The performances of solar water heating systems have improved steadily over recent years due to improved component quality and design. Solar water heating systems in Northern Maritime climates, however, are still not viable economically. In order to make a significant impact on the domestic hot water heating market, solar water heaters must be innovative, cost-effective, and adaptable for retro-fit installation. This paper investigates experimentally the performance of a novel heat exchanger (the “Solasyphon”) designed and developed for external connection to an existing traditional single-coil hot water cylinder. The connection of the novel heat exchanger to an existing system reduces the cost of installation as it avoids the need to replace the existing single-coil hot water cylinder with a twin-coil system. However, the connection of this Solasyphon to a solar water heating system has negative effects on heat losses especially at higher temperatures based on the size of the solar collector. The experimentation was carried out under outdoor and indoor control operating conditions. This paper presents the monitored performance of the Solasyphon compared with that of a conventional solar water heater with a coiled heat exchanger and the negative effects regarding the heat losses resulting from the small size of the solar collector especially at high inlet temperatures (greater than 70 °C).