How Open Loop Geothermal Works

Understand how open loop geothermal works, where it delivers the greatest benefits, and what determines whether it will succeed on your site.

Open loop geothermal systems use natural groundwater to provide heating and cooling, typically for large, energy-intensive buildings where consistent demand justifies the upfront work and cost.

At its simplest, the system abstracts water from an aquifer via a production borehole, passes it through a heat exchanger, and returns it to the ground via a reinjection borehole. Heat is either taken from the water (for heating) or transferred into it (for cooling). The groundwater itself never enters the building’s system it simply exchanges energy before being returned.

Unlike closed loop systems, there is no sealed circuit underground. You’re working directly with the natural water environment. That’s where both the opportunity and the risk sits.

For an open loop system to perform properly, the ground has to do the heavy lifting. It must provide sufficient yield, stable temperatures, and reliable hydraulic continuity between abstraction and reinjection. It also needs to sustain that performance over time, without depletion or interference. If those conditions aren’t met, a system may still be designed on paper, but it won’t perform in reality.

How Heat Transfer Works in Open Loop

The key component is the plate heat exchanger. This separates the groundwater from the building’s internal system while allowing thermal energy to pass between them. In heating mode, energy is extracted from the groundwater and upgraded via heat pumps. In cooling mode, excess heat is rejected back into the aquifer.

Because groundwater temperatures are relatively stable, this creates a consistent thermal baseline which is where open loop systems can outperform alternatives at scale. But that performance isn’t automatic; it depends entirely on how well a system is proven, designed and delivered against real site conditions.

Where Open Loop Works Best

Open loop systems tend to suit sites with high, stable energy demand and long operational horizons, places where performance gains justify the complexity. Typical examples include hospitals, universities, district heating schemes, and large commercial or industrial facilities.

These environments are more likely to accommodate the borehole infrastructure, licensing requirements, and testing needed to prove the resource properly.

How It Compares to Other Systems

All heat pump systems can look similar at a glance, but they behave very differently in practice. Air source systems are simpler to install but lose efficiency in colder conditions. Closed loop systems are more predictable but limited by the ground’s thermal conductivity, plus they can require significantly more space for potentially 100s of boreholes, this isn’t always available or feasible to access. Open loop systems, by contrast, use moving groundwater which can deliver significantly higher performance, but only where the conditions allow. And that’s the key point: open loop is not a default option, it’s a high-performance option where it’s viable.

What Actually Stops an Open Loop Scheme?

Most explanations stop at “this is how it works.” That’s the easy part. The harder but more important question is whether open loop can be delivered on a specific site.

Ground conditions are the first constraint. Yield may be insufficient, hydraulic connection between boreholes may not behave as expected, or water chemistry may introduce scaling and maintenance issues. Unknown ground conditions can also drive drilling risk and cost variability.

Regulation is the next hurdle. Abstraction and reinjection licences can limit what’s possible, and discharge consents during testing are often more restrictive than expected. Timelines for approvals don’t always align with programme pressures, which can stall projects before they properly begin.

Then there’s the site itself! Existing infrastructure, buried utilities, access constraints and operational environments all shape what can actually be built. Urban sites, in particular, introduce logistical and stakeholder challenges that aren’t visible at feasibility stage.

Finally, operational reality matters. Pumping requirements, long-term reinjection performance, and system degradation all need to be understood early rather than discovered after installation.

Start With What’s Deliverable

Open loop geothermal has huge potential. But it only works when the ground, the constraints and the delivery approach align. That’s why the most valuable early step isn’t designing a system. It’s understanding whether one can actually be delivered - properly, safely, and without unnecessary risk.

If you’re considering open loop geothermal, start with a clear view of what will work on your site. We can help you assess viability, understand the constraints, and explain what needs to happen next.

Book a feasibility discussion, or send us your site for an initial review and let’s see if open loop will work on your site.