Geothermal Design: Code In Conflict?
Uniform Solar Energy and Hydronics Code (USEHC) contains some requirements that may be inappropriate given certain conditions, climates, or simple designer discretion. That’s not a problem … unless your locality or state adopts USECH language as binding. Dig into the problem and note a timely opportunity to avoid having these issues set in stone for another few years.
As early as 3000 BC, civilizations began to create codes for public order and safety. The Code of Hammurabi, the old “eye for an eye” law many are familiar with, also included laws for safe building construction. The history of building codes, specifically in this country, date back to our founding fathers, George Washington and Thomas Jefferson, who believed that we should establish some minimum standards for how structures are built, simply to protect public safety. It wasn’t until the early 20th century, however, that anything actually happened in this regard.1
The history of the American Building Code development is pretty lengthy, and frankly boring, so let’s just start with this current period of time. The legacy codes in the United States are as follows.
- BOCA National Building Code (BOCA/NBC) by the Building Officials Code Administrators International
- Uniform Building Code (UBC) by the International Conference of Building Officials (ICBO)
- Standard Building Code (SBC) by the Southern Building Code Congress (SBCCI)
The BOCA/NBC was predominantly adopted on the East Coast and into the Midwest. The SBC was primarily used throughout the Southeast, and the ICBO on the West Coast.2 The two related codes most relevant to designers and engineers providing mechanical system design are the International Mechanical Code (IMC) and the Uniform Mechanical Codes (UMC). In California, the state I call home, the California Mechanical Code is based upon the UMC with a few local twists. This is not an uncommon practice (changing a national code to fit local jurisdictions), often done by larger cities such as Chicago, Los Angeles, and New York.
It is very important to note that these code-writing organizations do not have any legal force to them unless they are adopted by state and local governments.
Enter ANSI. About one hundred years ago, a group of folks got together with the intention of creating a national standards and conformity assessment process to assure the safety and health of consumers, among other things. The American National Standards Institute (ANSI) is a non-profit organization whose mission is, “To enhance both the global competitiveness of the U.S. business and the U.S. quality of life by promoting and facilitating voluntary consensus standards and conformity assessment systems, and safeguarding their integrity.”3
In a nutshell, the goals of the ANSI process are to ensure that all codes and standards follow a standard process which includes due process. Due process means that any person (organization, company, government agency, individual, etc.) with a direct and material interest has a right to participate by: a) expressing a position and a basis, b) having that position considered, and c) having the right to appeal. Due process allows for equity and fair play. In order to facilitate due process through the development of consensus, there are many things that must be incorporated into the code or standard development process. Openness, lack of dominance, balance, coordination and harmonization, consideration of views and objections, consensus vote and appeals — these are all elements of the consensus process and minimum requirements that the ANSI-Accredited Standards Developer (ASD) must employ. The process and intent of the ASD is noble, but on occasion things don’t go quite as planned.
With the movement in the building industry towards net zero energy buildings, sustainability, and the promotion of renewables, the geothermal heat pump industry has received the attention of several of these ASDs. In February this year, The Canadian Standards Association (CSA) Group released ANSI/CSA C448 Series 16, Design and installation of ground source heat pump systems for commercial and residential buildings. This standard is a bi-national effort between Canada and the United States that has gone far to build bridges between the countries and the geothermal (ground-source) heat pump community, providing a publically vetted standard that may be adopted by code writing organizations in the United States. During the same development period, the International Association of Plumbing and Mechanical Officials (IAPMO) Group was also working on a revision to the Uniform Solar Energy and Hydronics Code (USEHC) to update the code language for solar thermal systems, and to add chapters on photovoltaics and GSHP systems. This USEHC was published in the summer of 2015.
So Where’s The Conflict?
At this point, there is no conflict except if the USEHC is adopted by the state and local governments, the Authority Having Jurisdiction (AHJ). IAMPO’s website explains that the USEHC establishes the minimum requirements and standards for the protection of the public health, safety, and welfare.5
Should this USEHC code be adopted by YOUR state or local government, you need to be aware that there are several citations in the current USEHC code that conflict with current industry standards.
The USEHC cites specific design criteria relative to the ground loop portion of the system that are not appropriate for code language because the topics are not relevant to public safety.
For example, Section 703.4 states that the maximum borehole diameter shall not exceed 6 in. In the same section the code states that, “to reduce thermal interference between individual bores, a minimum borehole separation distance shall be not less than 20 feet (6,096 mm). Separation distances shall be permitted to be reduced where approved by the Authority Having Jurisdiction.”6
The topic of borehole size, spacing, and depth are design variables that are unique to each project, site, and geological formation. In some cases, these variables are defined by the AHJ who permit the drilling and who are responsible for protecting the ground water resource. It should not be prescribed by the building code because it is not related to public safety.
The USEHC also cites specific pressure testing requirements that may or may not be relevant to a specific system design. For safety reasons, it is important that the test pressure specified for the individual loops and system as a whole be based on lowest pressure rating component of the system, which in some cases could be a fitting. This code language should be more general, stating that the system should be tested and leave the details to the designer and the industry standards.
Sections 703.4, 703.4.2, and 706.1 all specify that the u-bend joint and pipe or system be filled with water and pressurized to not less than 100 psi (689kPA) for one hour to check for leaks.6 While this statement is similar to what is typically done in the industry at several points along the construction process for the ground loop heat exchanger system, it does not clearly identify the impacts of ambient temperature and solar gain on plastic pipe. It is common to use dimension ratio (DR) piping (e.g. DR 17, PE-3408) in the header system that may have a maximum working pressure of 100 psi (689 kPA) at 73˚F (22.8˚C ). If ambient temperature is sufficiently high or the solar gain on the black HDPE pipe is high, the surface temperature can exceed 73˚F (22.8˚C), reducing the maximum allowed working pressure. Likewise, in cold regions, the water in the pipe is susceptible to freezing, so there are justified exceptions to backfilling this pipe prior to final system pressurization, in the name of protecting the pipe.6
Section 703.4.2 includes a paragraph for DX systems, which are systems that circulate refrigerant through copper pipes in the earth instead of water through high-density polyethylene (HDPE) pipe. It states that “For DX Systems, each u-bend shall be tested and proved tight with an inert gas at not less than 315 psi (2,172 kPa and maintained for 15 minutes without pressure drop. The pressure reading after tremie grouting of the boreholes shall be maintained in the ground-heat exchanger for not less than 2 hours.”6
This reference is correct and is also duplicated again under 706.2 DX System Testing. However, there is an important detail missing from the System Start-up section under 708.0 that identifies nine items required for a water-source system and only one step for DX systems. This step only requires that the refrigerant liquid and vapor lines from the loop system be indentified and tagged.
More important is the requirement that in order to properly purge the DX system of air prior to adding the refrigerant, the DX loop system shall use a vacuum pump to evacuate the system down to 400 microns or less. After 400 microns or less has been achieved, the vacuum pump is removed and the digital vacuum gage is read. The system pressure should not exceed 500 microns within 15 minutes.
Depending upon the location of a ground loop system, the type of grout or “backfill,” as it is called in the USEHC, is often specified by a water district, county, or state, and is based upon the local geology. In some jurisdictions, only cementitious grouts are allowed due to local ground water or the AHJ’s preference. While thermally enhanced grout as specified in the code is used in more cases than not, it is not a one-size-fits-all solution. This information should not be put into a building code. The current citation in the USEHC currently reads as follows.
“Section 703.4.1 Thermally enhanced bentonite grout shall be used to seal and backfill each borehole. Grouting compound (bentonite-based and thermal enhancement compound) shall comply with NSF60.”6 Furthermore, “thermally enhanced” is meaningless unless it is defined with a unit of measure and a prescribed installation method to ensure it is properly mixed and installed.
Section 707.1 cites AHRI 870 as the standard for the equipment rating and performance testing for a DX heat pump unit.6 It does not however cite a standard for the equipment rating system and performance for the water-source heat pump unit. The proper reference for this equipment rating system is ARI/ASHRAE/ISO 13256-1 for water-to-air heat pumps and ARI/ASHRAE/ISO 13256-2 for water-to-water heat pumps. These equipment ratings systems for performance testing have been the industry standard since 1998.
So how did all this get past us, folks? Well, for starters, there was limited participation in the ANSI process for the previous USEHC revision cycle by stakeholders from the geothermal heat pump industry. Only two members of the voting members on the USEHC technical committee out of 23 had experience with this technology. As a comparison, the Canadian Standards Association (CSA) Group released what is now referred to as the “bi-national standard” or ANSI/CSA C448 Series 16, Design and installation of ground source heat pump systems for commercial and residential buildings. The development of this standard (not code) was revised with the collaborative efforts of 26 geothermal heat pump industry representatives from both the USA and Canada. It is recommended that in addition to the suggested language modifications proposed for the USEHC revision cycle, this ANSI standard should be used as one of the main references as the industry standard.
The opportunity to effect change to this code by the geothermal heat pump industry was missed during its first revision cycle. The next revision cycle has begun, and so those who have interest or concerns about this code need to respond to the call for proposals, which gives all stakeholders the opportunity to suggest alternate wording during the period of April 5 – July 1, 2016. Newly proposed language may be adopted for the 2018 version of this code.
“Now wait a minute,” a few of you might be saying, “what about the IGSHPA Standard? The International Ground Source Heat Pump Association created an industry standard over 25 years ago and it has served the geothermal heat pump industry well.” This is not disputed except when it comes to referencing the IGSHPA standard in the USEHC. The problem is that ANSI codes reference ANSI standards and the IGSHPA standard have not gone through the ANSI process. For this reason, over the past almost three years IGSHPA and other U.S. organizations such as the National Ground Water Association (NGWA) and Geothermal Exchange Organization (GEO) have worked with the CSA to incorporate much of the IGSHPA content into the CSA C448 document, so that both sides of the border may be better served.
So in summary, we do have a code that is in conflict if it is adopted by the AHJ. The collective industry has the next three years, beginning April 5 of this year, to change the language in this code following a very specific ANSI process. Get involved in the codes and standards review and development process, or spend another three years explaining to the building inspector why the ground loop system you designed or installed may not be “per Code” — right, wrong, or indifferent! ES
4. ANSI Essential Requirements: Due process requirements for American National Standards, Edition: January 2016. Copyright by the American National Standards Institute (ANSI), 25 West 43rd Street, 4th Floor, New York, New York 10036.
7. Closed-Loop/Geothermal Heat Pump Systems – Design and Installation Standards 2014 Edition, paragraph 1E.7.2, Published by the International Ground Source Heat Pump Association, Oklahoma State University.