Abstract: A heat transfer coil for exchanging heat between a primary fluid and a secondary fluid. The coil has a series of substantially parallel tubing runs. The primary fluid flows within the primary tubing. The secondary fluid flows outside the primary tubing and in contact with it. A heat pipe having a length has a first portion residing within a primary tube and a second portion residing outside the primary tube but adjacent and parallel to it. The secondary fluid thereby exchanges heat with the primary fluid both through the primary tube and through the heat pipe.