Air Flow windows (aka supply Air windows)

[fostertom]

Time I took the trouble to understand Air Flow windows - www.dwell-vent.co.uk with www.howarth-timber.co.uk. Can anyone explain? (These guys call them Supply Air windows, but the principle can also be applied as Exhaust Air windows)

[Chris Herring]

These windows work by allowing ventilation air to enter between panes, which can be 1+1 or 1+2.  In the UK they are 1+1 I think.  David Olivier informs me (I thought on the Forum, but can't find it, so maybe privately) that these have been around a long time in Finland.

When I looked into it, these are not suitable until airtightness approaches 5 m3/m2/h ie half current building regs.  On the other hand, MVHR is generally considered to come into its own at around 3 m3/m2/h.  Low energy building standards, such as AECB Silver, Passivhaus or AECB Gold generally advocate quite high levels of airtightness together with MVHR.  Use of MVHR obviously precludes use of these windows as they form part of a passive stack methodology.  So current regs are probably not airtight enough for these to be used efficiently - I understand they have experience problems of back ventilation.  And if we are really trying to build to best developed current methodologies, then these are probably ruled out. 

So, leaving aside the claimed 'effective' U values, which I have seen research work to support, never the less these windows have a relatively limited window of opportunity (sorry for the pun).

Now wait for a building physicist to post up a more systematic analysis!

[Nick Grant]

Yes Chris I'm sure we discussed this on forum unless it was email with you me and David, have searched but nothing.

Over to you David!!

[David OLIVIER]

They can work with mech. exhaust-only ventilation which is an option with the Silver Standard, espec. in flats. Yes, they've been around a long time.

I think the first I heard about them was a building in Finland about 1978. Due to the climate, Finnish buildings even then were airtight enough.

David.

[Andy Simmonds]

Chris,
AECB Silver standard is 1.5 m3/m2/h if MVHR is used.

[Mark Siddall]

I wonder what the cost differential is between MVHR and air flow windows.

I've read that airflow windows can achieve a centre pane U-value of 0.9w/m2K  (by Finish company Tiivi) to 0.6w/m2K  (Domus Domlux). The effective seasonal U-values could be much less. In fact McEvoy et al 2003 note seasonal U-values as low as 0.33w/m2K  compared with 1.4w/m2K  for comparable triple glazed window. Others report a 60% reduction in U-value i.e from 1.6/1.8w/m2K down to 0.6-0.7w/m2K.

Mark

[Jennifer Monahan]

Hi I'm relatively new to this forum malarky but couldn't help noticing this thread.  I to came across dwell vent earlier this year (project pilot was/is in Norwich where I'm based).

I was interested as it formed, with passive stack vents in wet rooms, a whole house passive ventilation system.  Personally, I like things to be as simple and as non mechanised as possible so was persuing it as an interesting idea. 

The pilot results indicated that occupant behaviour is fundamental in the success of the system (smokers and window/door openers were a bad combination as I understood it).

I'm no physisist so this next question may be very dumb indeed:  Assuming the perfect occupant and the building stays airtight as designed.  What happens  during periods of warm(ish) weather and the window vents are closed or, alternative scenario the vents are open and the external temperature/humidity approaches that of the inside?

Any discussion on this is most greatly received.

Jenn

[Mark Siddall]

Having thought about it I don't think MVHR is suited to air-flow windows (as the efficiency of the HV unit will be compromised). Perhaps air-flow windows are more suited to MEV.....

Not being a building physicist either I don't know quite what would happen. It strikes me that as long as the RH remains below the dew point I can't forsee any problems (furthermore, as long as there is negative internal pressure, condensation should be minimised as despite what may be a high external RH the moisture content will be low, thus as the air begins to warm as passes through the system RH will fall). As for "warm periods and closed vents" with no other nat vent it will get warm inside othre than that I can think of anything at the moment.

Mark

[Chris Herring]

Mark
Clearly they don't work with MVHR as I said earlier, but as David says would with MEV.  So not suitable for the more energy efficient builds moving towards PH.

[David OLIVIER]

To answer the question, as the temperature difference falls the ventilation rate of a PSV plus airflow windows system would decline towards zero. It's not ideal, because on a cloudy day with these temperatures people might not have the windows open.

David.