31 December 2011 at 11:25 am #31611
As an M&E engineer and building physicist, and (hopefully) soon to be certified Passivhaus consultant/designer, I'm feeling quite uncomfortable about Sue Roaf's page 4 article in the Winter 2011 Green Building Mag.
She is basicaly criticising the design approach of mechanical ventilation of all sorts. I feel she is being a little disingenuous in some respects, e.g. suggesting mechanical ventilation results in dryer air, and indulging in shooting fish in a barrel with regards to the building regs.
It seems to me that while these criticisms are valid, they can be resolved via good design. For example, a 'myth' of passivhaus is that you can't open windows. However, Designing a mixed mode MVHR system for houses is not a trivial task, because of the need to maintain ventilation (generally extract) for Part F purposes. Possible, but not usually with off the shelf kit, and it's therefore a more expensive exercise.
Fundamentally, if we are to build or retrofit buildings to a high energy standard, reducing heat lost via air, both infiltration and ventilation, must be reduced. Beyond a certain level, mechanical ventialtion of some sort must therefore be used. Sue Roaf seems to be suggesting that we heat air to 16 degrees and use local radiant heating to maintain thermal comfort to reduce heat loss via air.
Technically there are a few problems with this; first, you will still loose a fair amount of heat via air because it's warmer than external – but to be fair, it would be a bit less. Secondly, how do you achieve radiant heating? The cheap (capital) option is direct electric. There is not a big market currently for flued radiant gas heaters, but I suppose that could be possible, but you'd need one in every room, and unless you had several in every room, would thermal comfort be at risk due to drastically asymetrical heating? At least they can be turned off when people leave the room, to save energy. But we're bad enough at turning off lights! We have a long tradition of solid fuel stoves, which is what I live with, but they are difficult to control and therefore in herrently inefficient and not conducive to consistent thermal comfort.
There is also a big issue with exactly what is preferable in terms of thermal comfort. The approach suggested by Roaf is the polar opposite of passivhaus (and I'm not saying PH is Right or Perfect), which is to maintain very even temperatures over all internal surfaces – and this means everything at 20 degrees C, unless you cover everything with an electric heating mat or something.
As far as I can tell, a lot of this boils down to exactly what different individuals find thermally comfortable, if we assume all the technical issues can be resolved. So what should we as energy-efficient building practitioners take from this debate?
4 January 2012 at 9:18 am #38323Nick GrantParticipant
I think a lot of us wanted to respond to Sue's article but didn't know where to start!!! So many myths.
As you know, ventilation + heat + cold outdoor temp = dry air. MVHR just means its possible to have ventilation in cold weather without feeling cold.
Happy New Year
5 January 2012 at 12:11 am #38324Mark SiddallParticipant
Yup. I'm with you all the way.
9 January 2012 at 9:27 am #38325
Thanks guys, nice to hear I'm not missing something!
I've re-read the article but it's even more baffling now, it's mixing up a lot of issues.
On the other hand, if these complaints are genuinely arrising, what should we, as designers of the PH school, be doing better? No smoke without fire (an unfortunate analogy).
I am aware of some pretty poor systems (from a design point of view) and have narrowly avoided one or two of these on projects where we were borught in a bit late. Is it possible to out-compete the bargain-basement design-install outfits, or is there insufficient public appreciation of what we do?
Also the range of MVHR available is not great, controls and ability to do mixed-mode being two aspects that I've had painful experience with, at a domestic scale at least. How can we persuade manufacturers to raise their game?
11 January 2012 at 8:27 am #38326
Ha, designing a PH School – I wish! I meant us as designers who are members of the “School of PH”. But the point is worth making, that at larger scales, the flexibility and range of kit is much greater than the choice of domestic scale 'boxes'.
Totally agree on the kitchen issue, I simply don't believe these systems that claim they can handle air from an extract hood, which is a shame as there's so much heat to be recovered from that hot, wet air!
11 January 2012 at 9:57 pm #38327Mark SiddallParticipant
I'm working up some thoughts that I hope to edit down into an AECB SoapBox article. (With a bit of luck an extended version will appear in GBM to right a few wrongs.)
20 January 2012 at 7:32 am #38328Nick GrantParticipant
Sorry James, read your post too quick!!
For domestic it's usual to use a recirc hood for cooker, discussion on this elsewhere but not really sorted to my satisfaction yet. Recently saw a very cheap and cheerful one in a PH with no carbon filter and just very basic grease filters.
The MVHR wll deal with the moisture from cooking even if it can't keep up with it during the actual cooking time.
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