Indirect (anti-freeze) active solar thermal systems are probably the most common choice for freeze-prone areas in the U.S. Solar indirect systems circulate antifreeze fluid through the collector, and a heat exchanger transfers the heat from the antifreeze solution to the tank. The heat exchanger may be coiled around the tank, or it may be inserted inside the tank. Some systems utilize an external heat exchanger mounted and attached to the side of the tank (see Figure 1).

Propylene glycol is the most common antifreeze solution for solar thermal systems; however, this type of system requires periodic maintenance of the antifreeze solution (every 3-5 years).  The closed circulation loop is pressurized with appropriate charge pressure based on the height of the collector (e.g., 20-30 psi) to help minimize flow resistance against gravity.

Mounting

Solar collectors are usually mounted at an angle equal or close to the geographical latitude of a given site. In some situations, collector mounting may require brackets or racks to provide optimal inclination or southern exposure direction. Some manufacturers offer roof integrated collector designs; however, these require planning ahead for roof membranes that may be custom-specific to the installation. Generally, there are three methods for attachment to the roof: spanner, truss lag bolt and J-Bolt.  The truss lag bolt requires the least amount of components for the mounting assembly.

Metal and tile-type roofs may require specialized mounting depending on their architectural design.

Controls

Active solar systems with a circulation loop require a controller that determines pump operation. This is accomplished by a couple of sensors – one installed at the collector outlet and one at the bottom of the storage tank. A temperature differential of eight degrees (8°F), with the collector hotter than the tank, is usually the turn-on setting to activate the pump. A correct positioning and secured attachment of the temperature sensors for optimal temperature conduction is critical in addition to the wiring connection method. A water-tight connection on temperature sensors is important for an uninterrupted signal to the controller and to provide long-term reliable operation. Use the correct type of weather-proof connector with exterior wire connections. Common electrical wire twist ties are not suitable to withstand the outdoor elements.  Wiring used for sensors installed outdoors should have a UV-rated exterior jacketing.

Ball valves are the preferred design type for use in solar system isolation and servicing. Ball valves featuring a full bore internal diameter do not present added flow restriction in the solar hot water circulation loop. 

Documentation

Providing system documentation in form of a durable “Customer Manual” with instructions on proper operation and maintenance is highly advised. Written proof of warranty and product registration is also recommended.

Maximizing Savings and Monitoring Feedback

Minimizing electric auxiliary heating element activation of a storage water heater is of key importance for achieving maximum energy savings.  A digital or mechanical timer can be beneficial to cut-off heating element activation during selected daytime hours. Some installers provide a dedicated water heater auxiliary heat element cut-off switch (240 VAC, 2-pole, 30A) as a simple means to allow home owner to increase savings. An in-line sight glass flow meter with a visual indicator installed (added cost) in the solar circulation loop helps to determine if the pump is operating correctly. Builders and homeowners may also inquire about controllers with built-in monitoring capabilities and energy metering devices (e.g., Stecca, Metrima).

To ensure that other components of the home are ready for a solar thermal system, use the following guides:

• Utility Room Space
• Mounting Surface for Pumps and Gauges
• Solar Thermal Bypass Valve
• Solar Plumbing and Wiring Chase
• Architectural Drawing

Install an Indirect (Anti-Freeze) Solar Hot Water System

1. Select an approved manufacturer that has been certified and listed by an accredited institution such as the Florida Solar Energy Center – FSEC. Solar systems certified by SRCC (OG-300) may qualify for tax credits or additional rebate incentive programs. The North Carolina-based organization Database of State Incentives for Renewables & Efficiency (DSIRE) maintains a database of state, local, utility and federal incentives and policies that promote solar renewable energy.
2. Size the solar thermal system accordingly to provide at least 50% of the homes’ water heating energy needs. Solar system selection should be certified by the Solar Rating & Certification Corporation (SRCC), the International Association of Plumbing and Mechanical Officials (IAPMO), or be labeled with Energy Star.
3. A solar thermal collector is preferably mounted on an unshaded southern exposure orientation; however, eastern or western orientations are not to be ruled out. The use of a sun chart or approved analysis tool is recommended to determine seasonal shading.
4. Solar water heating system installations should comply with local building and plumbing codes. Installation should be executed by a trained certified installer. The North American Board of Certified Energy Practitioners (NABCEP) provides a national database on their website that lists certified solar contractors. In addition, the Solar Energy Industry Association (SEIA) provides a map listing of products, companies and solar services.
5. Collector mounting on a roof substrate requires special attention to avoid water intrusion or damage to the roof structure. Builders and installers should take into consideration mounting and positioning of the collector to comply with wind zones, particularly in coastal areas.
6. Plumbing lines to the collector are to be kept at minimal length, preferably < 25 feet, and are usually routed through attics where they are continuous with sleeved insulation. Exterior plumbing lines are also possible with an architectural chase for better visual appeal.