  Once in its non-equilibrium steady state, a nanoscale system coupled to
several heat baths may be thought-of as a quantum heat pump. Depending on the
direction of its stationary heat flows it may function as e.g. a refrigerator
or a heat transformer. These continuous heat devices can be arbitrarily complex
multipartite systems, and yet their working principle is always the same: They
are made up of several elementary three-level stages operating in parallel. As
a result, it is possible to devise external black-box testing strategies to
learn about their functionality and performance regardless of any internal
details. In particular, one such heat pump can be tested by coupling a
two-level spin to one of its contact transitions. The steady state of this
external probe contains information about the presence of heat leaks and
internal dissipation in the device, and also, about the direction of its
steady-state heat currents. Provided that the irreversibility of the heat pump
is low, one can further estimate its coefficient of performance. These
techniques may find applications in the emerging field of quantum thermal
engineering, as they facilitate the diagnosis and design optimization of
complex thermodynamic cycles.
