In 2008, Sara and I decided that we'd like to switch away from an oil-fired hot water heating system with a circulator and tank off our oil-fired boiler. We spent a fair amount of time researching solar hot water systems online and at Green Energy Options in Keene. We have a Siebel-Eltron 80 gallon hot water tank with a dual heating coil. The main heating coil heats the bottom of the tank using solar transfer antifreeze heated by the array on the porch roof. The array consists of 16 evacuated glass tubes that heat a manifold (small horizontal tank) and transfer that heat to the water tank in the house. When there's enough sun, this bottom coil heats all of the water in the tank and supplies domestic hot water (DHW) needs for the two of us. This probably amounts to approximately two showers, a load of laundry every few days and an occasional dishwasher load. This might be 30 gallons a day. On a cloudy day or high usage day, the other heating coil in the hot water tank kicks in with heat from the oil-fired hot water boiler.
Having had the system in operation for six years, we would suggest a few changes to the system if we were begin again. First off, make sure that the system is sized large enough to account for actual usage as well as the heat that is lost in circulating from the solar array to the hot water tank. Our array is a good 60' from our tank and the loss is pretty substantial. We need to add more “power” on the roof by adding more solar tubes to compensate for this distance. Of course, you want to use all of the hot water generated by the sun to avoid having to find a way to bleed off excess unused heat. A wide range will work, just don't size the system too small (or too large). Your installer will help you determine that range.
Secondly, our solar tank circulator (it pushes the solar antifreeze from the array through the heating coil in the tank) is 115 VAC or normal house current. If the power goes out and the sun is shining, the system will overheat in minutes and will blow the safety valve, spewing antifreeze in the cellar. A better plan would be to have a photovoltaic panel and a backup battery providing 12 VDC to a 12 volt circulator pump. That way the circulator system is independent of the grid. When the sun shines, the photovoltaic panel creates electricity (in combination with the battery) to run the circulator while the sun heats the solar array.
Since we burn wood to heat our house, I might use a different heat source for our back up heating coil. Our source is the oil boiler and often the only reason it runs is to provide heat for hot water on a cloudy or high usage day. With the high price of oil, it might be more efficient to have an electric top coil in the hot water tank. This is fairly common now and requires much less installation work than connecting to the oil boiler.
Another method that might work would be to have a single coil solar hot water tank using the sun to heat the DHW with an on-demand propane or electric hot water heater in series with the output of the solar tank. The on demand unit would be set to come on if the water from the solar tank isn't hot enough, otherwise it would not run.
Costs have started to come down since 2008 when our total cost was $7,200. We used to use a full tank of oil (275 gallons) to heat hot water all year before the solar hot water system was installed. That would cost $965 at $3.50 a gallon. Currently we use about 140 gallons for a savings of about $475 per year. Once we have additional tubes on the roof, our costs should come down. The payback period (which reflects the savings in oil to heat hot water) is about 15 years before rebates currently available. There are several other systems in town, some of which use flat panel arrays rather than evacuated tubes. A comparison of heating capability and costs between these two types of solar hot water heating systems would make a good topic for another article.