Summary Cooling Design Table

The Summary tab on the Cooling design screen shows a summary of maximum loads with some display options.

Summary Display Options

Totals method

You can sum total loads in 2 different ways

• 1-Building - loads are summed for the whole building.
• 2-System - loads are summed for each System type allowing you to calculate system (AHU)/block loads for sizing AHUs.

Peak method

Summary results can be displayed either for the time and month of the overall maximum building cooling load or for the individual zone.

• 1-Non-coincident - Summary results are for the time and month of maximum cooling load for each zone.
• 2-Coincident - All summary results are for the time and month of the maximum building total cooling load.

Coincident definition method

When the Peak method is set to 2-Coincident you can specify whether the coincident time is calculated for the building as a whole, i.e. for all zones, or whether it is calculated separately for each system. Select from:

• 1-Building, where the time of peak cooling for zones is calculated by summing hourly totals for all zones in the building, or,

• 2-System, where the time of peak cooling for zones is defined considering all zones in each system.

Summary Results

The Summary screen shows design cooling loads, flow rates internal and fabric gains for each zone and other related data.

• Design Capacity (kW or kBtu/h), shown in bold, is the maximum value of Total Cooling Load and Sensible multiplied by the Design margin to give the design cooling capacity of the equipment. This value is written into the Cooling capacity Model data on the HVAC tab if the appropriate Plant sizing Model Option is set.
• Design Flow Rate (m3/s or ft3/min), shown in bold, is the supply air flow rate required to deliver the Sensible cooling load using the Design supply temperature and the zone air temperature at the time of the maximum load multiplied by the Design margin.
• Total Cooling Load (kW or kBtu/h) is the total maximum Sensible + Latent loads for the zone at the time of maximum Sensible Cooling load. If latent load is negative then it is ignored here and the Total Cooling Load is the same as the Sensible load.
• Sensible (kW or kBtu/h) is the maximum Sensible Cooling load for the zone in the Design day.
• Latent (kW or kBtu/h) is the latent load for the zone at the time of maximum Sensible load, calculated as Total Cooling - Sensible Cooling.
• Air Temperature (°C or °F) is the Air temperature in the zone at the time of maximum Sensible load.
• Humidity (%) is the humidity in the zone at the time of maximum Sensible load.
• Time of Max Cooling - the time at which the maximum sensible cooling occurs.
• Max Op Temp in Day (°C or °F) is the maximum operative temperature in the zone (using radiant fraction = 0.5) over the design day including periods when the zone may be unconditioned.
• Floor area is the floor area of the zone.
• Volume (m3 or ft3) is the volume of the zone.
• Flow/Floor area (l/s-m2 or MBH/ft2) is the Design Flow Rate divided by the Floor area of the zone. It provides a useful check for the flow rates calculated.
• Design Cooling Load Per Floor Area (W/m2) is the Design Capacity divided by the Floor area of the zone. It provides a useful check for the design capacities calculated.
• Outside Temperature (°C or °F) is the outside temperature at the time of maximum Sensible load.
• Glazing Gains (kW or kBtu/h) are the gains through windows (excluding solar gains) at the time of maximum Sensible load.
• Wall Gains (kW or kBtu/h) are the gains through all external and internal wall (partition) surfaces at the time of maximum Sensible load.
• Floor Gains (kW or kBtu/h) are the gains through all floor surfaces at the time of maximum Sensible load.
• Roof and Ceiling Gains (kW or kBtu/h) are the gains through all roof + ceiling surfaces at the time of maximum Sensible load.
• Ventilation Gains (kW or kBtu/h) are the mechanical + natural ventilation gains at the time of maximum Sensible load.
• Infiltration Gains (kW or kBtu/h) are the infiltration gains at the time of maximum Sensible load.
• Electric Equipment Gains (kW or kBtu/h) are gains due to electric equipment (Computers, Office equipment, Miscellaneous, Catering, Process) at the time of maximum Sensible load.
• Lighting Gains (kW or kBtu/h) are gains due to lighting (General + Task) at the time of maximum Sensible load.
• People Gains (kW or kBtu/h) are the sensible gains due to people at the time of maximum Sensible load.
• Solar Gains (kW or kBtu/h) are the short and long wave solar gains through windows at the time of maximum Sensible load.
• Mechanical Ventilation Fresh Air Rate (m3/s or ft3/min) is the fresh air supply air flow rate at the time of maximum load.
• Fresh air % of supply air (%) is the % of the mechanical cooling design supply air that is from outside at the time of maximum load. The remainder of the supply air is assumed to be recirculated.

The image below shows an example of the summary results grid data when the Totals method is set to 1-Building. Note the totals and averages listed for the whole building at the bottom of the grid.

The image below shows an example of the summary results grid data when the Totals method is set to 2-System. Note the totals and averages listed for each system.

Tip: Use the File > Export > Export HVAC loads report command to view results in an industry standard PDF format. Alternatively, use the Export command to export summary data to spreadsheet format.

Why do the sum of the gains not equal the sensible cooling load?

 

It might be expected that the sum of the Glazing + Wall + Floor + Roof/Ceiling + Ventilation + Infiltration + Equipment + Lighting + People + Solar gains at design conditions would equal the Sensible cooling load. However, it is unrealistic to expect the sum of a mix of convective and radiant gains to equal the (purely convective) cooling load.

 

To understand more clearly we need to consider the nature of the building thermal resistance/capacitance network used in E+ (actually E+ uses a response factor method by default but the same principle applies). For each zone, you can think of the thermal network as a single zone air node and a series of nodes for each surface. All zone convective gains are accumulated at the air node and radiant gains at the internal surface of each wall, window, roof etc. Conceptually speaking there is a resistance between the zone air and surface nodes and heat flows between them (through Rconv in the diagram below). The key to understanding the apparent discrepancy is that heat transferred to/from the surface nodes is not equivalent to heat exchanged at the air node. So the surface heat gains can’t simply be combined together with gains at the air point to calculate cooling system sensible gain.

 

Why are some cells displayed in brown bold text?

 

In some cases, DesignBuilder may not be able to calculate a supply air flow rate, and in these cases, results may be displayed in brown bold text. The screenshot below illustrates.

 

 

This can occur when the cooling supply air temperature is the same as, or very close to, the zone air Cooling setpoint temperature which would lead to a divide by zero error. In such cases, the Design flow rate is set to zero and summary results are displayed in brown, even if there is a small cooling load.