5.6   Solar Water Heating    

The Water Heating Calculation Method allows water heating credits for solar water heaters. Solar systems save energy by using non-depletable resources to offset the use of conventional energy sources.

For single-family solar systems, all systems must be Solar Rating and Certification Corporation (SRCC) approved.  Accepted testing procedures include either fully approved system with OG-300 test results or built up system that use the collector (OG-100) rating.  For multi-family, only systems with OG-100 collectors can be installed.  For more detailed instructions on installation of solar water heaters refer to RA4.4.  The sortable database of SRCC-certified is located on the SRCC website.

Figure 5-13 summarizes the process flow for demonstrating compliance via both the prescriptive and performance compliances.

Figure 5-13 – Compliance Process for Solar Water Heating System

Under the prescriptive compliance path, to calculate SFF for OG-300 systems go to the following link.


and complete the use the compliance worksheet. To do this you will need the Solar Energy Factor (SEF) value from the SRCC website listed previously. To calculate SSF for OG-100 collectors under the prescriptive compliance use the compliance tool at the following link:


and complete the worksheet.

Regardless of the system type installed and compliance method chosen, mandatory requirements for pipe insulation and storage tank insulation apply as described in Section 0.

Mandatory Requirements

1.   Solar or Recovered Energy in State Buildings


Low-rise residential buildings constructed by the State of California shall have solar water heating systems. The solar system shall be sized and designed to provide at least 60 percent of the energy needed for service water heating from site solar energy or recovered energy. There is an exception when buildings for which the state architect determines that service water heating is economically or physical infeasible. See the Compliance Options section below for more information about solar water heating systems

2.   Solar Ready Buildings Requirements


There are new mandatory requirements for all buildings to be “solar ready.” The motivation behind having solar ready requirements is to encourage more future installations of both photovoltaic and solar water heating systems, even if these systems are not installed during the time of new construction. Details on these solar ready requirements are in the “Solar Ready Requirements” chapter of the compliance manual. In summary, the elements to being solar ready include:

      Designated solar zone

      Designated conduit and plumbing paths

      Documentation for solar zone and paths on construction plans, and

      Adequate electric busbar and panel capacity

5.6.1    Prescriptive Requirements

This section discusses when solar water heating is required prescriptively for systems serving single and multiple-dwelling units.

1.  Single Family

Solar water heating is prescriptively required for systems serving single family dwelling units with electric water heaters. Where no natural gas is available the standards allow the use of either electric-resistance storage or instantaneous water heater systems to serve single family dwelling units but only with the use of solar water heating.  To use this prescriptive option all of the following requirements and conditions must be met:

a.  If natural gas is unavailable to the home; and

b.  The water heater is located within the building envelope; and

c.   Recirculation pumps are not used; and

d.  The water heating system includes a solar water-heating system with a minimum SSF of     0.50.

In meeting the solar thermal system criteria the system or collectors must be certified by Solar Rating and Certification Corporation (SRCC) as described above in the introduction to 5.5.  Either OG-100 or OG-300 systems can be installed. Installation of a solar water heating system exempts single family homes from needing to set aside solar zone for future solar PV or solar water heating installation (§110.10(b)1A).

The collector installation must also meet the installation requirements detailed in RA-4.4.  These requirements specify that systems complying with the OG-300 system must be installed to the following guidelines:  

a.  Face within 35 degrees of due south; and

b.  Have a tilt slope of at least 3:12; and,

c.  Be un-shaded by buildings or trees.  See the residential appendix RA4.4 for details.

For built up systems using components and the OG-100 collector rating the collectors must be installed to match the specification entered when the solar thermal system was modeled.

For compliance with either option, you will need to print out the results and submit along with the completed compliance form.

2.  Multi-family, Motel/Hotels and High-Rise Nonresidential


Solar water heating is prescriptively required for water heating systems serving multiple dwelling units, whether they are multi-family, motel/hotels or high-rise nonresidential buildings. The minimum SSF is dependent on the climate zone: 0.20 for CZ 1 through 9, and 0.35 for CZ 10 through 16. The regulations do not limit the solar water heating equipment or system type, as long as they are SRCC certified and meet the orientation, tilt and shading requirement specified in RA4.4. Installation of a solar water heating system exempts multifamily buildings from needing to set aside solar zone for future solar PV and solar water heating installation (§110.10(b)1B). The following paragraphs offer some high-level design considerations for multifamily building solar water heating systems.

A high-priority factor for solar water heating system design is component sizing. Proper sizing of the solar collectors and solar tank ensures that the system take full advantage of the sun’s energy while avoiding the problem of overheating. While the issue of freeze protection has been widely explored (development of various solar water heating system types is a reflection of this evolution), the issue of overheating is often not considered as seriously as it should be, especially for climate conditions with relatively high solar insolation level such as California. This is especially critical for multifamily-sized systems, due to load variability.

The solar water heating sizing requirements for the Standards are conservative, the highest SSF requirement called for by the 2013 Title 24 at 50%. Stakeholders further suggested that industry standard sizing for an active system is 1.5 ft2 collector area per gallon capacity for solar tank. For more detailed guidance and best practices, there are many publicly available industry design guidelines. Two such resources developed by/in association with government agencies are Building America Best Practices Series: Solar Thermal and Photovoltaic Systems , and California Solar Initiative – Thermal: Program Handbook.Because of the new solar water heating requirement and prevalence of recirculation hot water systems in multifamily buildings, it is essential to re-iterate the importance of proper integration between the hot water recirculation system and the solar water heating system. Industry stakeholders recommended the recirculation hot water return to be connected back to the system downstream of the solar storage tank. This eliminates the unnecessary wasted energy used to heat up water routed back from the recirculation loop that may have been sitting in the solar water tank if no draw has occurred over a prolonged period of time.

Another design consideration is the layout and placement of collectors and solar tank. The idea here, similar to the discussions on recirculation system design in Section 5.3.3, is to minimize the length of plumbing, thus reducing pipe surface areas susceptible to heat loss and piping materials needed. This calls for the shortest feasible distance between the collectors themselves; furthermore, since solar tanks are typically plumbed in series with, just upstream of the conventional/auxiliary water heating equipment, the distance between collectors and solar tank should also be as short as practically possible.

5.6.2    Performance Compliance

Solar water heating systems with SSF higher than the specified prescriptive minimum or required mandatory level can be used as a tradeoff under the performance approach.  Figure 5-13 shows the compliance process needed for demonstrating compliance with solar water heating modeling. The new CEC compliance software integrates the capability of calculating an annual SSF. Users now input collector and system component specifications to calculate a corresponding SSF for the proposed system. The SSF along with other system parameters should be entered into compliance form which will be used to populate the certificate of compliance.