Accurate prediction of transient subsurface heat transfer is important in sizing downhole heat exchangers (DHEs) and making predictions of their thermal output, but quantification of these processes has been difficult and elusive in practice. As such, current DHE design methods rely on empirical data and rules of thumb. The work described in this paper makes use of so-called in-situ thermal response testing, in conjunction with a newly-adapted analytical solution that describes the coupled conductive and advective heat transport relevant to DHEs. The complex heat transfers within the well bore are described by a lumped thermal resistance parameter