uses a lot of power. Instead, they will connect intermittently, checking in for
new data. This can cause delays and result in situations where some interfaces
do not reflect the ‘correct’ state of the system. Network latency is also an
issue: it’s possible for the user to know that something has worked before the
UI does. For example, they may be physically sitting near a light that they
have just turned on and have to wait for a smartphone app UI to tell them what
they already know.
To return to the heating example: in the UK, it’s common for heating
controllers to run off a battery^16. So a heating controller may need to connect
via a low powered network like ZigBee to a gateway, and only connect
intermittently to check in for new instructions. There might be a delay of
perhaps two minutes between a setting being changed on the smartphone app
and the heating controller receiving that instruction.
This causes discontinuities in the UX. If a user changes the settings on the
smartphone app (say, turning the temperature up from 19C to 21C), there may
be a period of up to two minutes before the heating controller checks in to the
service and receives the updated instruction. During this period, the phone UI
could show that the system is set to 21C, and the controller UI will show that it
is set to 19C. If the user is standing in front of the controller with the
smartphone app, they will see two conflicting pieces of information about the
current status of the system (see figure 9.28). This violates one of the most
fundamental of Nielsen’s usability heuristics, visibility of system status: “The
system should always keep users informed about what is going on, through
appropriate feedback within reasonable time.”^17
(^16) UK heating engineers prefer battery powered wireless controllers as they can be
installed easily and sited anywhere without risk that rewiring will be needed. In the
UK, mains power is 240V AC and any mains electrical work must be done by a
qualified electrician. Even replacing an existing mains controller in the same location
requires an electrician. This isn’t an issue in the US, where HVAC controllers typically
run on a special low voltage circuit, making them safe for homeowners to install
themselves. This means that in the US, it’s feasible to offer a controller that maintains a
constant connection to a WiFi network, and the system can always be in sync.
(^17) Nielsen, J. (1994b). Heuristic evaluation. In Nielsen, J., and Mack, R.L.
(Eds.), Usability Inspection Methods, John Wiley & Sons, New York, NY.