  We propose a low-temperature thermal rectifier consisting of a chain of three
tunnel-coupled normal metal electrodes. We show that a large heat rectification
is achievable if the thermal symmetry of the structure is broken and the
central island can release energy to the phonon bath. The performance of the
device is theoretically analyzed and, under the appropriate conditions,
temperature differences up to $\sim$ 200 mK between the forward and reverse
thermal bias configurations are obtained below 1 K, corresponding to a
rectification ratio $\mathcal{R} \sim$ 2000. The simplicity intrinsic to its
design joined with the insensitivity to magnetic fields make our device
potentially attractive as a fundamental building block in solid-state thermal
nanocircuits and in general-purpose cryogenic electronic applications requiring
energy management.
