Our solar water heater is a HelioDyne Gobi 408, rated as one of the highest efficiency systems on the market. Not like we really need high efficiency in the summer, but this system will be able to provide hot water even in the winter months. For example, on a sunny December day, with an outside temperature of 50 °F, this collector will still output 150 °F water. It does this with only 32.3 sq. ft. of collector area, using diffusive glass to catch sunlight better at all angles, and an “infrared black” coating to absorb heat. During the summer, an automatic mixing valve keeps the hot water supply at a safe temperature.

The collector is permanently set at 51°, the optimum winter angle. Unlike photovoltaic panels, sufficient energy will be absorbed from the high summer Sun without lowering the tilt angle. You can see there are shadows cast on the collector by both the roof lights and weather station (not in photo). On a photovoltaic panel this would create problems. With a solar water heater it’s not really an issue, as the hot water output is directly proportional to the area not shaded.

The other half of the system interfaces with our hot water heater, a Lochinvar Sun Saver, model FTA 082-K. This is an electric water heater, specifically designed for use with solar water heating systems. It has all the extra connections needed, as well as super-insulation. The small appliance to the right is our water softener, not part of this system.

Between the rooftop collector and water heater is a heat exchanger — that complicated labyrinth of copper above the water heater. Fluid from the rooftop collector is a glycol-based mixture (like antifreeze) that can take the high temperatures without boiling, and never needs to be drained during those occasional freezing temperatures we get up here at 2200 ft elevation (maybe 2-3 days a year). The hot glycol flows through insulated pipes coming down through the roof, then through that U-shaped loop (heat exchanger) where it gives up its heat to the water flowing around it. There’s two pipes wrapped around each other in the U-loop, one containing hot glycol, and the other containing water to be heated. The water then enters the water heater where (if needed) an electric heating element tops off the temperature to 120 °F.

Unlike a purely convective system, this unit requires two small pumps to move the fluids. Of course this uses electricity, but the overall increase in efficiency is worth it. The system is estimated to save 2600 kWh each year over a conventional electrical water heater. That (conservative) estimate is provided by the utility, which rebated us $0.50/kWh ($1300), hence the adjective “conservative.” Actual output should be around 4000 kWh. That translates to $370 in savings each year. With federal and state tax credits, plus the utility rebate, this system cost us only $2450, and paid for itself in 6.5 years.