Air Flow Windows

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Apart from letting the light in windows do a good job at letting the warmth out. The energy leakage is lowed by double glazing and low-emissivity glass but if you are really looking to save energy at your windows you might like to consider an 'air flow' window. They're a bit rare at the moment but could get commoner when people learn what a difference they make. Most modern windows of the factory made sort, come with a 'trickle vent'. This is a slot in the window frame that lets cold air into the room and so compensates for all the draught proofing that we now use to keep the cold air out. Draught proofing sounds like a good idea until you remember that we need to breathe and get rid of water from washing and cooking. Hence the Building Regs. insist on either trickle vents in the windows or else some other form of ventilation being provided, air bricks or such like.

With an air flow window there are three layers of glass, a conventional double glazed sealed unit and a third sheet of glass. The air gap between the double glazed unit and the third sheet is not sealed. There is a vent hole to the outdoors at the bottom and another vent leading to the indoors at the top. Fresh air comes in at the bottom and move up between the panes of glass before entering the room. As it rises the air collects heat from the indoor pane of glass and carries it up and into the room instead of allowing the heat to escape outside. Furthermore, heat from the sun, trapped between the glass layers greenhouse fashion, is also collected and carried into the house. You get fresh air coming into the room but it is not such cold air, having recycled energy that would otherwise have escaped, and picked up more from the sunshine. The effective U-value is dramatically decreased.

Of course air will only come in through the window if it can go somewhere. The system will only work if there is a passive stack or open flue or some such thing sucking air out of the house. If there's an open fire (ooh, how inefficient) of a woodburner in the room, it should work just fine.

This is all a rather experimental area of window design (even if they have been using them for fifty years in Finland) so I'd be interested to hear from anyone with experience or thoughts on this subject.

Here's a press release from the European Commissions 5th Framework 3 year, research project RDPCLEVS (Who thought of that title?) coordinated by Dr Mike McEvoy, at the Martin Centre, University of Cambridge.

MEDIA RELEASE

Preliminary international research study findings point to potential savings of 10-15% on household fuel bills following installation of innovative ventilation system

20 January 2004:
Preliminary results from a European Commission-sponsored research project being carried out by scientists in Ireland, Poland, Denmark and the UK indicate that household fuel bills could be reduced by between 10% and 15% if homes were fitted with a new type of window ventilation system that has recently been developed by the University of Cambridge.

The study findings are particularly relevant in the light of the EU’s ratification of the Kyoto Protocol and the commitment to reduce carbon dioxide emissions by 13% by 2010. Currently, energy generated for household heating purposes accounts for up to 20% of total annual CO2 emissions/unit GDP in EU Member States.

The findings are also relevant in the context of growing concerns about air quality in houses, and its implications for human health. Air quality has been found to be poor in many EU countries where national building regulations and codes prescribe sealed construction as an energy-saving measure. It is widely accepted by scientists and health experts that domestic condensation and the detrimental health impacts of poor air quality are partly due to the draught sealing of properties.

The European Commission 5th Framework Programme research project RDPCLEVS, which is due for completion in May 2004, aims to develop a robust energy-efficient ventilation system to counter the poor air quality that has been exacerbated by tighter modern construction techniques coupled with increased draught sealing of buildings. The project is also aimed at addressing the inter-related problems of health, energy and comfort.

In order to establish the range of climates to which the system is most appropriate, the project field studies are being carried out in Denmark, Northern Ireland and Poland – locations that were chosen for their variation in terms of climate and geographical spread.

In each location, selected homes have been fitted with an innovative low-energy, whole house ventilation system that has been developed by the University of Cambridge.

Among many benefits recorded to date, the new system improves energy efficiency (thereby reducing CO2 emissions and generating lower fuel bills); it provides effective whole house ventilation; combats condensation and mould growth; reduces cold draughts; involves simple installation and requires minimal maintenance.

The window ventilation system can be manufactured relatively inexpensively. If fitted during construction, for example, it would add as little as 10% to the overall window costs of a house/apartment. It is also suitable for retrofitting in older buildings where windows are being replaced.

For further information, please contact:
Dr Mike McEvoy, the Martin Centre, University of Cambridge
Tel: + 44 1 223 331 705 (office) or +44 77 6660 0640 (mobile – Dr. Ryan Southall)/ email: mm366@cam.ac.uk

Dr Kirk Shanks, the Energy Research Group, University College Dublin
Tel: ( 01) 269 2750 (office) or 086 3700 764(mobile)/ email: shanks@erg.ucd.ie

Project website: http://erg.ucd.ie/rdpclevs

The following piece is from http://irc.nrc-cnrc.gc.ca/pubs/cp/win1_e.html

Emerging Window Technology A. H. Elmahdy and S. M. Cornick Originally published in "Construction Canada" 32(1), Jan. 1990, p. 46-48

Dr. A.H. Elmahdy and S.M. Cornick are researchers in the Building Performance Laboratory of the Institute for Research in Construction.

Air Flow Windows. An air flow window generally consists of a double glazed sealed unit with a single pane on the exterior side. Air is allowed to flow between the single pane and the double glazing. Interest in air flow windows has stemmed from the possibility of improved thermal performance and the trend towards providing controlled air changes within a building.

There are two types of air flow windows, the exhaust air window, and the supply air window.

In an exhaust air window, indoor air is forced through the window space and then exhausted outdoors. During the heating season, warm inside air would transfer some of its heat to the window, reducing the temperature differential across the inside panes, and therefore reducing the heat loss. When cooling is required, an exhaust air window would expel some of the solar heat gained by the window and lower the cooling loads. Solar blinds can be introduced in the window space to reflect additional solar heat.

Figure 1.Air flow windows

Supply air windows draw air from the outside, through the windows, into the building. During the heating season, cold outdoor air flows over the inside panes absorbing heat from the window and solar energy before being introduced into the building. The preheating of outside air tends to lower the heating requirements for the building. Test models indicate that the average air temperature rise inside the window is approximately 50% of the difference between interior and exterior temperatures.