Open loop solar thermal domestic hot water heater systems are common in arid regions (e.g., Arizona) because they are simple, inexpensive and permit easy retrofit of conventional systems since a heat exchanger is not required. The technique of circulating hot water from the storage tank through the collectors to prevent freezing is also common in arid regions because of reliability, simplicity and economy.

The open loop system can easily be powered by a photovoltaic (PV) module such that the system is completely independent of utility power. As with the conventional system, recirculation freeze protection can be obtained with the PV system.

The primary purpose of this paper is to describe a simple technique developed to provide recirculation freeze protection for the PV powered system, and to present operating results. It is assumed that the reader is familiar with the conventional 110 Vac powered system with recirculation freeze protection. It should be noted that the techniques presented in this paper may be easily applied to closed loop, drain down, drain back, and air solar domestic water heater systems.