Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
You describe only 1 form of energy required to condition the interior space of a building, in this case the sensible cooling or heating energy required to maintain a desired indoor temperature (Qhvac, sensible). This equation is used to calculate the HVAC energy. A more accurate equation for space conditioning would be Q_hvac = m_dot x (H_in - H_out) which would include the contribution of moisture in the equation.
A building would also required other forms of energy to provide a source for lighting and equipment. For lighting, this may be entirely provided by daylighting during the day, however, some energy may be required after sundown if the building required lighting. The energy sources for lighting and equipment would eventually result in a change in the requirement for space conditioning compared to a building without lighting and equipment. There may also be a need for heating water. Occupancy will also add energy in the form of heat and moisture. Additionally, if there are openings in the building shell, such as windows, solar gains (Q_solar) would also contribute to the building loads.
The equation shown (Q_source=m_dot x cp x deltaT) is used to calculate the source energy used to sensibly condition a building. However, you only know 3 of the 5 variables in this equation, T_in, T_out, and cp. To identify the fourth variable, modelers typically use Q_shell=UA(T_in-T_out) to define the building shell load. Knowing this value, your equation could then be used to determine the air mass flow rate required to condition the interior space of a building.
Q_load = Q_shell + Q_solar + Q_lighting + Q_equipment + Q_occupancy + Q_waterheating + ?
The energy balance equation would then be:
Q_hvac, total = Q_load = m_dot x (H_in - H_out) for total energy
| | 2 | No.2 Revision |
You describe only 1 form of energy required to condition the interior space of a building, in this case the sensible cooling or heating energy required to maintain a desired indoor temperature (Qhvac, sensible). This equation is used to calculate the HVAC energy. A more accurate equation for space conditioning would be Q_hvac = m_dot x (H_in - H_out) which would include the contribution of moisture in the equation.
A building would also required other forms of energy to provide a source for lighting and equipment. For lighting, this may be entirely provided by daylighting during the day, however, some energy may be required after sundown if the building required lighting. The energy sources for lighting and equipment would eventually result in a change in the requirement for space conditioning compared to a building without lighting and equipment. There may also be a need for heating water. Occupancy will also add energy in the form of heat and moisture. Additionally, if there are openings in the building shell, such as windows, solar gains (Q_solar) would also contribute to the building loads.
The equation shown (Q_source=m_dot x cp x deltaT) is used to calculate the source energy used to sensibly condition a building. However, you only know 3 of the 5 variables in this equation, T_in, T_out, and cp. To identify the fourth variable, modelers typically use Q_shell=UA(T_in-T_out) to define the building shell load. Knowing this value, your equation could then be used to determine the air mass flow rate required to condition the interior space of a building.
Bringing all possible building loads together yields:
Q_load = Q_shell + Q_solar + Q_lighting + Q_equipment + Q_occupancy + Q_waterheating + ?
The energy balance equation would then be:
Q_hvac, total = Q_load = m_dot x (H_in - H_out) for total energy
