With the use of wind towers placed on the roof of ventilated spaces, supply and exhaust can be placed close to each other on opposing sides of the small towers. [9] These systems often feature finned heat pipes although this limits the theoretical maximum heat recovery efficiency. [10] The Passive Air Vent is an environmentally friendly condensation control vent, designed to minimise if not eliminate condensation. Calautit, J. K., O'Connor, D. & Hughes, B. R., 2015. A natural ventilation wind tower with heat pipe heat recovery for cold climates. Renewable Energy, I(87), pp. 1088-1104. Under typical comfort ventilation conditions with a heat recovery efficiency of 80% and a SFP of 2000 J/m 3 we get:

Gan, G. & Riffat, S., 1999. A study of heat-pipe heat recovery for natural ventilation. AIVC, 477(12), pp. 57-62.The Condensation Control Vent contains a vapour permeable membrane that allows vapour out the building without letting draughts come inside. a b Schultz, J. M., 1993. Naturlig ventilation med varmegenvinding, Lyngby: Laboratoriet for Varmeisolering, DTH. (Danish) The following design guidelines are selected from the Whole Building Design Guide, a program of the National Institute of Building Sciences: [4] Different methods of passive ventilation are effective in different environments, which obviously impacts on their popularity.

McWilliams, Jennifer (2002). "Review of air flow measurement techniques. LBNL Paper LBNL-49747". Lawrence Berkeley National Lab. The RUK Condensation Control Vent contains a vapour permeable membrane that allows vapour out the building without letting draughts come inside.In climates where the mean absolute difference between inside and outside temperatures exceeds ~10K the energy conservation argument for choosing natural over mechanical ventilation might therefore be questioned. It should however be noted that heating energy might be cheaper and more environmentally friendly than electricity. This is especially the case in areas where district heating is available.

Later attempts have primarily focused on wind as the main driving force due to its higher pressure potential. This however introduces an issue of there being large fluctuations in driving pressure. Autodesk, 2012. Passive Heat Recovering Ventilation System. [Online] Available at: sustainabilityworkshop.autodesk.com/project-gallery/passive-heat-recovering-ventilationsystem Passive ventilation is the process of supplying air to and removing air from an indoor space without using mechanical systems. It refers to the flow of external air to an indoor space as a result of pressure differences arising from natural forces. Since the internal heat gains which create temperature differences between the interior and exterior are created by natural processes, including the heat from people, and wind effects are variable, naturally ventilated buildings are sometimes called "breathing buildings". While natural ventilation eliminates electrical energy consumed by fans, overall energy consumption of natural ventilation systems is often higher than that of modern mechanical ventilation systems featuring heat recovery. Typical modern mechanical ventilation systems use as little as 2000 J/m 3 for fan operation, and in cold weather they can recover much more energy than this in the form of heat transferred from waste exhaust air to fresh supply air using recuperators.Passive ventilation makes use of natural forces like the buoyancy of hot air and wind to encourage airflow through your property. The result is that moist stale air is extracted and replaced with fresh clean air. Kitchens, utility rooms, bathrooms and toilets are rooms where conventional domestic extract fans are normally found. The natural ventilation principle of Freshvent 100 is particularly suitable for ‘clean’ living spaces such as bedrooms, lounges and work rooms. Wind driven ventilation can be classified as cross ventilation and single-sided ventilation. Wind driven ventilation depends on wind behavior, on the interactions with the building envelope and on openings or other air exchange devices such as inlets or windcatchers. Liquid coupled run around loops have also been tested to achieve indirect thermal connection between exhaust and supply air. While these tests have been somewhat successful, liquid coupling introduces mechanical pumps that consume energy to circulate the working fluid. [11] [12]