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Integrated Collector Storage

    Scope Images
    Integrated collector storage (ICS) passive solar hot water system

    Install an integrated collector storage (ICS) solar hot water system. 

    • First determine that the roof and utility room space are suitable for solar hot water components.
    • Follow the requirements for all local codes.
    • Choose an accredited solar water heating installation company.
    • Size the system according to the home’s hot water demands. 
    • Mount the system on an unshaded, southern exposure if possible. 
    • Ensure that the roof mounting system avoids water intrusion and damage to the roof structure. 
    • Choose an ICS system if you are installing a solar hot water system in a southern or warm climate that does not experience freezing temperatures at any point during the year.  (See the Climate section for more information.)

    See the Compliance Tab for related codes and standards requirements, and criteria to meet national programs such as DOE’s Zero Energy Ready Home programENERGY STAR Single-Family New Homes, and Indoor airPLUS.



    A passive integrated collector storage (ICS) solar thermal water heating systems are common in the southern–belt areas of the U.S. An ICS is a self-contained unit integrating the solar collector and hot water storage of 30-50 gallons.

    Integrated Collector Storage (ICS) System.
    Figure 1. Integrated collector storage (ICS) system. (Source: NREL.)


    These systems are passive type since no pump or controllers are needed. These units pre-heat mains incoming water before it is routed to the primary water heater storage unit supplying hot water into a building. Some people prefer the simplicity of these systems because there is little or no maintenance required. ICS units can also be plumbed in series to feed tankless electric of gas water heaters where it is boosted to end point of use. Storage on these systems may be a large single tank or a series of four inch (4 in.) tubes, usually copper with selective paint acting as the absorber. Because of the thickness and volume of water contained, chances of freezing are minimized. ICS units do not usually require freeze protection if installed in lower southern regions.  However, outdoor exposed pluming lines to and from the ICS unit may require larger diameter with heavy insulation or freeze valve for protection.  If an ICS unit is used for direct hot water consumption it is mandatory to use an anti-scald valve, whereas used in series with a secondary tank it may not be required.

    Installation of an integrated solar water heating collector 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:

    Install an Integrated Collector Storage 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.
    Ensuring Success

    Installation of an integrated collector storage (ICS) solar hot water system is no more difficult than the assembly of any other building component when proper design precautions are taken into consideration. Builders are encouraged to work with solar installers and manufacturers to select a package for high consumer acceptance. Building aesthetics and minimal use of floor space are a high priority to customers in new homes.





    The DOE Zero Energy Ready Home PV-Ready Checklist (Revision 07)  is required only under the following condition related to climate (See the Compliance Tab for other exceptions):

    • Location, based on zip code, has at least 5 kWh/m2/day average daily solar radiation based on annual solar insolation using the PVWatts online tool. See map below. 
    Average Daily Solar Radiation Map.
    Average Daily Solar Radiation Map.


    Water heating energy use varies according to the region and amount of hot water gallons used in a residence. 

    Expected annual energy use and annual cost ($), for a typical household using 60 gallons/day, for selected states.

    Table 1. Expected annual energy use and annual cost ($), for a typical household using 60 gallons/day, for selected states.
    *Simulations performed in EGUSA (Colorado and New York water heater location in basement, others located in garage).



    Collector and pipe freezing is a major obstacle solar thermal systems face to avoid operation interruption and to ensure a long-term service life. Increased probability for pipe freezing is higher in mid and northern states, as shown in the figure below. Integrated collector storage (ICS) units with 4 in. diameter tubes have an advantage over thermosyphon systems.  ICS collectors avoid freezing under prolonged low temperature periods due to their large mass and with the help of a glazed layer and insulated frame walls. 

    Probability of at least one pipe freeze in 20 years.

    Figure 3. Probability of at least one pipe freeze in 20 years.


    Pipes should be insulated with a minimum of 1 in. insulation (closed cell preferred) and those exposed to ultraviolet (UV) should be protected by using a form a jacketing (UV inhibited acrylonitrile butadiene styrene (ABS), polyvinyl chloride (PVC) or aluminum). Passive integrated collectors may require a freeze valve for extra protection in regions above central Florida. Freeze exposed potable water piping may be avoided by using heavier insulation such as R-12.

    Publication Date
    Cold Climate Housing Research Center
    Video describing the photovoltaic system at the Cold Climate Housing Research Center in Fairbanks, Alaska, which includes several types of pole-mounted tracking solar panels and inverters.


    The Compliance tab contains both program and code information. Code language is excerpted and summarized below. For exact code language, refer to the applicable code, which may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

    Installation of a solar thermal system requires building code compliance. Requisites for code compliance may differ by area, city, or county. Refer to the local governing code ordinance for specific plumbing and electrical rules. 


    DOE Zero Energy Ready Home (Revision 07)

    Exhibit 1 Mandatory Requirements.
    Exhibit 1, Item 1) Certified under the ENERGY STAR Qualified Homes Program or the ENERGY STAR Multifamily New Construction Program.
    Exhibit 1, Item 7) Provisions of the DOE Zero Energy Ready Home PV-Ready Checklist are Completed.

    DOE recommends but does not require solar thermal water systems. See the DOE Zero Energy Ready Home Solar Hot Water-Ready Checklist (Encouraged)


    2009, 2012, 2015, 2018, and 2021 International Energy Conservation Code (IECC)

    Section 401.3 A permanent certificate shall be posted on or near the electrical distribution panel that lists types and efficiencies of water heating, heating, and cooling equipment, as well as insulation R values, and window U and SHGC factors.

    Retrofit:  2009, 2012, 2015, 2018,  and 2021 IECC

    Section R101.4.3 (in 2009 and 2012). Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with this code. (See code for additional requirements and exceptions.)

    Chapter 5 (in 2015, 2018, 2020). The provisions of this chapter shall control the alteration, repair, addition, and change of occupancy of existing buildings and structures.


    2009, 2012, 2015, 2018, and 2021 International Residential Code (IRC)

    M2301 Solar Energy Systems (Solar Thermal Energy Systems in 2015, 2018, and 2021 IRC) - See requirements for solar water heating systems.

    20152018, and 2021 IRC

    Appendix U (Appendix T in 2018 and Appendix AT in 2021 IRC) Solar Ready Provisions - Prepare the home for solar installations in accordance with these specifications.

    Retrofit:  2009, 2012, 2015, 2018,  and 2021 IRC

    Section R102.7.1 Additions, alterations, or repairs. Additions, alterations, renovations, or repairs shall conform to the provisions of this code, without requiring the unaltered portions of the existing building to comply with the requirements of this code, unless otherwise stated. (See code for additional requirements and exceptions.)

    Appendix J regulates the repair, renovation, alteration, and reconstruction of existing buildings and is intended to encourage their continued safe use.


    2009, 2012, 2015, 2018, and 2021 International Mechanical Code (IMC)

    Follow the requirements for solar water heating systems found in the IMC, Chapter 14, Solar Systems (Solar Thermal Systems in 2018 and 2021 IMC).

    Existing Homes


    Installation of a solar thermal system on an existing roof is similar to installation on the roof of a new home. See the Scope, Description, and other tabs for guidance.

    Consult a structural engineer and manufacturer guidance to determine if the existing roof can carry the load.

    Ensure that all penetrations through the roof are adequately sealed to prevent water leaks. New flashing should properly integrate with existing roof underlayment and cladding layers.

    See the U.S. Department of Energy's Standard Work Specifications for more information about solar water heating.


    More Info.

    Access to some references may require purchase from the publisher. While we continually update our database, links may have changed since posting. Please contact our webmaster if you find broken links.

    References and Resources*
    National Renewable Energy Laboratory
    Publication Date
    Provides consumers with general information on solar water heating systems and how to select a contractor.
    North Carolina Solar Center
    North Carolina Solar Center
    Publication Date
    Brochure with information on residential solar hot water systems.
    U.S. Department of Energy
    Publication Date
    Website describing how solar water heaters -- also called solar domestic hot water systems -- can be a cost-effective way to generate hot water for your home.
    *For non-dated media, such as websites, the date listed is the date accessed.
    Contributors to this Guide

    The following authors and organizations contributed to the content in this Guide.

    Florida Solar Energy Center, lead for the Building America Partnership for Improved Residential Construction (BA-PIRC), a DOE Building America Research Team

    Building Science Measures
    Building Science-to-Sales Translator

    Solar Hot Water = Solar Hot Water System

    Technical Description

    Solar water heaters use energy from the sun to naturally heat water. These systems usually include one or two collectors that typically sit on a house’s roof. System designs can vary depending on many factors such as aesthetics, rigor of freezing conditions and cost. Homes ready for solar water heating systems should be designed with adequate space on the roof that is unshaded and with enough space in the utility room for a water storage tank and any needed pumps and controls.

    Solar Hot Water System
    Sales Message

    Solar hot water systems use the free energy of the sun to produce hot water. What this means to you is all the hot water you want at lower cost. Isn’t it time homes used advanced technology components?

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