ASHRAE Journal - January 2021 - 29

TECHNICAL FEATURE

speciﬁcally for users and are often physically separated
from the mechanical or electrical systems they control. Here we consider a much broader deﬁnition of
interfaces to include anything that occupants routinely
interact with that affects a building system. Long before
modern HVAC, there were ﬁreplaces to feed with wood.
What was the interface there? Perhaps the opening of
the ﬁreplace and the bellows to feed it air. Interfaces can
be integrated into the building design in a structural or
an architectural way (such as operable windows or ﬁreplace openings) or located far from the system/actuator
and connected by electrical or mechanical means.
Consider the following conceptual model for humanbuilding interaction (Figure 1). The human input (1)
interacts with the interface (2) by means of physical contact with the interface, gesture or voice command. This
interaction can be triggered by discomfort, habit, views
to outside, desire for privacy, etc. The interface has some
context (3), such as location, visibility and accessibility.
The connection between the interface and the actuator
(5) involves control logic (4). This control logic could be
as simple as closing an electrical circuit or disengaging
a window latch, or as complex as adaptive occupantcentric controls with machine learning.1
Like the interface, the actuator has context (6).
Actuator context includes characteristics such as allocation in the room and portion of the building that is
affected by the system. And ﬁnally, the frequently missing piece is feedback to occupants (7). Occupants need
to be reassured immediately that their input has been
reﬂected in system operations so that they can return to
their activity with conﬁdence that the system is working.
For actuators that are physically separated from occupants, actuator status can be communicated through
electronic means via the interface (e.g., an indicator
light). Figure 1 shows that from the occupant's perspective, while the interface and feedback tend to be tangible
or at least present, the control logic and actuator are
often nebulous and intangible.
For example, as shown in Figure 1, when an occupant
turns on a light (1) by ﬂipping a switch (2), the interface
input is subject to control logic (4), which can have varying levels of complexity. The occupant is then able to
observe the effect of their input through tangible feedback (2→7) and hopefully see that the light is on and
the space is illuminated (5→7). In fact ANSI/ASHRAE/
IES Standard 90.1-2019, Energy Standard for Buildings

FIGURE 1 Conceptual model for building systems, starting with input to the interface and ending with feedback to the occupant.

The Tangible
1 Human Input
Interface
3 Context
2
Interface
7 Feedback

The Intangible

4 Control
Logic

6 Actuator
Context
5 System/
Actuator

FIGURE 2 Bank of light switches without clear mapping to luminaires.

Except Low-Rise Residential Buildings, requires that occupants be able to see the illuminated space from the light
interface. This is a rare instance of a building interface
usability requirement in a building energy standard.
If implemented well, the familiar interface, combined
with an easily observable connection between interaction and effect, allows the occupant to quickly construct
a reliable mental model of how the controls affect the
system.
However, the situation shown in Figure 2 is all too familiar: multiple switches placed on the wall for occupant
engagement without clear mapping to the controlled
luminaires (not shown in photo). What if multiple
switches control a single luminaire such that the functionality of a given switch reverses depending on the
state of its counterpart(s)? And imagine an open-plan
ofﬁce where one switch controls the lighting for dozens
of people. How likely is a user to have the conﬁdence to
turn those lights off, even if there is adequate daylighting? Socially pressured situations such as this highlight
the importance of ﬁne-grained individual controls. One
way to overcome this social issue is to have the building
itself signal users when it is appropriate to interact with
the interfaces.2
JAN UARY 2021

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ASHRAE Journal - January 2021

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