ASHRAE Journal - January 2021 - 30

TECHNICAL FEATURE

At least for lighting, the feedback is generally instantaneous, and the average user's mental model is normally adequate to grasp the situation, even if they must
memorize illogical mapping between switches and
luminaires. Now, consider a thermostatically controlled
radiant floor heating system. The lag between a setpoint
change and a perceptible change in thermal sensation
in the space could be hours and could involve a central
plant, a district energy network, multiple heat exchangers and a thermally massive floor-all of which are
opaque to the human senses.
Jakob Nielsen's sixth heuristic for user interface design
is " recognition rather than recall. " 3 In the case of the
radiant floor system, we force occupants to recall that
a setpoint increase eventually causes the space to feel
warmer. This raises the question of whether our continuous attempts to conceal building systems and their
operations are really the way forward. Moreover, does a
trend toward energy-efficient, low-carbon technologies
lead to reduced usability? We do not think this needs
to be the case, but without deliberate intervention,
reduced usability may become an inherent quality of
such systems.

Feedback: Letting the Occupant Know It's Working
The average occupant does not understand the physics
represented by the right side of Figure 1, but they do not
need to. People form mental models that provide them
with a functional understanding of the system to the
extent they need to use it. For example, when dimming
a light, we don't need to understand the electrical circuit
or the exact path of the wires within the wall; we just
need to know that the knob should be turned one way to
brighten the light and the other way to dim it. We need
to know that if the light does not turn on, something
is broken, and that an obvious first action is to check
whether the bulb is burned out. The problems arise
when users' mental models are insufficient and users
become frustrated and possibly overcompensate in
energy-intensive ways. As emphasized by design expert
Don Norman, users have an unfair tendency to blame
themselves rather than the system.4
Figure 1 shows two ways people receive feedback:
through the interface, as represented by the arrow
between items 2 and 7, and through the actuator's
effect on the environment, as represented by the arrow
between items 5 and 7. In some cases, such as latches
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ASHRAE JOURNAL

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JAN UARY 2021

for operable windows, the interface and actuator are
one, and there is no distinction between feedback from
the interface and feedback from the actuator. More
advanced interfaces may provide digital feedback (e.g., a
thermostat may indicate that the heating system is activated). We note one additional nuance about feedback
from the actuator: it does not necessarily come in the
form of the service the actuated system provides. The
hot air from a furnace may not be felt immediately, but
the sound of the fan still tells us the furnace is working
as expected.
Building interfaces frequently rely on the actuators'
effect on the environment as the only form of feedback
for the occupants. However, in cases where systems are
slow to respond or where the system is malfunctioning, a lack of feedback from the interface can cause
confusion and frustration. One of the first principles
for the usability of a designed system is that it provides
instantaneous feedback. Norman suggests that feedback
should be provided within 0.1 second.4 Contrast this
recommendation with the thermal response of buildings, which can be measured in minutes or even hours.
With advances in communications technology, consideration of feedback in interface design is of increasing importance. Whereas interfaces and actuators were
once necessarily in close proximity to each other, it is
now conceivable for an interface to be miles from a heating or cooling plant, with many intermediate systems,
each causing lags and impacting decisions.
We believe that direct feedback from the actuator combined with immediate feedback from the interface is
ideal. This provides users with multiple ways to engage
with the system, and users can be sure the entire loop
has been completed successfully. We acknowledge that
feedback from the actuator is not always possible; in
those cases, immediate feedback through the interface is
even more important.

Standardization of Building Interfaces
Consider the experience of arriving in a warm, dark
hotel room in a foreign country after a long flight. From
our experience, it is common to have trouble finding
light switches (so much so that lights are often left on
by cleaning staff) and difficulty adjusting the temperature settings. Add to that ignorance of local interface
conventions (e.g., is a light switch in the up position on
or off?). Contrast hotels to the experience of renting

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