Collectors and storage tanks come in many sizes, and the most appropriate ones for your application will depend on your household hot water usage and the amount of solar radiation at your site. The average American uses about 20 gallons of hot water per day, so it seemed reasonable to assume that 40 square feet of collectors and 80 gallons of hot-water storage would be ample for a two-person household in the Pacific Northwest. But because our country lifestyle entails heaps of dirty work clothes, bathing muddy dogs and horses, and hot baths after a long day of chores, I decided to supersize the system rather than rely on our existing electric hot water tanks to make up for production shortfalls. I settled on 80 square feet of flat-plate collectors and selected a 120-gallon storage tank for extra storage capacity, to tide us over during cloudy spells, winter months, and high-use periods.
After specifying the collectors and storage tank, I needed to choose a circulation pump for moving the antifreeze solution through the system (see “Pick the Right Pump” in HP121). Without circulation, temperatures in the collectors can climb as high as 400°F and overheat the antifreeze solution—causing the glycol to break down. As a result, the system loses its antifreeze protection, and the solution must be drained and replaced.
The less expensive option is a standard 120-volt AC pump, switched by a controller that receives input from two or three temperature sensors, one at the collectors and one or two at the storage tank. The more expensive option up-front is a 12-volt pump driven by a solar-electric module that powers the pump when the sun shines. Though both of these pumping strategies reportedly work well, I put my faith in the sun and a 21-watt PV module. In our remote location, where utility outages can be counted on, the sun is a more reliable power source.
Once I’d specified the components, deciding where to mount the collectors was tricky. A variety of factors came into play, including limiting shading of the collectors, roof strength, insulation, and aesthetics. In our area, getting a local building permit to add weight to an existing roof is either difficult or impossible. With retrofitting out of the question, and to preserve views and avoid disturbing some nearby landscaping, I decided to mount the collectors on the roof of a purpose-built shed just south of our home.
The shed’s insulated interior accommodates the storage tank and other mechanicals. This saved space inside the house and simplified the plumbing between the collectors and storage tank, making it shorter and limiting heat loss. As an unexpected benefit, the collectors’ position on the shed allows the early morning sun to hit the exposed backside of the collectors, which causes a slight temperature rise in the closed loop even before the sun hits the glazing.
Although no two DIY installations are ever identical, systems do share some similarities. The first mantra for a successful installation is that good planning goes a long way. Here are some valuable lessons I learned from my installation.
Map out your plumbing route. Put some thought into how your new feed line will run from the SHW storage tank to your pre-existing hot water tank. Look for a route that will provide short-term benefits (ease of installation) and long-term dividends (shorter lines for less heat loss).
While connecting the two existing tanks and the new tank in the shed, I spent a lot of time getting cozy with tight crawl spaces while soldering pipes, pressure-testing connections, adding support for and insulating the pipes, and then installing temperature sensors. If possible, use an above-floor route that is more installation-friendly than mine.