Codes & Certifications

for Solar Hot Water Systems

Inside this Article

International Code Books.
International Code Books.
U.S. Code Books
U.S. Code Books
U.S. Solar Hot Water Certification Programs.
U.S. Solar Hot Water Certification Programs.
International Code Books.
U.S. Code Books
U.S. Solar Hot Water Certification Programs.

Building trades code books have a history of being difficult to understand. Sometimes, instead of a reference on how to do things right, they can seem more like codes used to send secret messages. And if you don’t know the key, the information can be obscure or undecipherable. Confusion and conflict are often the result of different interpretations. 

SHW code books are getting better, but they are not without their failings. In addition to a bunch of codes, there are also certifications for solar collectors, systems, and installers—and where do these fit in the industry?

Here are some insider insights and background on codes and certifications most applicable to SHW systems.

Why Codes & Certifications?

Codes are published and enforced for the health and safety of the public. They specify acceptable materials, sizes of materials, and other information for constructing safe buildings and the electrical, mechanical, and plumbing systems within. Plumbing, heating, cooling, and solar codes are usually published by two national code-writing organizations, but sometimes local government agencies write their own codes. 

Whether a government writes its codes or adopts those written by someone else (as most do), they have the rule of law behind them. Government agencies regulate the building trades through building permits, code enforcement, and licensing qualified individuals and companies.

In the United States, each state administers its code enforcement and licensing. Many large municipalities have additional code requirements instead of or in addition to state requirements. The codes used, and licenses and permits required, can change significantly across state lines. Licenses in most trades are only recognized in the state of issue.

Wading into the Alphabet Soup

In the United States, five code books give guidance for installing solar water heating systems. The International Code Council (ICC) publishes the International Plumbing Code (IPC) and the International Mechanical Code (IMC). The International Association of Plumbing and Mechanical Officials (IAPMO) publishes the Uniform Plumbing Code (UPC), the Uniform Mechanical Code (UMC), and the Uniform Solar Energy Code (USEC).

The books are published on a three-year cycle, with the most recent being the 2009 version. The ICC codes are used in the majority of the U.S. states, especially in the East and Midwest, but there is no precise dividing line where the different books are used—it all depends on the jurisdiction.

Having two sets of published codes for the same trade is a hassle. If you move across a state line, or even into another local jurisdiction, you may find yourself under a different set of rules—although they are similar, the nuances can be nuisances. It is also a reason why plumbing and HVAC licenses don’t have reciprocity like the electrical trades with a single national code.

There may come a time when the United States will have a single set of code books for solar water, pool, and space heating. The ICC codes seem to be gaining ground. It could be that their books are easier to read and understand, or that the International Energy Conservation Code gets a push from the government.


The IPC and UPC are the senior references for all issues dealing with potable water, and specify materials and other requirements for supply piping systems. Likewise, the same is true for heating and cooling systems with the IMC and the UMC. For solar thermal systems, however, the plumbing and mechanical code books don’t offer much specific detail, and that’s where IAPMO’s Uniform Solar Energy Code (USEC) steps in. 

Until recently, the little-known USEC has not been widely adopted. However, the 2009 edition was adopted as a standard by the American National Standards Institute (ANSI), which publishes standards for almost everything. (For instance, if you want the specifications and standards for a certain kind of bolt, ANSI is the place to look.)

The USEC specifies materials for collectors and balance-of-system components. The 88-page book covers material specifications and some design and installation requirements for collectors, tanks, heat exchangers, piping, insulation, and more. 

Conflicts & Controversy

Compared to the faster-paced PV industry, the thermal industry has a snaillike pace of innovation. For the most part, SHW installers are installing the same basic types of systems they did 30 years ago. Although this makes the industry seem stagnant, the reality is that flat-plate collectors are simple devices that already approach maximum theoretical efficiency. Except for improved longevity, collector technology hasn’t changed in decades. Strides in lowering costs, and overcoming fragility and longevity issues are being made for evacuated-tube collectors, but, again, the systems are nothing new.

Heat exchanger design has been the only big change in the last few years. The ICC and IAPMO books are now in relative agreement when it comes to heat exchangers—and this is a first. Both the IMC and USEC now allow single-wall heat exchangers with nontoxic heat-transfer fluid; the USEC also requires a pressure-relief valve of no more than 30 psi if a single-wall exchanger is used—probably because the pressure-relief will actuate below the municipal water pressure.

The ICC’s IMC was the first to allow single-wall heat exchangers, while the UPC and USEC required double-wall, vented heat exchangers if a system design calls for anything but potable water in the collector loop. 

This requirement made even nontoxic-antifreeze-based systems that have less-expensive and higher-efficiency single-wall exchangers illegal in those jurisdictions using the UPC and USEC and subject to a red-tag correction notice—essentially a notice that work or use must be stopped because of unsafe conditions.

Now, a change in the 2009 USEC allows single-wall exchangers, so long as the collector-loop pressure relief is a maximum of 30 psi and a caution label is provided to warn of the fluid in the system. The 30 psi relief provision stems from municipal water systems, which typically have higher pressure than that. The logic is that any leakage will travel from higher to lower pressure. 

However, not all jurisdictions are on board. The health department of Louisiana, for instance, believes that single-wall exchangers are not fail-safe enough to prevent municipal water supplies from being contaminated by a leaking system with the wrong (toxic) antifreeze installed. While health officials also were skeptical of double-wall exchangers, they appeared to see the logic that it was impossible for a system with a double-wall exchanger to threaten city water systems. The department’s solution is to require backflow preventers on any building where a SHW system is installed. This decision may also require hiring qualified people to check the backflow preventers at least annually. The irony is that these costs make installing systems with single-wall exchangers more expensive than ones with double-wall exchangers.

Codes & Installing Your Own System

If you are installing a SHW system in any jurisdiction using ICC code books, the 132-page IMC is the book you that will be the most valuable to you. The IMC contains a small section on boilers and water heaters in Chapter 10 and two pages on solar systems in Chapter 14, which also has a paragraph on heat exchangers in Section 140. Reading all this takes less than a half hour. 

The IPC addresses rules regarding potable water and waste systems. Most of what might pertain to a SHW system is in Chapter 6: Section 607 (Hot Water Supply Systems) and Section 608 (Protection of Potable Water Supply). The IPC also has rules for heat exchangers that are a little different than the IMC and are in Section 608.16.3. It just takes a short time to read these sections in their entirety. 

The difference in the code publishing organizations is evidenced by the ICC putting water heaters in the mechanical code book and IAPMO putting water heaters in the plumbing code book. In the UPC, Chapter 5 (Water Heaters) and Chapter 6 (Water Supply and Distribution) provide guidance for SHW systems. Heat exchangers are redundantly addressed in the IAPMO books with Sections 506.4.2 and 603.4.4.1 in the UPC, and Section 405.1 in the USEC.

In IAPMO jurisdictions, the USEC is usually the governing code book, but this, as in all things relating to codes, is up to the individual location. This book contains more detailed information about SHW systems than the ICC books. The first three chapters of the USEC are just the standard boilerplate of administrative procedures, definitions, and regulations common to all code books. But, even here, the detail about SHW systems is evident in that definitions are given for things like drainback systems, closed-loop, collector tilt, and many other terms that are not given in the IMC’s definitions.

Here’s a breakdown of pertinent SHW section in the 88-page USEC:

  • Chapter 4—Piping (includes heat exchangers)
  • Chapter 5—Joints and connections
  • Chapter 6—Thermal storage (water storage and expansion tanks)
  • Chapter 7—Collectors (materials suitable for liquid and air collectors)
  • Chapter 8—Insulation (pipe and duct insulation)
  • Chapter 9—Ductwork (a single sentence; it just gives reference to the UMC chapter on ductwork)
  • Chapter 10—Electrical (This chapter is sometimes not adopted by jurisdictions since it is just a copy of the National Electrical Code’s Article 690, which local building officials have probably already adopted.)
  • Chapter 11—Material standards (10 pages of references on standards for piping, etc.)

Many sections of the USEC refer back to the UPC or UMC, as does the whole of Chapter 9. These references have little to do with an SHW installation. Keep in mind, the authority having jurisdiction (AHJ or “inspector”) is the ultimate decision-maker of what is safe or not—and arguing with them may not be wise. However, if you have a justification for a component or technique in writing, inspectors’ opinions can often be swayed.

I’ve noticed that inspectors often concentrate on two things: pressure-relief valves and heat exchangers. It makes sense because a pressure-relief valve on an antifreeze system will prevent excessive pressure buildup and a heat exchanger of the right type will prevent water supply contamination in the event the heat-transfer fluid is toxic. Public health and safety is the reason we have codes in the first place.

Beyond the Codes—Certifications

Certifications for collectors & systems. When energy conservation and RE tax credits hit the mainstream in the late 1970s, much SHW system marketing went over the top. This first tax credit era brought out the worst, with some fly-by-night manufacturers claiming outlandishly high SHW system performance. In addition, unscrupulous businesspeople wanting to cash in on the trend became inexperienced installers, installing substandard and shoddy systems. The newly established Department of Energy told the solar thermal industry to clean up the problems and threatened to have the government do the janitorial work if the industry hesitated. The Solar Rating and Certification Corporation was founded in 1980 as the solution. This nonprofit organization certifies solar thermal collectors and SHW systems based on testing by accredited independent test laboratories. The SRCC is funded partly by fees levied on manufacturers.

If you plan to take advantage of federal tax credits for SHW systems, the collector or system must be SRCC certified. Many state and utility incentive programs also require SRCC system certification for eligibility. The SRCC merges the system certification data with climate data for hundreds of locations in the United States to achieve fairly accurate estimates of system performance. Many incentive programs use these estimates to calculate system rebate amounts, since hot water isn’t metered like electricity. Incentive program administrators like to base the incentive rewards on system performance as much as possible and the estimates are the next best thing to a meter.

You would think 30 years of history would mature an agency like this into a well-oiled organization. Not so. When the last federal tax credits were implemented in 2006, resulting in an onslaught of new SHW collector and system manufacturers, the SRCC was caught off-guard. Because it recognized the test results of only two laboratories, a two- to three-year backlog of certifying collectors and systems ensued. And, because a lack of a certification made a product ineligible for incentives, innovation and new product development was hindered. The backlog has diminished with the accreditation of many new laboratories last year. More information on the certifications is available at

Professional certification. The nonprofit North American Board of Certified Energy Practitioners (NABCEP) certifies installers in the PV, thermal, and wind industries based on passing a written exam. The thermal certification tests individuals on their knowledge of solar hot water and pool heating systems—in particular, safe installations. There are numerous tracks to be eligible to take the certification test, but all require candidates have a minimum of two documented installations. NABCEP has always had the policy of keeping the certification voluntary. With the bottleneck created by the SRCC, which was unable to quickly respond to a growth spurt, keeping the NABCEP certification voluntary seems wise. Note that while the NABCEP certification can give consumers confidence in a contractor’s technical expertise, the certification rarely takes the place of a license issued by the appropriate government agency.

Both the SRCC and NABCEP certifications are voluntary —up to a point. Uncertified collectors can be made, sold, and installed but the SRCC certification is required to receive the 30% federal tax credit. Plus, in many cases, the SRCC’s more comprehensive system certification is required to take advantage of state and utility incentives. The NABCEP certification also is required by a few state and utility incentive programs, even though NABCEP has a policy of discouraging mandatory certification. 

Related National Codes

The International Energy Conservation Code (IECC) is used to help calculate federal tax credits for energy-efficient homes and energy-efficiency standards for federal residential buildings and manufactured housing. The code has the support of the U.S. Department of Energy (DOE) due to the efficiency gains that the code promotes and relates to federal law through the Energy Policy Act of 1992. The DOE claims that the 2009 version will result in approximately a 15% improvement in residential energy efficiency compared to the previous edition. The IECC contains a small amount of information on solar thermal systems—it allows waiving the requirements for a pool cover if a solar thermal system provides 60% or more of the pool’s heating. This code is downloadable at 

The 2009 Uniform Swimming Pool, Spa and Hot Tub Code published by IAPMO is also an ANSI standard. The code governs the installation, inspection, alteration, and maintenance of swimming pool, spa, and hot tub systems and their components. In 2009, the ICC announced it would publish the International Swimming Pool and Spa Code in 2012, and is working with the Association of Pool and Spa Professionals to develop the code. 


Contributing editor Chuck Marken (chuck.marken at is a plumber, electrician, and HVAC contractor licensed in New Mexico. He has been installing and servicing solar thermal systems since 1979. Chuck is a part-time instructor for Solar Energy International and the North Carolina Solar Center, and works under contract with Sandia National Laboratories supporting the DOE-sponsored Solar Instructor Training Network. 

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