Open Loop Geothermal Drilling

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Open Loop Well System

Here on the Delmarva Peninsula, the two standard geothermal systems installed are open loop and closed loop systems. Customers who are doing their research and trying to determine the system that will work best for their project are still sometimes left without enough factual answers or information to make an appropriate decision. We receive many calls regarding this question, so I will address each system in detail in the hope that we can clear up misconceptions and provide the answers you need. Throughout our articles we will be using a 4 ton unit as our example unit, along with a 4” supply and recharge well.

Open loop systems have been around for many years on the Delmarva Peninsula. An open loop system uses water from a supply well and runs this water to the geothermal unit(s) in your house. The water is then returned back to the aquifer into a second well called a recharge well. These two wells must be a minimum of 50’ apart, which can sometimes be difficult on smaller lots. The supply well also must be at least 100’ away from the septic area. This is a classic system and is great if you need a new well anyway in the instance of replacing an old well or new construction with no existing well. A common question is, “We have an existing 4” well…can we use it as a geothermal supply well?” The answer is mostly about your own well attributes and depends on:

Well capacity (how many gallons per minute it will produce)?
How many gallons per minute do you need for both domestic water AND your geothermal unit?
Size of your existing pump?

Most 4” residential wells in this area are set up with a ˝ HP pump, that averages 10 – 12 gallons per minute, or GPM, and these gallons must be dedicated to your home usage, not your geothermal unit. Figuring gallons per minute usage for a geothermal unit is a rule of thumb: 3 gpm for every ton of load. Using our 4 ton unit example, your house would need an additional 12 gpm for a total of approximately 22 gpm coming from the well. Knowing that figure, we can tell that the ˝ HP pump will need to be upgraded to at least a 1 HP pump that will provide 25 GPM, so that water is effectively provided to both the unit and the house. Before any pump upgrades, though, a well capacity test should be done to accurately determine just how many gallons per minute your well is able to provide and to verify that upgrading your pump size is a proper move to make. (A test on well depth will also need to be performed as the recharge well being drilled is required by the State to return water into the same aquifer.)

Speaking of recharge wells, the water that comes out must go back. As easy as it sounds, replacing water back into a well can be risky. While the majority of recharge wells we have drilled here on the Peninsula have worked successfully, there are enough that have refused to accept the water back that we had to note this in our major concerns list. Sometimes even drilling an additional return well does not correct the problem. This is especially critical to note since Delaware and Maryland laws mandate how the water must be returned, so if you have no functioning return well, you will not be permitted to have an open loop system.

While open loop systems can be a great choice for new home construction with well needs, it is critical to get feedback from your well driller on your property location and whether it is within an area that has shown a minimal amount of issues for open loop systems.

Pros: Lower upfront cost for units over 5 tons

Cons: Mechanical parts failures

Water quality concerns such as bacterialized iron

Could be affected by drought

Recharge well may not accept water

Well may fail

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