the CEPHEUS final technical report from 2001. 0.45 Wh/m3 is the limit for certified products and the average achieved in practice was 0.26 Wh/m3.

    By all means let's have a debate, but base it on realistic numbers please.

    Oh, and Passivhaus standards are required as policy in some German cities now, so the UK won't be first. MVHR is already compulsory in many other countries, so again the UK won't be the first. I would have expected you to mention those facts in the interests of a balanced article?

  11. I stayed in a hotel that had air conditioning, that would turn off/on when you opened/shut the door. So people opening windows shouldnt be a problem, as a simply solution exists(dunno if simply means cheap).

  12. I wasn't trying anything sophisticated in my calculation. I just assumed a fan of 100w going 8760 hours a year, and divided it by 15kWh/m2/a, the Passiv Haus standard, and reckoned that running such a fan would provide enough heat for 58m2 of PassivHaus (876/15=58ish). If you can MVHR to power off lower rated fans, all well and good. The CEPHEUS figures seem to suggest fans of no more than 10w, which seems unbelievable for a whole house system.

    Yes, I am aware that Frankfurt (isn't it?) is demanding all new homes be built to PH standard, but this has only just happened, and it doesn't mean that it will work as anticipated. I think they should also have trialled on a non-elective user group. (Maybe they have?)

    And as for MVHR - where exactly has it been made compulsory?

  13. "And as for MVHR - where exactly has it been made compulsory?"

    Here in Quebec the building code is such that all new construction that is connected to the electrical grid must have a HRV system that is activated at least when the heating is on.

    Reading the comments here is fascinating: you guys are being so over cautious that it's really quite surprising.

    HRV systems are mechanically very simple (and hence reliable) and even with "airtight" construction, there's enough background leakage (especially in winter) that no-one is going to be asphyxiated if the system fails. We have one and never think about it - it gets turned off in the shoulder seasons when it's warm/cool enough to have the windows open, otherwise it's controlled by a humidistat with 20 minute "boost" switches in the bathrooms in lieu of regular extractors. Maintenance is simple: clean the filters four times a year and clean the HR core once a year.

    An HRV is not so much about saving energy as ensuring good IAQ - but if you haven't built airtight in the first place, then it's a waste of money (i.e. better than about 2ACH@50Pa). It should be straightforward to interlink the controls to primary windows and/or the heating system (if any - I guess in a passivhaus there wouldn't be one ... except that most seem to have one anyway).

    There are hundreds of thousands of HRV systems here in Canada and I don't hear of any problems with using them so I'm not sure a special study is required. If you're completely obsessed by the fact that an HRV uses electricity, the load is small enough that a small PV could easily offset and carbon requirement.

    Paul in Montreal.

  14. Thanks Paul, that's a really useful contribution. And I believe that you are right: MVHR is not about saving energy, it's about IAQ. My issue isn't that it doesn't work, but that it mat simply be a step too far for the UK climate - i.e. an expensive waste of money and resources. That's why I think there needs to be a trial before it's rolled out as mandatory. Sussex is not Quebec.

  15. I am puzzled by this thread:

    Your initial concern was about occupants not noticing that the MVHR had stopped:
    - Since controls usually have indicator lights (even remote ones are possible if requested), where’s the problem?

    Or what happens to the air quality if it stops working:
    - Again, the practical experience of occupants seems to rule out that issue.

    Then it was, that people might not accept it:
    - Our building regulations are not based on users’ acceptance, but on health, efficiency and safety.

    Or that they might not like it:
    - A rather large number already do, it appears. And by nature of the mixed housing market, not all of these will be “elective” owners.

    Next it was high running costs:
    - Figures not up-to-date, it seems.

    Now the MVHR is too expensive:
    - Compared with? Central heating?

    “Let’s have more trials” … :
    - And committees, and consultation papers, and. … ? We have wasted 20 years, have run out of time, don’t have to test something that has already been proven practically just because of “not invented here”. We cannot afford further delays.

    An even better system that you may invent, demonstrate and trial in your house will be of great interest. Until that is developed and fully tested, the latest proven generation of MVHRs seems the “best buy” to me.

  16. Ventilation divides people into almost religious or political camps!

    My own daily experience is living is an airtight and super-insulated home with hi spec’ double glazed windows and natural ventilation. What I would change if we built our own house again would be:

    1.Design using the PHPP spreadsheet to optimise form, shading, glazing etc.
    2.make it even more airtight
    3.use triple glazed windows installed within the insulation layer
    4.install heat recovery ventilation.

    Our house is comfortable in winter and summer and our energy bills are a non-issue but I regret not implementing the above because they would make the house even more comfortable and easy to live in.

    If we don’t care about energy and want a less efficient house beware we need to specifically design a traditional high-energy house to avoid very serious problems! Ensure the n50 pressure test is no less than 10 air changes, allow space for large radiators under the windows and cut back on insulation to reduce the risk of condensation damage through the leaky structure. If you deviate from this and are tempted to reduce drafts or fit fancy windows you could have serious problems as many thousands of households have discovered, you need to go all the way for this to work!

    However if we are serious about sorting energy use this is the logic I’ve followed:

    1.We must insulate which means we need to really sort the thermal bridges to avoid cold spots (mould, we are not putting in enough heat to warm up all the surfaces) and so we don’t waste the thick insulation by bypassing it.

    2.The insulation increases the risk of moisture damage in the structure due to air movement so we need to be really airtight. The airtightness is also needed to prevent cold spots (mould), drafts and of course heat loss. Hence the non-negotiable maximum airleakage of the Passivhaus standard

    3.The low heat input means that we don’t have hot radiators under windows so they will be the coldest surface leading to discomfort (sometimes we light a fire when room temp it is an acceptable 20C because the sky has cleared and the windows have cooled and we feel that, you get to be a sensitive soul when you live in an insulated house).

    4.We now need to ventilate to control moisture etc. Opening the windows in winter for background ventilation is not an option as we loose all our heat and it is drafty. Juggling trickle vents and passive stack is what we do and it’s a pain if you want to maintain good air quality AND low heat loss as weather and household activity vary considerably hour by hour. So I don’t really believe in passive ventilation any more and that leaves MEV and MVHR.

    MVHR is still fairly expensive in terms of energy saved per Euro invested but it has enough benefits such as improved comfort and air quality to make it attractive to me and it is far more cost effective than say PV, heat pumps or solar thermal if comparing with zero carbon bolt on bollocks. In time it could become a lot cheaper as it is a very simple technology compared with say a washing machine or boiler.

    But yes with MVHR the building needs to be really tight if there is to be a net energy saving compared with MEV (where air is sucked through any leaks in the fabric). Also the MVHR needs to be really good kit with high thermal efficiency, low fan power and low noise.

    So this is where I believe it makes sense to be heading (rather than anything you will find on the BRE Innovation Park). How we get there is another question and I do not underestimate how hard that will be. Intermediate steps are possible with care, e.g. AECB Silver standard (Passivhaus Lite). Bolting on a mix of measures to score points in a Code is a recipe for disaster. The leap is probably similar to that between a Trabant and the latest Skoda (with or without VW badge) but I don’t hear so many people arguing for Trabant technology because it is cheaper and easier to understand and fix.

    Bit long, hope it makes sense.


  17. Nice discussion:

    1. Passive Houses in central Europe are not "elective". Just 350 families moved in in Innsbruck in the sofar biggest passive house social housing elevation. Last project > 150 families in Frankfurt. These are "normal" persons (what is normal?).

    2. The climate in UK is even more favourable for Passive Houses than the climate in Germany is. Your problem just is, that you did not have experience in really airtight construction.

    3. But: Keeping the buildings as untight as they are in Britain will
    - not call for MHRV, I agree: you just do not need it in a drafty dwelling
    - never be able to come even close to a really good energy efficiency. Humans need fresh air, period. The amount is not to be reduced (this is a health issue!). So: it is easy to see, how high the ventilation losses will be.

    Aiming for zero carb without efficiency measures? That is like wanting to go to the moon with a hot air balloon.

    Kind regards: Wolfgang Feist, The Passive House Institute

  18. I wish people would stop talking such poppycock re. the allegedly soft gentle English climate and the harsh German climate.

    The need for heating in kWh per annum for a given house is much the same in Frankfurt or Vienna as in Manchester. Try it in PHPP.

    If you start manually entering climate data for Scottish towns and cities in PHPP, you'll note how cold they are compared to Scandinavia or indeed the cities in southern Germany which border the Alps.

  19. Absolutely

    And as we know PH approach is also applicable in warm climates. Some of the general rules of thumb such as U values (inc glazing) will vary between climates but for UK it seems we need triple glazing and well controlled ventilation to meet comfort criteria without resorting to excessive energy use.

    PH approach is actually very flexible but does set backstops on critical issues such as indoor surface temperatures and airtightness for reasons of health and comfort. Yes you could have a more leaky building with more expensive windows and thicker insulation but that would be silly and it would probably rot.

  20. Maybe you get a similar annual heat demand, but a much more favorable heat load. In your climate you can in general easily achieve a heating load of less then 10W/m2. That makes heating with the ventilation system possible. This saves also installation costs. This simplicity makes the Passivhaus standard so attractive.

    Don't lower your demands on airtightness and ventilation - they are crucial to the internal comfort of a house. Do you really think people are going to be happy about spending money on energy saving measures and ending up with draft or stale air?

    The main reason people enthusiastically embrace the Passivhaus standard all over Europe is because of their high comfort and building quality (read: value). And it comes with a bonus: low energy consumption at an affordable price.

    Try to achieve energy savings with no added value and you will fail. Which will be the case of houses with low airtightness and no MVHR system...

  21. Love the last sentence Bjorn

    However at the risk of being failed in the Passivhaus Designer exam in a couple of weeks, I have to say I remain sceptical of air heating for most buildings even though it is at the heart of the whole PH concept, but that's another topic!


  22. @Nick Bad boy! Great danger! (:-). Well, that is a misunderstanding: PH is not based on air heating; you are still allowed to use each heating system you want - but please do not complain about costs. You could see it this way: Passive house is the energy standard, with wich the heating system used does not matter any longer - you could even use fresh air heating. And you will love it in a passive house, even if you do not like it anywhere else.
    Beeing sceptical is one of the most important principles in science. I admit, that I was sceptical on fresh air heating at the beginning, too. But I learnd from experience, it works and it works well.

  23. Paul,

    You "don't hear of any problems with using them" because many (most?) people don't know how to properly use them. I've seen many installs where the unit is run at low speed 24/7 and kicks into high speed with a humidistat. Combined with 2ACH@50 and cold winters (-10C days & -20C nights) gives an unhealthy indoor RH of 25% or less which promotes virus transmission (like H1N1), causes nosebleeds, irritated skin, more dust in the air, annoying static, ...

    If I could find bathroom exhaust fans with reasonably airtight dampers I wouldn't put an air exchanger in a new house...

    I posted this in another thread, and would link to it if I knew how:
    During my 2nd winter I never ran my HRV after the end of November, and the humidity stayed around 40% for most of the winter. The house volume is 1530m^2 (54,000cf), tested 1.1ACH@50Pa, and is occupied with 2 adults & 4 children (and often 2 extra adult guests).

  24. I live in a climate with 4520 18C degree-days and my heating energy use is <20kWh/m^2. Walls are 2x6 + 1.5" of foam, windows are dbl-glazed lowE/argon.

    Air tightness is 1.1ACH@50, and my MHRV is OFF, as it was most of last winter and likely will be most of this winter. RH & temp in the upstairs bedrooms is currently 47% & 18C.

    I heat with geothermal, so if I had no HRV negative pressurization would not be a problem. In houses with fuel-burning appliances I think a balanced air exchanger (not necessarily with heat recovery) should be mandatory; the risk of CO poisoning is too high.

    With no fuel-burning appliances I see no convincing reason to require an air exchanger, let alone a MHRV.

  25. I just got a phone call to say I was in trouble!! :-)

    Wolfgang, I'm not against fresh air heating and do believe it works (in a Passivhaus). As you say it is not a requirement of PH and what I have to say on the matter will confuse this discussion so will be somewhere else.

    Ralph I can't comment on your low heating requirement without MVHR but would like to point out for anyone following this discussion that the PH designer is very aware of over ventilation and low humidity although low winter humidity should be less of an issue in UK than Germany.

  26. Ralph,

    I don't think there is any correlation between dry air and virus transmission.

    Your RH is so low because you have a very large volume house for the number of humidity generating occupants - so you may be correct that a HRV is not required. Typical UK homes are probably 1/4 of the volume of yours and so humidity levels are likely to be higher from this fact alone. 1ACH of a large volume is a lot more fresh air than 1ACH of a small volume - so ACH figures are likely misleading. This is why ASHRAE and CSA specify ventilation rates in litres/second (or equivalent) per number of occupants. Of course, the rates may be met through natural infiltration without need of an HRV, as your case demonstrates.

    Passivhaus airtightness standards are higher than even your good figures. The advantage of using an HRV is that you can guarantee the ASHRAE (or equivalent) ventilation rates are provided for, no matter what the occupants do. Also, a well designed HRV will provide the ventilation air where it is needed, rather than hoping the leakage is occurring in the right places.

    To Mark et al, I still don't understand the fear of HRV systems - it's not like they are new or haven't been used much in other places. As for climate, the UK climate is surprisingly (large number of heating degree days) so to say the climate is mild is reflected in the actual data. And given the dreariness of the weather, solar gains in are also disproportionately low, making a Passivhaus even harder to achieve than other climates where the temperatures may be lower but there are more utilizable solar gains.


  27. Paul,

    Here's a few reports demonstrating the link between flu virus transmission & humidity:

    I find the research to be convincing, and the physics of virus suspension time in the air quite logical; low humidity causes droplets containing virus particles to shrink in size, increasing the time they remain airborne (to >24hr around 5 microns). At >80% RH, the droplets act as condensation nuclei, grow in size, and fall to the floor in seconds.

    As for ventilation rates, according to Hot2000, the F326 Required continuous ventilation rate for my house is 211.9 cfm (0.24 ACH). That's more than double the rate required to maintain CO2 below 900ppm with 6 occupants.

    I agree that the climate in the UK is generally milder than Canada, but doesn't make much sense to treat a house built in Cork, Ireland the same as one built in Newtonmore, Scotland. I suggest staying away from opinion (HRVs are good/bad), and stick to the science. If the natural air infiltration rate during Jan/Feb is even close to providing ventilation for the occupants, then there's no need to install a MHRV; once you factor in normal occupant activities like running bathroom & kitchen exhaust fans, open/closing doors, etc. there will be lots more fresh air getting into the house.

    I think part of conservation is allocating resources as efficiently as possible; in some cases the MHRV is an inefficient allocation of resources.

  28. PaulM,

    there is experimental evidence that flu viruses survive on indoor surfaces longest at low RH (20%-40%) [2], and animal experiments suggesting that we may produce more viruses for longer at low relative humidity [1]. These are among the suspected reasons for why flu is much more common in winter, along with other "seasonal factors such as melatonin and vitamin D levels; seasonal changes in host behavior, such as school attendance, air travel, and indoor crowding during cold or rainy weather; and environmental factors, including temperature [..] and the direction of air movement in the upper atmosphere." [1]

    [1] Lowen AC, Mubareka S, Steel J, Palese P: Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature. PLoS Pathog 3(10): e151, 2007.

    [2] Schaffer FL, Soergel ME, Straube DC (1976) Survival of airborne influenza virus: effects of propagating host, relative humidity, and composition of spray fluids. Arch Virol 51: 263–273.

  29. Thanks for the links about virus lifetime and RH - I'm happy to stand corrected!

    I think ventilation is less of an issue in houses with forced air heating than in those with other methods as the heating system naturally moves air (and hence humidity) around. Part of the problem in the UK is that people tend to be very zonal in their heating and tend to have bedrooms that are quite cool, leading to mold growth behind furniture etc. This, I believe, is less of an issue in North America where air-based heating is the norm rather than the exception.

    Maybe one good thing about Passivhaus standard construction is avoiding such zonalities and having all the internal areas of the house warm enough that mold will not form - thus improving the IAQ as a side effect. I remember many places I lived in the UK where mold would form behind furniture in bedrooms in particular.

    Nice to have such an interesting discussion!

  30. Poor Mark, the geeks have taken over your forum!!

    Ralph and Paul, the virus thing is interesting but as I mentioned PH is really clear on maintaining optimum humidity and stresses that people are more likely to complain about dry air than high CO2.

    Conventional UK M&E for non domestic buildings is far more likely to over ventilate and I have watched sandwiches curl and go crispy in a 1 hour meeting.

    Re heat loss, the physics can't lie and if we need around 20-30 m3/(p.hour) for clean air then this needs heating up in winter.

    Today a cold wind is howling and I'm struggling to juggle ventilation. Could probably close all vents but would need to do a calculation to work out the infiltration given current windspeed and my n50 pressure test - was there a mention of normal people???

  31. Nick,

    You are right. It's turning into a forum in its own right.

    To all, all I was ever trying to say is/was that I am not convinced that MVHR is a fit and forget solution. I have anecdotal evidence of people who have turned it off/taken it out/adjusted it and messed up the balance/closed off certain registers/getting mouldy bathrooms. And these were all people who chose to fit it. If we make it mandatory, how can we be sure that it will work as planned?

    I accept that it's currently probably the best solution available. That doesn't make it perfect....

  32. Mark, were these MVHR units used in ‘conventional’ houses or in passive houses?

  33. I'm talking about half a dozen homes I have visited over the years, all selfbuilds, all well insulated, but none of them Passivhaus standard.

    Interestingly, one had a conservatory which was included in the MVHR network as an inlet. The owner tried to isolate the conservatory in winter and to manually shut down the register, but this simply threw the whole system out of balance. It shows how inflexible MVHR can be, and how difficult it can be to tell if it's working correctly. Call him stupid, if you like, but we are all capable of being stupid.

  34. Glad you didn't run away Mark

    I can imagine the thought process, recover heat from warm air in the conservatory to warm the house - err on hot days.

    Not stupid, just not done the maths or thought it through. If he had discharged the boiler flue indoors to recover the heat from that would this be evidence that boilers are an inappropriate technology for our DIY culture?

    Suggestion for next blog post, is anthropogenic climate change a New Labour conspiracy?

  35. Now from this conservatory example, you might forsee an intelligent MVHR with self-regulating registers which respond to the conditions. This is what ITHO's Demand Control Ventilation system essentially is driving at. This ought (in theory) to be a vastly improved solution, but it would be technically even more challenging than regular dumb MVHR, and there is still little in the way of user-feedback to indicate whether it's all tickety-boo.

  36. This comment has been removed by the author.

  37. I agree with Nick, there's no need to make the whole HRV system overly complex. All it has to do is supply some fresh air (the amount is debatable depending which authority one speaks to, whether it is the ASHRAE, CSA or whoever) and expel a similar amount of stale air with some degree of heat and/or energy transfer to the incoming air. If it's not exactly perfectly balanced, it's not a big deal unless there are unsealed combustion appliances. If the user turns it off, it's not going to kill them. Maintenance is simple - certainly less complex than any gas boiler etc. Even the controls don't have to be complicated: just a few boost switches for the bathrooms and that's all that's really needed and maybe a humidstat. But if you don't build airtight in the first place, then there's no point in putting one in.

    And if you have built airtight, you might as well enjoy a home that can be heated to a comfortable temperature throughout, rather than having to have a couple of warm rooms and risking mold in the ones that aren't heated. After all, comfort is still an important factor even in low energy homes isn't it?

    Paul in Montreal

  38. In terms of detecting if the MHRV is running effectivly or needed at all we should reflect on the future changes which might occur in the home

    My Morris 1000 of 1964 had 1 sensor - oil pressure

    My Galaxy of 2007 has in excess of 250 -

    both worked - both drove - and the only impovement in that time is from 25 MPG to about 38 MPG (of course this is not apples and apples)

    So what we need to do to validate these arguments is to have some detailed, mesured, objective tests

    but given that these take time, resources etc etc then a sealed, semi-climate-controlled house has to be better than the 80 or 90% or houses currently which allow for most of the heat to seap out very quickly

    A potential (and soon to make a decision) Passive house builder in Wexford, Ireland

  39. 1. There is no requirement for MHRV in a Passive House - there is however a requirement for evenly distributed fresh air and high levels of comfort. So far I haven't figured how to do this manually, but if anyone can come up with a way, they can have a PH without MHRV.

    2. MHRV has been used in a number of extensively researched social housing apartment blocks in PH Standard in Europe. Some Austrian federal states even require it for social housing projects. There has never been a noteworthy degree of malfunction detected - but a high degree of user satisfaction. Now, these occupants did not choose this technology at all, and for example in an intensively researched apartment block in Kassel, Germany, when asked at the time of moving in whether they would recommend it to their friends, some were quite reluctant to do so. The picture changed in a repeat survey after the first winter. Practically no one wanted to live without it any more. Proof of the pudding is in the eating - it's just brilliant to live in a well designed mechanically ventilated house - I am testament to that as well!

    3. As for bang for the buck: a COP of 16.5 was measured! (as opposed to calculated - what you get for most strategies) at the Hanover row houses estate. For 1 kWh invested electrical energy you get back 16.5 kWh heating energy. Compare this to a top-of-the-line heat pump (4)! There is no technology in houses that uses energy as efficiently!

  40. I have to echo Barneys defence of the CSH - a holistic look at how the house can influence the choices and lifestyle of the occupier to reduce their carbon footprint and is not just narrowly focussed on the insulation levels, air-tightness and orientation. On another point, my understanding was that a gas fired boiler was permissible under CSH Level 6 with emissions offset by a renewable energy?

  41. We are currently looking at the ecorefurb of a very large 1970's dwelling. The intention is to wrap in insulation and make very air tight. We are looking at using MVHR especially as we have 9 WCs/Ensuites. We do have some reservations though. There is no way the owners will want to keep the windows shut during the summer. We are therefore keen to provide a mix mode solution where during the winter a MVHR strategy is follwed and during the summer the supply fan is switched off and extract only is provided. This seems sensible to minimise wasting energy on the supply fan. Has anyone come across an MVHR unit which can provide this type of control option?