ASHRAE Members around the world recently received their 2017 Fundamentals Handbook. Most of us likely received the digital version on a CD. Many of the new updates will impact the HVAC load calculations, performed by mechanical designers day in & day out – there are new design LPDs, revised kitchen equipment resources, and additional climate data, among many other additions!  Something that hasn’t changed: load calculation methods.

The 2017 edition of the Fundamentals Handbook defines only two load calculation methods: the Heat Balance Method (HBM) and Radiant Time Series (RTS). As part of the introduction, the radiant time series is categorized as “a simplification of the [heat balance] procedure”, but what exactly is the simplification?

Heat gain from the sun offers an illustrative example. Solar energy is absorbed by the exterior wall of a building and transfers to the inside of the building by conduction. Because the wall has thermal mass, that heat isn’t transferred immediately – there’s a conductive delay.

Solar energy is transmitted to the inside of the building through the window. Some of the heat is absorbed and reflected by the window, and the remainder is absorbed by the interior surfaces.  When those surfaces later emit that heat by convection, cooling load is created in the room – there’s a radiative delay.

The Heat Balance Method calculates these time delay effects explicitly with some basic assumptions like uniform surface temperatures. There are no arbitrarily set parameters.  Conductive, convective, and radiative heat balance is calculated directly for each surface within a room.

By contrast, the radiant time series (RTS) calculation uses radiant time factors and conduction time factors to distribute hourly heat gains over a 24 hour period instead of iteratively calculating the time delay effects. A table of time factors published in the Handbook determines how long heat is held by a wall or other surface before creating a cooling load in the space.

Whether heat balance method or radiant time series is used to calculate heating & cooling loads, computer software is nearly always employed. IES Virtual Environment (VE) software performs ASHRAE load calculations using the non-simplified Heat Balance Method, calculating heat balance and the resulting loads directly without time factors. To learn more about calculating loads for HVAC design using the VE, check out

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