Monday, December 8, 2008

Natural Ventilation

The house has been designed to be comfortable at certain times of the year without heating or air conditioning. The owners prefer to use HVAC as little as possible, and, in this climate, with proper ventilation, none is required for extended periods in the spring and fall. The house is made comfortable without air conditioning via cross-ventilation, incorporation of the chimney effect in higher spaces, ceiling fans, and excellent insulation. Additionally, a screened porch off of the Living Room has been incorporated into the design of the house. The screen porch is a traditional southern house feature that allows for mosquito-free outdoor comfort and privacy much of the year.
The house itself has been designed for natural ventilation with strategically placed operable windows with screens. Remotely operated windows in the clerestories create the chimney effect.
Documentation and Verification: In the primary living areas natural ventilation is achieved as follows:

• Over 12% in operable window area/conditioned floor area in the Master Bedroom in addition to a pair of screened French doors to a covered deck.

• Over 12% operable area/conditioned floor area in the remaining primary living areas (Living Room, Dining Room, Kitchen, Bed Rooms). Operable area achieved through inclusion of one door from the Living Area to the Screened Porch.
• At least two windows in each habitable room are on opposite or adjacent walls.

• Insect screens installed on all operable windows.

• Ceiling fans in all primary living areas.

Quantifying the contribution of natural ventilation:

• There are physical principles that control the flow of air in a house that is referred to as natural ventilation. Our discussion can focus on “Stack driven ventilation”. Wikipedia has a good discussion under “Natural Ventilation” and refers to an equation in ASHRAE. We have verified that our ASHRAE Fundamentals are the same equation. This equation (see attached) estimates the stack flow in cfs (cubic feet per second) as a function of parameters. Inside and outside temperatures are two controlling variables. If the inside temperature equals the outside temperature the delta T goes to zero and there is no stack flow. The temperature contribution is expressed as ((Ti- To)/ Ti) where temperature is expressed as Absolute Temperatures (Deg R) i.e. (460 + degrees F). 

• To have a stack flow the T inside must be greater than the outside temperature. A comfortable inside temperature range is, say, 70º to 85º F. To have a stack flow the outside temperature must be lower than these temperatures, say, 65º to 80º F. The weather bin temperatures tell us historically the hours per year that Atlanta can expect to have outside temperatures in this range. These hours are expressed in five-degree F bands as follows for the following temperature: (Note: our earlier Equipment Performance Bins reports show these same values.)

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