ASHRAE Journal - August 2013 - 40

technical feature

Energy Codes & Standards
Partial, Full, Multiple & Total System Performance
By John hogan, P.E., MEMBEr aShraE

Virtually all energy codes contain multiple compliance options to accommodate a range of users who have a variety of needs and technical knowledge.
Part 1 of this article in the July 2013 issue covered prescriptive vs. performance compliance.
Partial System Performance Compliance Options
The next step up in complexity is the partial system performance compliance option. These options address more
than one component of a building system. However,
they only address portions of one system and only allow
trade-offs within those portions of that building system.
These options are used almost entirely by designers, but
the designers often need the component performance
information provided by the manufacturers.

Building Envelope
A variety of partial system performance compliance
options exist for the building envelope. One type allows
compliance to be shown for the overall building envelope for either the maximum heat loss criteria or for
the maximum heat gain criteria (but not for both). For
example, a UA (U-factor × area) trade-off allows the
designer to take the U-factor for each of the building
envelope components (e.g., skylight, opaque roof, window and glazed doors, opaque door, opaque wall, floor,
etc.) multiplied by the area of that component and show
compliance with the maximum heat loss criteria on an
overall building envelope basis. The simpler the tradeoff
system, the less accurate it can be for some systems.
For example, in climates where the outdoor temperature fluctuates above and below the balance point of the
building (where the building requires neither heating

nor cooling during some portion of the day for a significant portion of the year), the UA trade-off method
does not accurately or entirely reflect the performance
of components with thermal mass. Conversely, a more
accurate trade-off system, for instance, one that would
account for fenestration orientation and area as well as
opaque envelope components including thermal mass,
would most likely be more accurate but would require
more user input. The UA trade-off also does not account
for solar gains into a space from fenestration.
The SHGCA trade-off (SHGC × component area) is a simplified parallel on the heat gain side, allowing the designer
to take the SHGC for each of the fenestration products
(e.g., windows, skylights, and glazed doors) multiplied by
the area of that product and show compliance with the
maximum heat gain criteria on an overall basis for the
fenestration portion of the building envelope.
The overall thermal transfer value (OTTV), contained
in the earliest versions of ASHRAE/IESNA Standard
90.1, is a more complete parallel on the heat gain side,
as it considers the heat gain through the opaque building envelope components as well as the fenestration
components.
However, all of these compliance options are only
considered to be partial system performance options
because they do not allow trade-offs between maximum heat loss and maximum heat gain. Therefore, it is

aBout thE author John Hogan, P.E., is a consulting engineer and registered architect in Seattle. He is a member of ASHRAE SSPC 90.1, and worked 30 years for the City of Seattle
developing and implementing energy codes.
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ASHRAE Journal - August 2013

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