Load Modeling Terms

Load is the summation of various gains or losses in a space or building that must be counteracted in order to maintain constant conditions. Loads are defined at a point in time, often called peaks, in which the space or building experience the highest summation of the gains or losses. Loads are a measure of power with common units being Btu/h and watts. Important types of loads to know are Cooling and Heating Load.

Cooling Load

Defined as the sum of internal gains such as lights and people, external gains such as solar, and mechanical gains such as fan motor pick up. Cooling loads are a combination of sensible and latent components. They are calculated in various forms include peak and block to inform different portions of the mechanical system.

Heating Load

Defined as the sum of losses such as envelope conduction and infiltration from a space. Heating load is sensible only. Heating loads are taken at a single worst-case time which represent the peak for the space or building.


Gain is a specific type of heat generated or entering a space. Examples include lighting gain from lights within a space and solar gain from the solar energy entering a space through glazing. Gains are positive values and can be either sensible, latent or both. Gains and Loads are sometimes used interchangeably


A Loss is a specific type of heat leaving a space. Examples include conduction losses through a building envelope or losses due to infiltration. Losses are negatives values and can be either sensible, latent or both. Losses can also sometimes be referred to generically as loads.

What are Internal vs. External loads?

What is Block vs Peak Load?

Peak load is the absolute highest load for a room, systems or building ignoring time that individual gains and losses occurs. Peak is often used to size room level equipment like diffusers and fan coil units. Peak is not recommended for sizing systems like AHU or Chillers because it will result in oversizing

Block or coincident load is the highest load for a system or building taking into account the time that individual gains and losses occurs. Block loads are critical to right sizing equipment as the highest gains in one side of a building verse the other will not occur at the same time. This means that the central equipment such as AHU and chiller don't need to sized for the sum of these peak loads, but rather a Block load that will actually occur in the building.

Further Reading

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