Advice provided by: Sarah Gudeman, LEED AP, EIT, Morrissey Engineering, Inc.

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Timothy asks: Can you explain at least one way how a building can regulate each of the following – air exchange, cooling, heating and indoor humidity – without mechanical means? Thank you.

Answer: Hi Timothy, you’d think that being a mechanical design engineer by trade I’d be able to rattle off a quick and detailed response to your question. But believe it or not, in my four years of design work I haven’t yet come across a job that utilized non-mechanical means as a primary method of comfort or ventilation control (this strategy is oftentimes utilized in apartment buildings and residential applications). However, the one topic we do discuss oftentimes if natural ventilation.

Codes and standards such as ASHRAE Std. 62.1-2007 dictate how much outside air (OA) for ventilation we as engineers are required to deliver to commercial buildings. This quantity depends on a complex calculation including the variables of space types, number of occupants in each space, floor area of each space, the type of HVAC system and delivery methodology for air to each space.

In a simple example, a building is served by a rooftop unit (RTU) and outdoor air is mechanically brought into the building via the RTU, then mixed with return air and delivered to the space. The alternative, natural ventilation, by definition, does not use mechanical means (such as a fan) to deliver OA to the space. In most common cases, the discussion begins with operable windows. Section 5.1.1 of Standard 62.1 requires that “Naturally ventilated spaces shall be permanently open to and within 25 ft. of operable wall or roof openings to the outdoors, the operable area of which is a minimum of 4% of the net occupiable floor area.”

Other natural ventilation openings include doors, dormer openings and skylights, roof ventilators, stacks and specifically designed inlet or outlet openings. Roof ventilators provide a weather-resistant air outlet. Natural-draft or gravity roof ventilators should be positioned that they receive full unrestricted wind. Stacks or vertical flues should be located where wind can act on them from any direction.

Natural ventilation can and is used to control both ventilation and regulate temperature effectively in mild climates, but is not considered practical in hot and humid climates or in cold climates. Temperature control by natural ventilation is often the only means of providing cooling when mechanical air conditioning is not available (ASHRAE Handbook: Fundamentals 16.12).

I suppose an example of natural heating would involve combustion of some sort, or a passive solar heating set-up. Again, climate would play a role in this strategy. Examples of non-mechanical humidity control would include the use of desiccants for dehumidifying and something as simple as this mast humidifier (http://www.treehugger.com/files/2010/01/mast-humidifier.php). You can read plenty more about natural ventilation in the aforementioned Standard 62.1 or in “Natural ventilation in buildings: a design handbook” by Francis Allard, Matheos Santamouris.