Solar Water Heater

The Idea

My water heating bill was running about $40/month in a house where I was the only occupant. A passive solar collector seemed like the kind of project that would pay back in two or three years and be interesting to build. It has been both of those things.

Design

The collector is a simple flat-plate design: a 4x8 sheet of 3/4-inch plywood as the backing, painted black inside, with 1/2-inch copper tubing run in a serpentine pattern across it. A sheet of tempered glass salvaged from a sliding door sits on top, creating a greenhouse effect. The whole assembly is tilted at roughly my latitude (35 degrees) facing due south.

Water circulates via a small 12V pump powered by a 10W solar panel — no grid power involved. A differential temperature controller turns the pump on when the collector is more than 8 degrees hotter than the storage tank and off when the difference drops below 3 degrees.

Results

In summer, the collector heats my 40-gallon storage tank from ambient to around 120F by early afternoon on a clear day. In winter, useful gain is limited but the system still handles maybe 30-40% of heating load on sunny days. I added a bypass valve so the system drains back into the storage tank overnight in freezing weather, avoiding the need for antifreeze.

Cost

Copper tubing: $60. Glass: free. Plywood and lumber: $35. Pump and controller: $45. Miscellaneous fittings: $30. Total: roughly $170 and two weekends. Payback at current electricity rates is about 14 months.