The installers of solar water heating systems have to navigate a range of plumbing and mechanical codes, which sometimes seem to be at odds with each other.
Installation of solar water heating (SWH) systems requires significant expertise in a number of trades. Installers need to know carpentry—critical for mounting solar collectors—as well as electrical systems, necessary for installing system controls. They also need a thorough grounding in plumbing and heating systems to properly integrate SWH equipment with standard water and space heating systems.
Each of these trades has a set of requirements that installers must follow to ensure code compliance. In many jurisdictions, this requires SWH installers to be knowledgeable about specific portions of the local building, plumbing, mechanical, energy conservation and electrical codes.
Among jurisdictions in the US, some code adoption is relatively uniform. The International Residential Code (IRC), International Building Code (IBC), International Energy Conservation Code (IECC) and National Electrical Code, for instance, are dominant. Some variation exists because each jurisdiction decides which edition of a code to adopt, but the contrast in requirements for these codes is not as stark as it is for plumbing and mechanical codes.
There are two primary plumbing codes in the US: the Uniform Plumbing Code (UPC) and the International Plumbing Code (IPC). Most jurisdictions have adopted the UPC or IPC. Exceptions include states that have adopted unique state plumbing codes, including Louisiana and Massachusetts, and states that have adopted the National Standard Plumbing Code (NSPC), such as Maryland and New Jersey.
While the International Mechanical Code (IMC) and the Uniform Mechanical Code (UMC) share some similarities, they differ significantly in their treatment of SWH systems. In addition, the national mechanical codes have variations that can create confusion and inconsistency in local requirements.
In this article, I explore differences among the plumbing and mechanical codes, and clarify some of the more confusing code requirements that impact SWH integration professionals. I also explore and explain portions of the international codes and the Uniform Solar Energy Code (USEC) that have posed a challenge for installers, designers and building inspectors.
Unique Characteristics of Solar Heating Systems
Plumbing and mechanical codes must be broad enough to accommodate all types of heating systems, including gas and oil boilers, ground-source heat pumps, radiant heating distribution, electric water heaters and solar heating systems. Sometimes the industry introduces new techniques or technologies faster than the code organizations can respond. For instance, some SWH system installers have begun to use corrugated stainless steel tubing (CSST), a material that the major mechanical codes have yet to address.
Committees of volunteers, each with specific expertise, create the plumbing and mechanical codes. A plumbing code committee must have members who are knowledgeable in topics such as water supply, sanitary drainage and plumbing fixtures. The code reflects the expertise and foresight of these individuals. If committee members have limited experience with a particular technology, they may approve requirements without full consideration of whether they apply to all the technologies that the specific code governs. The resulting code requirements may be too restrictive or may be irrelevant for systems that operate differently from the norm. This has been the case in several US jurisdictions, where applying requirements for standard hydronic systems to SWH systems has created onerous installation procedures.
There are two significant differences between antifreeze solar heating systems and the majority of standard hydronic systems: First, most hydronic systems have an automatic fill valve that adds water to the system to maintain its pressure, while many SWH systems do not. Second, unlike solar heating systems, most standard hydronic heating systems automatically shut off the heat sources when temperatures exceed design parameters.
Without an automatic fill valve, antifreeze and drainback solar heating systems contain a fixed amount of liquid. If a component fails or if an overpressure situation occurs, it is a singular occurrence that the introduction of more fluid into the system does not exacerbate. This is an important difference, as hydronic heating systems with make-up water supplied via an automatic fill valve can experience numerous overpressure events if they are not immediately identified.
In addition, when gas, oil or electric appliances serve as a heat source for a hydronic system, thermostatic controls can regulate fuel or power delivery. These controls can turn the energy source off once the system reaches the design’s target temperature. However, because it is not possible to turn off the energy source for antifreeze solar heating systems, the design needs to build in measures for alleviating overheating. Certain equipment may require higher temperature ratings than standard hydronic systems would call for. Also, solar heating systems may allow for higher operating pressures.
To address these variations, some code organizations have developed subcommittees or stand-alone codes for solar technologies. For example, the International Association of Plumbing and Mechanical Officials (IAPMO), which issues the UPC and UMC, developed the USEC to specify requirements for SWH system installation.
Heat Exchangers and Backflow Prevention
The most controversial code requirements related to solar heating systems—and those that have had the most impact on the industry—concern cross connection. Cross connection occurs when you connect a potable water system to piping containing a fluid that is not potable or that may contain contaminants. A common example of cross connection occurs in hydronic heating systems that pipe an automatic fill valve into the mechanical loop to provide make-up water. In this case, code requirements ensure that water from the hydronic heating system does not contaminate the potable water supply through backsiphonage or backflow.
Backsiphonage. When the water supply is under negative pressure, suction-—or backsiphonage-—can pull fluid back into the water supply. For example, the opening of a fire hydrant can cause nearby homes and buildings to experience back-siphonage. Installers can prevent backsiphonage by installing a vacuum relief valve or an approved dip tube on the cold water supply to the storage tank. These measures provide an air gap that prevents suctioning of the tank’s contents into the potable water supply.
Backflow. Differential pressure between the potable water system and the water supply causes backflow. It occurs when the pressure in the nonpotable system is higher than that of the potable system to which it connects. Unless the installer takes preventative measures, nonpotable fluid can push into the potable water supply. For this reason, standard hydronic heating systems in which potable water connects directly to a system with nonpotable fluid require backflow prevention devices. Indirect SWH systems, however, do not connect directly to the potable water supply. Instead, the interface between the mechanical piping and the potable water supply occurs at the heat exchanger.
If the heat exchanger fails in a standard hydronic or SWH system, under certain conditions heat transfer fluid could enter the potable water system at an indirect water heater. The fluid behavior depends upon the operating pressures on either side of the heat exchanger. For example, hydronic heating systems typically use pressure relief valves that are rated for discharge at 30 psi and operate at pressures of 15–20 psi. These pressures are commonly below the standard street pressure for a public water supply. If a heat exchanger leaks under these circumstances, the potable water will push into the hydronic piping and likely activate the pressure relief valve.
In contrast, antifreeze SWH systems typically utilize pressure relief valves rated to discharge at 75–150 psi, depending upon system design. These higher relief valve ratings are due primarily to the possibility of stagnation, which can significantly increase system temperature and pressure. Stagnation occurs when there is sufficient solar radiation and the fluid in the system does not circulate due to factors such as a power outage or a pump failure, or when the tank has reached the maximum temperature and the pump turns off to protect the tank from overheating. System designers can use relief valves rated at higher pressures, such as 150 psi, to permit larger pressure fluctuations in the system and reduce the required expansion tank volume. For most systems, these high system pressures are infrequent or may not occur at all. For a majority of the system’s lifespan, the standard operating pressure stays below the potable water pressure.
The differences between SWH systems and standard hydronic heating systems make it challenging to develop effective code requirements, since codes provide an overarching set of requirements for all systems, while the pressure conditions that occur during stagnation are unique to solar heating technologies. In light of this issue, code committees and local jurisdictions should make an effort to adapt code requirements in a flexible manner that promotes public health without putting an unnecessary burden on a particular product or technology, including solar heating.
Code developments in Louisiana over the last few years illustrate this challenge. In 2010, Louisiana’s Department of Health and Hospitals issued a letter of intent that required all SWH systems in the state to utilize double-wall heat exchangers, nontoxic heat transfer fluid and a reduced-pressure backflow preventer that needed at least annual testing. This decision increased the cost of systems so significantly that it limited the growth of the state’s SWH industry. Recognizing that the use of a double-wall heat exchanger and nontoxic heat transfer fluid adequately protects the public water supply, the department issued a revised letter of intent in 2011 that revoked the backflow preventer requirement. The current code requires the use of a double-wall heat exchanger regardless of the type of heat transfer fluid used. This requirement includes drainback systems, which typically use water as a heat transfer fluid and have less risk of causing cross contamination than any conventional heating system that utilizes an indirect water heater.
US code organizations take varied approaches to the cross-contamination issue. The major plumbing and mechanical codes include installation requirements related to the type of heat transfer fluid, the type of heat exchanger and the operating pressure of the solar loop to minimize any cross contamination that might occur if a heat exchanger were to fail.
IPC 608.16.3 (2015) and USEC 406.1.1 (2012) require the use of essentially nontoxic heat transfer fluid with single-wall heat exchangers. If a system uses a toxic heat transfer fluid such as ethylene glycol, these codes require a double-wall heat exchanger with an air gap. The air gap ensures that the toxic fluid leaks onto the floor below the heat exchanger rather than into the potable water supply. Since a pool of toxic fluid in a residential or commercial setting presents another health hazard, it is best to avoid the use of toxic heat transfer fluids in SWH systems.
The IPC and the UPC define essentially nontoxic differently. The 2012 IPC definition refers to “fluids having a Gosselin rating of 1,” while the 2012 UPC definition refers to “fluid having a toxic rating or Class of 1.” Propylene glycol meets both definitions (per 21 CFR 184.1666). The corrosion inhibitors commonly used in solar heat transfer fluid also meet the definitions of essentially nontoxic. Since mixing several nontoxic ingredients does not ensure that the final solution will be essentially nontoxic, manufacturers, public health officials and code officials are developing mechanisms that can better assess the nontoxicity of heat transfer fluids.
USEC 406.1.1 (2012) allows the use of single-wall heat exchangers only if the FDA has recognized the heat transfer fluid as safe and the maximum pressure in the solar loop does not exceed the maximum pressure of the potable water supply. The intent here is to ensure that a heat exchanger leak causes the potable water to push into the solar loop rather than vice versa. To comply with this requirement, the installer must know the potable water pressure, which can vary significantly by jurisdiction and even within the same jurisdiction. For example, a facility located near a public water supply pump house may see higher pressures than a facility located at the end of a distribution branch. Additionally, homes on a private water supply such as a well may have water pressures that would necessitate the use of a double-wall heat exchanger.
Though these requirements have evolved over several code cycles, challenges continue to arise when common industry practices conflict with existing code requirements. For example, in Oregon, code requirements formerly dictated the use of a double-wall heat exchanger when the pressure relief valve on a SWH system exceeded 30 psi. In 2008 the Oregon Building Codes Division issued an alternate method ruling that allows installers to use pressure relief valves rated up to 150 psi with single-wall heat exchangers as long as the operating pressure of the system remains “below the normal minimum operating pressure of the potable water system in the building.” The state made this determination after considering common industry practices, including the fact that standard system operating pressures are below 30 psi.
The major codes grant local jurisdictions considerable responsibility for interpretation, allowing the AHJ (commonly represented by the local code official) to accept alternate designs. For example, Appendix C of the 2012 UPC and Appendix A of the 2012 USEC detail local control for heat exchanger designs.
However, some requirements confuse even code officials and solar professionals. For example, IMC 1401.2 (2012) stated that “potable water supplies to solar systems shall be protected against contamination in accordance with the International Plumbing Code.” The 2012 IRC had a detailed requirement stating that “the potable water supply to a solar system shall be equipped with a backflow preventer with intermediate atmospheric vent complying with ASSE 1012 or a reduced pressure principle backflow preventer complying with ASSE 1013” (2012 IRC P2902.5.5). However, this requirement was ambiguous because it does not clearly define what constitutes the potable water supply to a system.
The IPC definition of cross connection refers to a physical connection between two piping systems where a pressure differential could initiate cross contamination. In addition, the devices certified under ASSE 1012 are required when you install an automatic fill device in a standard hydronic system. This requirement does not distinguish between systems that utilize single-wall or double-wall heat exchangers, nor does it specify the type of heat transfer fluid used in the system. It is unclear whether this section applies to standard SWH installations.
Requiring an ASSE 1013 device on the potable water supply to a solar indirect water heater would support public health only if you used an essentially toxic heat transfer fluid, such as ethylene glycol, with a single-wall heat exchanger. Considering that the code already prohibits such an installation, this portion is superfluous. Industry practice supports that interpretation, and the 2015 International Codes clarify this issue by specifying that “water supplies of any type shall not be connected to the solar heating loop of an indirect solar thermal hot water heating system.” As a result, indirect SWH systems do not require backflow preventers, nor do most direct systems.
With each code adoption cycle, committees review the public health risks associated with the use of a single-wall heat exchanger with an antifreeze SWH system. For a code-compliant SWH system to contaminate the potable water supply, the following combination of events would have to occur: the heat exchanger leaks, the solar loop stagnates, and the expansion tank(s) cannot protect the system from exceeding the water supply pressure to the building.
Under these conditions, the antifreeze solution could potentially push back into the public water supply. The probability of such an occurrence is low, considering that the potable water in the indirect water heater, and any water consumed at hot water fixtures in the building located after the heat exchanger breach, would dilute the solution. Stakeholders are considering proposals for the 2015 code cycle that would provide further clarification of this issue and of what constitutes an essentially nontoxic fluid.
ASME Certification for Tanks
In many residential SWH systems, designers and installers work with 80–120-gallon tanks. Commercial applications or residential combination, or “combi,” systems that also provide space heating often utilize much larger tanks. However, although larger volumes can increase a system’s heat storage capacity, they also increase the risks associated with storing pressurized water. Code requirements are much more stringent for pressurized vessels that exceed a certain volume threshold.
Both IMC 1003.1 (2015) and USEC 603.7 (2012) require the construction of pressure vessels in accordance with the ASME Boiler and Pressure Vessel (B&PV) Code, Section VIII. The B&PV Code specifies requirements for the design, construction and installation of containers for pressurized water. For solar heating systems, this includes water heaters and expansion tanks. ASME must certify pressure vessels that fall within the scope of the B&PV Code. That means the manufacturer must construct the tank in accordance with specifications detailed in the B&PV Code, and an ASME-recognized inspector must verify the manufacturing of each pressure vessel. An ASME certification may double the cost of a storage tank and increase the cost of an expansion tank by a factor of 20.
Water heaters that exceed 120 gallons require ASME certification unless their operating pressure does not exceed 15 psi. Since the water pressure from a public water supply and from most private water supplies does exceed 15 psi, a pressurized water heater that is greater than 120 gallons in volume needs an ASME stamp if it contains potable water.
When a system requires more than 120 gallons of storage, designers can avoid the costs associated with ASME certification and remain code compliant by using multiple 120-gallon tanks or by using an unpressurized storage tank, which contains water at atmospheric pressure. Heat exchangers transfer heat to the potable water and heating distribution system. According to USEC 302.1 (2012) and IMC 301.7 (2015), an approved agency must list and label these tanks. However, USEC also recognizes third-party certification.
Thermal accumulators. The introduction of thermal accumulators into the US market provides opportunities and challenges for designers, installers and the code enforcement community. Thermal accumulators are buffer tanks that contain the nonpotable water used to distribute heat in a hydronic heating system. These tanks typically have either an immersed heat exchanger at the bottom of the tank or a sidearm heat exchanger that transfers heat from the solar collectors to the buffer water. Ports at various heights in the tank supply heat to the distribution system and provide auxiliary heating from high-efficiency boilers. A large heat exchange coil or an immersed tank in the buffer tank transfers heat to the potable water.
Since combi systems need to supplement larger heating demands than a simple residential SWH system handles, they require larger storage capacities. Thermal accumulator tanks provide a simple solution for integrating solar with auxiliary heating sources for domestic water and space heating. One of the thermal accumulators available in the US market, Lochinvar’s Strato-Therm+, offers 125–900-gallon capacities. Each tank has an ASME Section VIII stamp to comply with the B&PV Code. Any application requiring storage tank volumes of 125–900 gallons can use these tanks.
Triangle Tube offers storage capacities of greater than 120 gallons without ASME certification due to the unique tank-in-tank design of its Smart Multi Energy line of storage tanks. These models meet compliance since neither the stainless steel potable water tank nor the surrounding buffer tank contains a volume greater than 120 gallons. One of the models achieves a total storage volume of 171 gallons by surrounding a 105-gallon inner tank with 66 gallons of buffer water in the outer tank.
Several other companies offer thermal accumulators that exceed 120 gallons but are not ASME certified. ASME B&PV Code Section VIII, U-1(C)(2)(h), requires that the “internal operating pressure must not exceed 15 psi.” The only way to ensure that the operating pressure does not exceed this level is to utilize a pressure-relief valve rated at 15 psi, but this may cause complications. The modulating, condensing gas-fired boilers that are most appropriate for integrating with these tanks often require a minimum hydronic system pressure of 15 psi or higher. As a result, it is critical to confirm that all of the equipment operates properly under the design conditions that B&PV Code requires when a system installation includes large non–ASME-certified tanks.
Expansion tanks. ASME has separate requirements for expansion tanks in solar heating systems. B&PV Code Section VIII states that an expansion tank must be ASME certified if the design pressure exceeds 300 psi or the design temperature exceeds 210°F. Expansion tanks that buffer the system from pressure fluctuations are designed to operate below these limits. As a result, the B&PV Code does not require ASME certification of thermal expansion tanks.
The requirements relating to expansion tanks in antifreeze SWH systems require further interpretation, however. While the B&PV Code does not have an explicit definition for design temperature, other codes do. The 2012 USEC, for example, defines the design temperature as “the maximum allowable continuous or intermittent temperature for which a specific part of a solar energy system is designed to operate safely and reliably.” Based on this definition and considering that collector temperatures in antifreeze systems may reach 340°F–420°F during stagnation, a strict interpretation of the code indicates that you must either use an ASME-certified solar expansion tank or install the tank in a location within the system that ensures antifreeze temperatures do not exceed 210°F. During normal operation, the fluid temperature in an SWH system does not exceed 210°F. Since designers commonly position a solar expansion tank on branch piping from the main collector loop, fluid contained in the expansion tank generally remains at a much lower temperature than the rest of the system fluid.
Stagnation events, when the fluid in the collector turns to steam and forces the liquid contents of the collector array into the system piping, pose a design challenge. When stagnation occurs, the membrane in the expansion tank stretches to accommodate expansion. In a properly designed system, the steam should remain in the collector array and adjacent piping. The volume of fluid between the collector array and the expansion tank determines the maximum fluid temperatures in the expansion tank. If this volume is less than the volume of the collector array, temperatures in the expansion tank may exceed 210°F. To alleviate this situation, designers can use heat dissipation strategies on the piping between the primary solar circuit and the expansion tank. For example, they could add heat dissipation fins like those used in standard baseboard radiators or install a prevessel on the piping that contains enough fluid to maintain temperatures below 210°F in the solar expansion tank.
The ASME requirements for expansion tanks in SWH systems illustrate how the traditional categories within the codes do not reflect the unique nature of solar heating systems. For example, the B&PV Code assumes that expansion tanks that experience temperatures exceeding 210°F are connected to power boilers. ASME defines a power boiler as “a boiler in which steam or other vapor is generated at a pressure of more than 15 psi (100 kPa) for use external to itself.” A standard SWH system does not produce steam for the purposes of providing heat to another source. In fact, when a SWH system does produce steam, it is not operational. This contrast is a significant one.
Corrugated Stainless Steel Tubing (CSST)
In recent years, several manufacturers have introduced insulated CSST linesets for use in the collector loop of antifreeze solar heating systems. Manufacturers typically sell these products in coils that include two lengths of CSST encased in pipe insulation, which also contains a two-wire sensor cable for connecting the collector sensor to the system’s differential controller. The insulation generally has a coating to protect it from degradation due to UV radiation when used in outdoor applications.
The fittings used to transition from CSST to copper tubing and other components comprise several parts: a union nut, a clamping washer installed between the CSST corrugations to secure the union nut to the CSST, an adapter that links the CSST and the copper tubing, and a washer or sealing ring between the union nut and the adapter. These fittings, unique to each manufacturer, may target a particular application. For example, some manufacturers utilize the same CSST product for both potable water and solar applications. In this case, the fittings for each application may differ significantly. Since the temperatures in solar heating systems may exceed 400°F, you may need to employ a different material for the washer or sealing ring.
Though CSST is a well-established product in the fuel piping trade, it is fairly uncommon in plumbing and hydronic piping applications. As a result, the major mechanical codes have yet to include CSST as one of the allowable piping materials, and neither IMC 1202.4 (2015) nor USEC 407.1 (2012) allows its use.
Since CSST sizes vary by manufacturer, there is no listed standard for this type of tubing. As the code organizations incorporate CSST for hydronic applications into their mechanical codes, they will face the challenge of identifying a standard for use of these products in solar applications.
The fact that the mechanical codes exclude CSST does not necessarily prevent its use in solar heating systems. Both the IMC and the USEC include provisions designed to address situations in which technology responds more quickly to change than the 3-year code cycles do. IMC 105.2 (2015) and USEC 302.2 (2012) allow the AHJ discretion for such cases. A code official can review technical documentation specifying that the particular brand of CSST and its fittings are compatible with the fluid in the system and are rated for the design temperatures and pressures. If satisfied, the official can approve its use.
Many solar CSST manufacturers have chosen to have their products tested to ASTM A240, which is useful if a code official requires conformance to a recognized standard. ASTM A24 specifies the required chemical composition for stainless steel in pressure vessels and general applications.
If the code official approves CSST for a project, the installer must support the tubing in accordance with the manufacturer’s installation instructions, per IMC 304.1 (2015) and USEC 307.1 (2012). If the instructions do not include the maximum support intervals and method of support, it is the installer’s responsibility to ask the manufacturer. The manufacturer’s instructions may provide guidance on the minimum allowable bend radius for its style of CSST. This guidance is important because too tight a bend radius could weaken the tubing wall, ultimately causing it to fail.
SWH designers and installers who are looking to incorporate CSST must carefully consider flashing and penetration sealing details where the CSST enters a building. Manufacturers design most conventional pipe flashing products for use with smooth rather than corrugated tubing. It is important to use flashing designed specifically for use with CSST to ensure code compliance.
Becoming Part of the Process
The evolution and clarification of code requirements is important to the growth of the solar heating industry. Developing clear expectations provides a level of uniformity and predictability that is helpful when training new design, installation and inspection professionals and empowers those already working in the field.
While it is unlikely that the solar heating industry will reach the same level of uniformity in code adoption as the PV industry, it is still important for industry professionals to engage and educate local building officials and get involved with the code-making process. Consistent and standardized plumbing and mechanical code requirements make it easier for SWH system installers to work in multiple jurisdictions, and ultimately drive down the cost of deploying SWH.
Vaughan Woodruff / Insource Renewables / Pittsfield, ME / insourcerenewables.com