Building Envelope Details; Envelopes Inside of Envelopes

In my previous post, I outlined how my awareness of the building
envelope as an integral part of the HVAC system
evolved as my
career evolved. The diagram below illustrates a similar awareness
that rose out of my evolving interest in the building envelope and
its interactions with the environment and the HVAC systems it

The drawing is a section cut through the building pictured below
with the section cut taken so that it hits every major shaft in the
building (the cut line is the long dashed line on the building plan
in the upper right corner). Jim Brooks, one of my mentors from my
McClure Engineering day, taught me this technique as a way
to understand how an air handling system functioned in the
context of the building it served.

The building is served by 16 air handling systems located on the
upper mechanical levels.  Four of the systems are 100% outdoor
air, direct/indirect evaporative cooling systems serving the public
lobbies.  The remaining 12 systems are conventional,
economizer equipped, VAV reheat systems.  Most systems serve
multiple floors and also serve perimeter and interior

I made the section cut because I was trying to understand a
problem we were having with the direct/indirect evaporative cooling
systems during start-up, which I will discuss in a subsequent
post. The focus of this post is to say that I’ve come to
believe that a drawing like this may be one of the most important
ones that I can make in terms of diagnostics and documenting how a
building works, especially for complex high-rise buildings for the
reasons illustrated and discussed below.

Atriums Interconnect Floors: 
This building had a number of atriums, which are striking
architectural features.  But, they also interconnect the
various systems serving the floors that are interconnected by the
atrium at the zone level.  If you think about it, if one
of the conventional air handling system is on 100% outdoor air as
the result of its economizer process and it is serving a zone that
is interconnected with a zone served by one of the direct/indirect
air handling systems, which are also 100% outdoor air units, then
the two systems are in parallel and will interact just like any
other parallel fan or pump circuits.

Stairs and Elevator Shafts Act Like
  This building is approximately 475
ft. tall and when you think about it, a lobby that has access to a
475 ft. tall shaft to the top of the building is not that different
than a conventional fireplace, hearth and chimney;  the taller
the shaft, the better the draft.  In the case of the building,
the lobby becomes the hearth and the occupants subject to the whims
of the stack
created by the tall shafts acting like chimneys.

Smoke Shafts Interconnect
  This building had an engineered smoke
control system that incorporated large shafts that ran up through
the building, interconnecting the ceiling return plenums on various
floors, creating effects similar to those discussed in the
preceding bullets.

Return and Relief Plenums Interconnect Systems and
  In this building, the equipment rooms
served as common return and relief plenums.  As a result, they
become a common point of connection for all of the systems, adding
another twist to the system dynamics under certain operating
cycles.  In addition, all of the air from all of the systems
is mixed together.   A subtle effect related to this is
that the air returning from the zones served by the direct/indirect
cooling systems, which is cool, but relatively humid compared to
the conventional systems and which is on its way out of the
building, can impose a latent load on the conventional systems
under certain operating conditions.

Under floor and Above Ceiling Cavities Serve as
Supply Plenums:
  The scale of the drawing is too
small to highlight these features, but there are quite a few areas
in the building where underfloor and above ceiling cavities created
by architectural and structural elements also serve as part of the
HVAC system in that they are supply plenums.  This means that
they need to meet the functional criteria associated with a duct
system in terms of leakage and thermal characteristics, parameters
that are likely not a part of the normal structural or
architectural design target.  This is also true for the return
and relief plenums discussed in the preceding bullet, but the
thermal criteria are not as extreme when compared to the thermal
criteria associated with an architectural or structural
element.  However, in both cases, the leakage criteria are
quite different.

The bottom line is that the interior structural
components and separations in many buildings are often critical
components of the HVAC systems in the building either by intent, as
in the case of a plenum, or simply as a result of the
configuration of the building and its systems.  In this
example, the application was a bit more exotic than
normal.   But the fact is that buildings with under floor
air plenums or ceiling supply or return plenums that we are
frequently encountered in may buildings rely on the architectural
components to provide an HVAC function.  Paying attention to
these details can be just as important as paying attention to how
the envelope is fabricated and how the HVAC systems controlling the
environment inside the envelope are designed and installed.

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