Thermosiphon systems are passive with a storage tank located higher than the solar collector. They rely on passive natural convection where more dense cold water falls, moving hotter water to the storage reservoir at the top. Some systems come prepackaged with tanks pre-mounted to collectors. In these systems the tank sits on the outside of the roof. Other systems have tanks located inside attic spaces above the collectors. In the case of a direct system, potable water is used as the heat transfer fluid.

Passive direct themosiphon systems are usually limited for use in coastal areas of Texas, southern California and Central Florida. (See the Climate section for more information.) 

Passive indirect thermosiphon systems are also available which utilize polypropylene glycol with a heat exchanger to avoid freezing.  In warm climates, care must be taken to avoid stagnant conditions that can lead to the glycol overheating. In climates that freeze, the water pipes and tanks containing water must be in a conditioned space.

Installation of a thermosiphon solar hot water system on a roof requires proper connection to the roof substrate.  The manufacturer-designed attachment kit provides long-term and secure attachment to the collector.  It is inappropriate to use other building materials such as wood blocks to mount an integrated collector system. 

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 Bypass Valve
• Solar Plumbing and Wiring Chase
• Architectural Drawing

Install a Thermosiphon 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 credit or additional rebate incentive programs. The North Carolina based organization Database of State Incentives for Renewable Energy (DSIRE) maintains a data base map for 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 at 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 appeal.