NIght Flushing

Maintaining the internal climate of buildings and structures while minimizing the ecological footprint as well as reducing power consumption has always been a very important part of architectural engineering. To achieve this goal of climate control an important technique being used extensively, especially useful in areas with relatively large temperature difference between days and nights; and in relation to buildings and structures constructed of material with high specific heat capacity such as bricks masonry and concrete, is called Night Flushing.

Night Flushing is an interesting use of natural ventilation. By using the natural cooling effect of the night and the cooler air at night simply allowing the cool night air to circulate a structure during the night allows the loss of the heat buildup, or heat mass gathered by the structure during the day. In order to achieve this cooling one simply needs to allow the night air to circulate the building , in simple structures by simply opening the windows during night time. The cool night air carries away the heat absorbed by the structure during the day. The very nature of concrete or other high specific heat capacity materials makes them perfect to use in conjunction with Night Flushing as the structure will take a long time to absorb enough heat during the day to change its temperature and thereby not only decreasing the cost of maintaining a stable internal climate during the day, but would also drastically decrease the cost of cooling as most of the heat absorbed during the day is lost during the night via Night Flushing. Thermal mass is a property enabling structures to absorb, retain and then release heat energy, this coupled with a high specific heat capacity means that buildings made up of concrete need to absorb a substantial amount to heat to effect a change in temperature. 1

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This simple use of natural cooling if properly incorporated in a building’s design could drastically reduce the load on HVAC and can result in substantial savings of electricity.

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This technique is not without its limitations and one of the most important cons which comes to mind immediately is security, especially in residential buildings or buildings occupied during the night. Similarly Night Flushing requires a climate with a marked difference between night and day temperatures, so its implementation is very climate dependent.

For those interested in further reading the subject here a few links to get you started.

http://en.wikipedia.org/wiki/Thermal_mass

http://www.concretethinker.com/solutions/Thermal-Mass.aspx

http://sustainabilityworkshop.autodesk.com/buildings/natural-ventilation

http://digitallibrary.usc.edu/cdm/ref/collection/p15799coll127/id/314242

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