Friday, December 12, 2008
Tuesday, December 9, 2008
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.)
Energy Recovery Ventilator
RainShine is equipped with an energy recovery ventilator that draws air from areas of the house typically exhausted to the exterior, efficiently recovers heat from the exhaust stream during cold weather and pre-cools and dehumidifies incoming air during hot, muggy weather. Indoor air quality (IAQ) is improved and the risk of moisture related damage to the house is reduced.
RainShine’s exterior envelope is airtight to reduce air and moisture infiltration. The ERV thus provides a controlled method of moving fresh air into the house, dehumidification, removal of stale or contaminated air and reclamation of energy invested in exhausted conditioned air.
The ERV operates through a dedicated ducting system, replacing bathroom exhaust fans as well as Laundry exhaust. Outdoor air is warmed close to room temperature with heat that would otherwise be lost with the exhaust air. Water vapor transfer moderates extremes in humidity levels which helps prevent moisture damage or over-drying of the house.
GeoThermal Heat Pump
RainShine’s ground source heat pump system takes advantage of the steady state temperature of the earth for heating and cooling. In winter heat is extracted from the earth and delivered to the house. In summer heat is removed from the house and absorbed by the earth. Through the heat pump system the earth’s stored energy is accessed with minimal energy expenditure.
Well system: Because of the RainShine limited site area a closed vertical loop system was employed to access steady state temperatures. For the vertical system, 5 wells (approximately four inches in diameter) were drilled approximately 200 feet deep. Two high-density polyethylene pipes were installed in each of the holes and are connected at the bottom with a U-bend to form a loop. The vertical loops are connected with horizontal pipe (via a manifold), placed in trenches, connected to the heat pump in the basement and filled with an environmentally friendly antifreeze/water solution that acts as a heat exchanger. All joints were heat-fused and pressure-tested before and after installation. The majority of the depth of
RainShine’s wells were drilled through solid granite, an excellent conductor of heat thus positively impacting the efficiency of the system. The bores around the vertical loops in the wells were filled with bentonite clay.
Heating: In heating mode, the geothermal system uses the closed loop well system to extract heat from the ground. The heat pump takes heat from the loop and distributes warmed air through a conventional duct system.
Cooling: In cooling mode, ducted air is cooled by reversing the heating process. Instead of removing heat from the ground, the heat pump extracts heat from air flowing through the ducts and dumps it in the heat sink of the loop wells.
Hot Water: Desuperheaters transfer excess heat from the geothermal heat pump’s compressor to a hot water tank. In the summer cooling period, the heat that is taken from the house is used to heat the water for little cost. In the winter, water heating costs are reduced by about half.
Geothermal heat pumps save money in operating and maintenance costs. While the initial purchase price of a residential GHP system is often higher than that of a comparable gas-fired furnace and central air-conditioning system, it is more efficient, thereby saving money every month.
Unlike standard heat pump compressors that can only operate at full capacity, RainShine’s two-stage scroll compressors operate close to the heating or cooling capacity that is needed at any particular moment. This saves large amounts of electrical energy and reduces compressor wear. Two-stage heat pumps work well with RainShine’s zone control systems using automatic dampers to allow the heat pump to keep different rooms at different temperatures. RainShine’s heat pumps are equipped with variable-speed motors on their fans (blowers). The variable-speed controls for these fans keep air moving at a comfortable velocity, minimizing cool drafts and maximizing electrical savings. It also minimizes the noise from the blower running at full speed.
Rainshine’s geothermal system is eligible for a $2,000 state tax credit and a $2,000 federal tax credit.
Please see the simple diagram below taken from this informative website.
Ground-source (geothermal) eat pump in cooling mode.
Ground-source (geothermal) eat pump in heating mode.
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