This is a brief overview of how Energy Scheming calculates energy flows (loads), which can help you interpret the results of the energy evaluation reports.
Reference: For a more rigorous technical explanation of the formulas, refer to Appendix A, "Algorithms."
The Evaluation tells you the amount of load your building will have for heating and cooling for up to four representative days in an analysis year. It calculates the heat flows for up to 24 hours in each day and gives you a breakdown by source. For example, it will tell you how much of your heat loss is due to windows and how much of your heat gain is due to equipment.
For each hour or selected interval, for each of four climate days, the computer calculates the net energy flow in the building. This can be generalized as:
NET LOAD = Total Heat Gain - Total Heat Loss
The final energy analysis shows you the consequences of each of these factors and tells you how much net load remains to be served by auxiliary systems.
Total Heat Gain =
- Gain from People +
- Gain from Lights +
- Gain from Equipment +
- Solar Gain through Glass +
- Conduction Gain through Roof +
- Conduction Gain through Walls +
- Conduction Gain through Floor +
- Conduction Gain through Glass +
- Transfer from Mass +
- Ventilation/Infiltration Gain
Total Heat Loss =
- Conduction Loss through Glass +
- Conduction Loss through Roof +
- Conduction Loss through Walls +
- Conduction Loss through Floor +
- Ventilation/Infiltration Loss +
- Transfer to Mass
People generate heat in proportion to their metabolic rates. So the more people there are, and the more active they are, the more they contribute to the heat gain.
Energy Tip: Excessive heat gain can be alleviated by scheduling peak crowds for cool times of the day or by staggering activity schedules.
Electric lights, particularly incandescents, generate a great deal of heat.
Energy Tip: A savings in electricity can frequently be achieved by reducing the need for lights through daylighting.Reminder: Be careful that your daylighting strategies do not lead to overheating through solar gain!
Electrical equipment generates heat that can be either useful (when the building experiences a net loss) or harmful (when the building is overheating).
Energy Tip: The harmful effects of heat gain from equipment can be reduced by staggering activity schedules so that equipment use does not coincide with peak diurnal climate-related heat gains.
Direct sunlight can be a large source of heat gain, both wanted and unwanted. It can be modified by changing the amount and orientation of the windows or with the use of external or internal shades.
Energy Tip: In general, winter-time solar gain will be the most useful if it comes through unshaded south-facing glass and hits an associated thermal mass. Summer-time solar gain will be the least harmful if it is reduced by shading devices and does not coincide with other cooling loads.
Gain through the roof is a function of the color of the roof and its insulation level. It is "driven" by the exterior air temperature and by the effective temperature caused by the sun beating down on the roof.
Energy Tip: You can decrease heat gain through the roof by increasing its R-value.
Gain through the walls is a function of the color of the walls and their insulation level. It is "driven" by the exterior air temperature and by the effective exterior temperature caused by the sun striking the walls. Massive walls (with high lag times) will continue to transfer heat into the space long after the exterior surface has cooled off.
A floor whose underside is exposed to outside air, such as a cantilevered floor or a floor over a crawl space, will gain heat, like a wall, as a function of its area, R-value, and temperature differential. Direct sunlight is negligible in these cases, so there is no increase in effective temperature due to insolation.
This conductive heat gain is due to the temperature differential between the inside and outside air temperatures and should not be confused with the heat gain from direct solar radiation coming through the glass.
Energy Tip: Heat gain via conduction can be reduced by increasing the R-value or reducing the glazing area.
When there is a net heat loss in the building, previously stored heat is released from the thermal mass to heat the building. The amount of heat available in the mass is a function of the Diurnal Heat Capacity (DHC) of the massive material and the solar and other gains that occurred earlier in the day. During night ventilation of mass for cooling, the heat release from the mass will also appear in the energy flow summary as a heat gain in the building.
If the building is "closed" during hot periods, a minimum amount of outside air will either infiltrate through leaks in the construction or be introduced by code-required ventilation air. In either case, Energy Scheming calculates the resulting heat gain based on building type. Natural ventilation is not an option when the outside air is above the maximum allowable interior temperature because this will make the inside of the building hotter.
This heat loss is a function of the R-value of the glazing, the area of glass, and the temperature differential between inside and outside.
Energy Tip: It can be reduced by increasing the R-value or reducing the glazing area. Heat loss can be reduced by means of nighttime application of an insulating material.
Conduction Through Roof:
Loss through the roof is a function of the color of the roof and its insulation level. It is "driven" by the exterior air temperature and by the effective temperature caused by direct sun on the roof, if any.
Energy Tip: You can decrease heat loss through the roof by increasing the R-value or darkening the color of the roof.
Loss through the walls is a function of the color of the walls and their insulation level. It is "driven" by the exterior air temperature and by the effective exterior temperature caused by direct sun on the walls, if any. Massive walls (with high lag times) will continue to lose heat from the space even after the exterior surface begins to warm from sunlight.
A slab-on-grade floor loses heat primarily through its edges, so the loss is a function of the perimeter length, rather than its area, and whether or not the perimeter is insulated. A floor whose underside is exposed to outside air, such as a cantilevered floor or a floor over a crawl space, will lose heat, like a wall, as a function of its area, R-value, and temperature differential. Direct sunlight is negligible in these cases, so there is no increase in effective temperature due to insolation.
If the building is "closed" during cold periods, a minimum amount of outside air will either infiltrate through leaks in the construction or be introduced through code-required ventilation air. In either case, the computer calculates the resulting heat loss based on building type.
Energy Tip: If the building is "open," then natural ventilation can be used to replace overheated interior air with cooler outside air. The two methods are:
Cross-ventilation, which requires wind movement directed into some of the apertures, and
Stack-ventilation, which requires a vertical separation between inlet and outlet in order to work.
Both methods are enhanced by an increased aperture size and temperature differential.
When the ambient interior air temperature is at or above the maximum allowable, some of the excess heat may be stored in the thermal mass. While this normally occurs during periods of overheating, the Transfer to Mass is located in the Heat Loss portion of the equation because its effect is to reduce the interior air temperature.
Most of the calculations in Energy Scheming work by using an area of an element or material, its resistance to heat flow, and a temperature difference on opposite sides of the material or element.
Therefore, the heat flows which Energy Scheming models to calculate loads can be affected by three categories of design variables:
Manipulation of these categories (individually or simultaneously) will allow you to quickly explore those aspects of your design that affect its energy use.
