The downhole heat exchanger (DHE) is used to provide space heating and domestic hot water from a single geothermal well. The most common construction of DHEs has been black iron, which is subject to failure by corrosion. This paper describes the design, installation, and monitoring of a new type of DHE constructed of cross-linked polyethylene plastic (PEX), a material known for its relatively high temperature and pressure rating, durability, and chemical resistance. The PEX DHE was installed as a retrofit at a residence in Klamath Falls, OR and a data logger was used to record system temperatures at 5-minute intervals for the 2004-2005 heating season. Observations thus far show the PEX assembly to be an acceptable DHE