Basics of internal gains

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Internal gains refer to components inside the building that generate heat and therefore will have an impact on the comfort conditions in the space, and usually will be a key factor when sizing HVAC systems. Internal gains consist of lighting, equipment and people.

Internal lighting

Internal lighting heat gains - radiant and convective components. (Source: IBPSA-USA BEM Workshop)

Light fixtures contribute heat gains to a space by both radiation and convection. The total amount of electric input to a light fixture is converted to a portion of visible radiation, thermal radiation, and the convective gain.

Radiation gains occur when heat is transferred through electromagnetic waves (visible or non-visible spectrum emitted from the lights) - when the waves strike a surface, some of the energy is absorbed and converted to heat. Convective heat gains occur when the lights heat up causing the surrounding air to become warm. The magnitude of the gains is largely dependent on the wattage of the light fixtures.

The proportion of these gains is dependent on the lighting type. For example, the total electric input to typical incandescent lights is converted to 10% visible radiation, 80% thermal radiation, and 10% convective gain. In contrast, the electric input to typical fluorescent lights is converted to 20% visible radiation, 20% thermal radiation, and 60% convective gain.[1]

For light fixtures that are installed in a ceiling with a plenum located above, some portion of the heat gains will be contributed to the occupied space whereas some portion may go directly to the plenum. Space conditioning should mainly account for the heat gains directly in the space, but note that as the plenum heats up, some of that heat will be transferred back into the space, and the HVAC zone equipment will need to account for this. If air from the plenum is returned to a central HVAC system, then the plenum heat will be part of the return air stream and the HVAC system will need to respond to this additional heat.

The ASHRAE Fundamentals Handbook, Chapter 18, Table 3 contains a summary of typical values for radiant/convective split and space/plenum split of lighting heat gains for a range of lighting types.[2]


Equipment heat gains - radiant and convective components. (Source: IBPSA-USA BEM Workshop)

Plug and process loads (PPLs) are building electrical loads that are not related to lighting, heating, ventilation, cooling, and water heating, and typically do not provide comfort to the occupants.[3] PPLs account for a growing percentage of a building's total energy use as the continued focus on efficiency of HVAC and lighting drives down those energy end-uses.

Similar to lighting gains, plug loads have a radiant and convective portion associated with the heat gains to the space.

Plug Loads

Most spaces have equipment that is plugged into receptacles (e.g. plug sockets in a wall) that use electricity and also give off heat – computers, televisions, table lamps, etc. Most frequently, equipment heat gains are input by space type in Watts, Watts/ft2 or (W/m2 ). It is important to study the heat gain operation for a typical occupied weekday separately to weekends or holidays. E.g. a school’s summer holidays.[4]

Plug loads may also be referred to as miscellaneous electrical loads (MELs).

Process Loads

Process loads refer to loads within the building that are important/critical to its operation. There is a bit of gray area between plug loads and process loads. Plug loads are generally things that the occupants can plug/unplug, whereas process loads tend to be somewhat "permanently" installed (though they may be replaced after a period of time, and they may be plugged into a wall receptacle!).

Some examples of process loads include kitchen equipment, refrigerators, water fountains, vertical transportation (elevators, escalators), computer IT equipment, and manufacturing equipment.

Gas equipment

Some process loads use gas (natural gas or propane) as the energy source rather than electricity, or some use a combination of electricity and gas. Some examples include gas cooking appliances, laboratory equipment, and manufacturing equipment.


Occupant heat gains - Latent and sensible (with radiant/convective split). (Source: IBPSA-USA BEM Workshop)

With the exception of some mission critical buildings (e.g. data centers) and storage facilities, most buildings exist for people to occupy. Each occupant adds both sensible and latent heat to the space depending on their activity level, metabolic rate, and age. Occupant heat gains are either calculated per person, or accounted for by occupant density, e.g. person per floor area.[4]

Typical values for the sensible and latent heat gains associated with the building occupants in a range of different building types and activity types can be found in COMNET Appendix B.


  1. "EnergyPlus Engineering Reference".
  2. "2021 ASHRAE Handbook - Fundmentals".
  3. Sheppy, Michael; Lobato, Chad; Pless, Shanti; Polese, Luigi; Torcellini, Paul (2013). Assessing and Reducing Plug and Process Loads in Office Buildings. NREL.
  4. 4.0 4.1 "Building Performance Modeling Student Handbook" (PDF). IES Ltd.
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