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Ground-source Heat Exchange (GHEX) Borehole Drilling
Geothermal heat pumps systems (sometimes referred to as GHEX, GeoExchange, earth-coupled, ground-source, or water-source heat pumps) have been used for several decades in Iowa to help Iowans’ utilize the natural thermal properties that exist under their property.
Ground-source Heat Exchange Systems (GHEX systems) use the constant temperature of the earth as the exchange medium instead of the outside air or well water source to maintain a constant temperature. The GHEX system takes advantage of this by exchanging heat between the home or business and the ground through the use of a series of ground heat exchanger loops. This allows the heating and cooling system to operate at a very high efficiency using a minor amount of electricity.
There are four basic types of geothermal heat exchange systems found in Iowa.
Three of these are: horizontal loop, vertical loop, and pond/lake loop are closed-loop systems. Each of these systems use heat exchange fluid that is circulated in a loop of piping placed in either a borehole in the earth or in a large body of water. The heat exchange fluid is used to exchange heat with the ground or a body of water.
The fourth type of system is actually a water-source heat exchange system and is called an open-loop system. This option uses one or more water supply wells to pump groundwater out of the earth and run the water through a heat exchanger inside the building. The water that has been heat altered in the process is then placed onto the land surface for absorption and evaporation or it can be placed back into the same aquifer through one or more additional water wells.
The type of system you install will depend on the contractor, the size of the application, the soil and geological conditions of the site, available land to place a system, and local site specific conditions that may limit the site for drilling boreholes like areas of contamination, local ordinances that limit or prohibit drilling, or easements.
All closed loop systems that are 20 feet or greater in depth and all open loop systems require the issuance of a private well construction permit before any loop drilling, trenching, or boring takes place. In Iowa, GHEX closed loop systems currently make up 25% of the private well construction permits issued. These systems require the installation to be done by a certified well contractor and the use of defined construction standards. Depending on the actual drilling location, there may be additional requirements put in place to help manage fragile geological conditions, confine localized contamination, and protect the state's drinking water aquifers.
Closed loop boreholes require minimum setbacks from other wells and sources of contamination. The current setbacks can be found at the following web link: Web link for GHEX borehole setbacks.
Heat Transfer Fluids Requirements
All heat transfer fluids used in ground heat loop applications must meet groundwater safety requirements as found in 567 Iowa Administrative Code Chapter 49.29(5). Essentially, pharmaceutical or food grade propylene glycol is required as the fluid. In addition, all heat transfer fluid additives needed to prolong fluid life or limit corrosion must be certified as NSF 60 safe. Never use ethylene glycol (common automotive radiator fluid) or other non-approved fluids or additives in your ground loop system as it constitutes a groundwater hazard that must be addressed at the loop owners expense. If you suspect your system needs fluids/antifreeze, call an authorized system service provider as adding the wrong fluid to the system will result in the need for total system flushing and fluid replacement, and repair costs that may run as high as thousands of dollars.
Before You Plan Your GHEX Project and Start to Drill GHEX Boreholes
It's important for you, the department, and the driller to know if the proposed GHEX borehole site can actually be drilled for the installation of GHEX loops. Even though all sites in our state have the potential for GHEX loop borehole systems to function properly, there are many sites in your area that shouldn't be drilled because of pre-existing conditions like onsite contamination.
One example is when your property is near one or more public water supply wells. If you have a public water supply well within 400 feet of your property, it may be difficult for you to install a GHEX systems due to the federally required setbacks between public water wells and private drilling activities. In addition, some communities may not allow the installation of GHEX boreholes due to local covenants, ordinances or other restrictions.
Additional examples are sites that are currently used for or were historically used for various industries. Iowa has a long history of industrial growth, technological advancements, and progress. This type of growth doesn't happen without the use of raw materials, chemicals, and fuels required by industry - things that are potential contaminants. One of the common issues that we encounter when looking at individual sites is that the site contains some form of contamination that may be limited to the footprint of the site or possibly even extend beyond property lines and onto nearby properties. When this happens it is difficult to find a location where the contamination does not pose a threat to the groundwater and/or the piping used in GHEX systems.
Here are some of the situations that may limit drilling on your site:
All proposed GHEX sites that may contain contaminants require a more thorough review and construction plan. There are even cases when drilling vertical boreholes may not be an option due to the type and extent of contamination on the property or adjoining properties near where the drilling is proposed. If detailed review is not performed in these scenarios, installation cost, inconvenience and liabilities increase for both the property owner and the drilling contractor and can ultimately result in increased monitoring requirements, fines and associated clean-up costs. It is important to remember that a GHEX system may not be the most economical long-term solution on a site that requires extensive review, greatly increased construction standards, ongoing monitoring requirements and questionable long-term durability of the system piping when exposed to contaminants.
It is the responsibility of the property owner to initiate the site review process and to work with a groundwater professional and the department to determine the actual risks of drilling at the proposed location. There may be properties that will not be able to utilize GHEX systems due to pre-existing site contamination issues.
The Karst Geology Variable
One additional consideration for the construction permitting and placement of GHEX loop borehole fields is Karst geology. Karst areas may be subject to additional requirements before the site can obtain construction permitting. In general, Karst sites require the use of more stringent construction standards to ensure that poor quality surface water and shallow groundwater do not reach the deeper protected drinking water supplies.
The additional standards can include a more detailed site survey and site analysis, geological investigation and profiling, the installation of test boreholes to collect and interpret geological and aquifer information, hydrogeological modeling, increasing setbacks from sources of contamination, setting specific minimum construction and grouting standards tailored to the geology and hydrology present under the construction site, and the use of long term monitoring to watch for ground settling and groundwater contamination.
When looking at the overall protections that all GHEX boreholes require, full depth grouting is one of the most important steps you can take to protect the groundwater resource. It also helps your geothermal system perform with high efficiency which reduces your total operational lifecycle costs. You should only hire contractors who agree to utilize full depth grouting for your installation as they're vested in providing a quality service that helps your system work as designed and helps protect the groundwater used by your family and your neighbors.
Additional information regarding Karst geology in Iowa can be obtained by contacting the department and asking to speak with one of our geologists, or by visiting our Karst website.
For additional information on GHEX systems please consider viewing the following pages:
- For more information contact -
Erik DayWater Supply Engineering Section Wallace State Office Building 502 E. 9th Street, Des Moines, IA 50319-0034 Phone 515-725-0237 Fax 515-725-8202