ASHRAE Journal - January 2021 - 31

TECHNICAL FEATURE

a car, where user interfaces have
been extensively tested by designers and are generally standardized.
Imagine the havoc if the gas and
brake pedal positions in cars were
not standardized.
One of the authors lived in an
overseas university residence for
several months. Each suite had a
toggle button like the one shown in
Figure 3 outside the main doorway.
Given his past association with buttons outside homes, the author
assumed that this was a doorbell
of sorts. Accordingly, he didn't
dare push it, though he noted the
unnerving experience of navigating
the hallway in the dark. Months later
he discovered that the button was a
light switch that turned the hallway
lights on for 10 minutes before a
timer turned them off again.

Feedback and Mental Models
In most consumer products, the
interface is integrated and directly
connected to the rest of the system,
which means that the designer(s)
can optimize the interface for that
system (all seven parts of Figure 1).
In buildings, designers can mix and
match components; moreover, different professionals are normally
responsible for selecting different
components (e.g., interfaces, luminaires, HVAC, blinds, etc.). Technical
compatibility of components does
not automatically lead to a usable
building. In commercial buildings
particularly, we can add software
customizability to the list of components. Modern building automation
systems (BASs) allow technicians to
completely customize the control
logic. Moreover, digital interfaces
(especially those with touch screens)
can be customized-possibly by

people having no training in human
factors or feedback from dissatisﬁed
occupants. With this programming
power comes great room for failure
in usability.
In one academic building we
studied, three ofﬁces-each with a
thermostat-were grouped within a
single HVAC control zone. The system was designed to supply heating
to any ofﬁce that required it, regardless of the effects on other rooms,
meaning that the thermostat with
the highest setpoint temperature
controlled the other ofﬁces' supply
airﬂow rate and temperature. As a
result, the other ofﬁces frequently
overheated, and their thermostats
had no effect on overheating. Given
the conventional layout of the thermostats in these ofﬁces, there was
a mismatch between the implied
control of allowing each ofﬁce user
to adjust the setpoint and the real
control situation.
This mismatch, combined with
long thermal response times and no
instruction manual, inhibited occupants' ability to construct a functional mental model of the system.
Even the experienced researchers
with access to historical data and
the control logic took several hours
to ﬁgure it out. Karjalainen found
homes to be more comfortable than
ofﬁces and to have greater controllability, in part because occupants
understand residential HVAC
systems better than commercial
systems.5
Interfaces that promote useful
mental model development through
natural mapping (described next)
and clear and sufﬁcient feedback
allow users to interact with and
learn system functionality. In many
cases, particularly with increasing

FIGURE 3 A light switch to activate hallway lights for
10 minutes.

trends towards advanced BASs, a
building's systems are complex and
difﬁcult for the occupants to understand immediately. Therefore, users
need to be able to interact with the
system in an explorative way to learn
how they can operate the system
effectively.

Natural Mapping
A fundamental concept in usable
design is " natural mapping " : the
notion that interfaces and their
associated actuators should have an
intuitively correlatable geometrical
layout. It is normally up to the trades
to get this interface context (item 3
in Figure 1) right.
Don Norman suggests that the
best natural mapping is for the
interface to be physically connected
to the actuator.4 The next best is
geometrical congruence between
the interface and the actuator,
whereby the geometrical layout of
the interface features or interfaces
matches that of the actuator(s).
And worst is neither of these (e.g.,
Figure 4). For buildings, we don't
necessarily agree that the interface
should be connected directly to the
actuator because remote control is

JAN UARY 2021

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ASHRAE Journal - January 2021

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