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Load Sizing Methodology
Load Sizing Methodology

How does the platform calculate loads?

Patrick Chopson avatar
Written by Patrick Chopson
Updated over a week ago

loadmodeling.tool utilizes the full-featured EnergyPlus whole building energy simulation engine for calculation of all building load results. EnergyPlus utilizes the industry-standard Heat Balance method defined in the ASHRAE Handbook Fundamentals. The Heat Balance method calculates heat gains and losses for each surface of a room, along with the instantaneous internal loads. By using sub-hourly time steps, complex interactions between the environment and rooms, and rooms to neighboring rooms is captured.

The primary inputs into zone load calculation for both heating and cooling are

  • Accurate building geometry captured via drawing.tool

  • Detailed construction performance for all surfaces

  • Internal loads, including daily variability via schedules assigned by Room Templates

  • External environment defined by a standard or user-specified .epw weather file

EnergyPlus runs several levels of calculations for each zone, system, and plant within a model producing design heating and cooling loads along with flow rates for air and other fluids. In loadmodeling.tool all levels of calculations are performed producing cooling, heating, and airflow results for every room, zone, air system, and mechanical plant in the model. Load is calculated for each time step of the design days specified and the peak value total cooling or heating values are reported. A breakdown of the components of these peak loads is also calculated via the EnergyPlus Sizing Manager. For full information on the EnergyPlus sizing methods used by loadmodeling.tool refer to this Engineering Reference.

How can I assign safety factors?

Sizing criteria or safety factors can be assigned on the Simulation Settings page. These default to 1.25 for heating and 1.15 for cooling.

How can I assign diversity to internal loads for room sizing?

When sizing room level equipment it is standard practice not to apply diversity, but rather to 'saturate' internal loads such as lighting or equipment to generate the peak cooling load of an individual room. For peak heating load, the opposite is standard where the internal loads are turned off to produce the highest heating load. This is handled via the Summer Design Day and Winter Design Day assigned in each project schedule. If desired these defaults can be modified.

How can I check the load sizing methodology?

The best way to check any engineering calculation is to compare against rules-of-thumb or other basic hand calculations. For example, checking the cooling load per area against known standard values is a great way of confirming results make sense. Room checks are included on the Results page to make this important step easy.

How are Room Loads produced by EnergyPlus?

EnergyPlus does not include Rooms directly and for every model will only report the Zone results. To ensure engineers have all the information they need for their designs tasks, the loadmodeling.tool platform performs a second simulation in which each room is assigned to a single zone and the Ideal Air Load system. The settings for this second simulation will match the primary which are set on the Settings Page. The results of this simulation are then included as the Room Results.

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