Geothermal and geo-exchange systems have moved from niche sustainability discussions to a mainstream development strategy. With that momentum comes a persistent challenge: separating strong strategy from oversimplified assumptions.
For developers, engineers, and owners, the question is no longer whether geothermal can work. It is how to make it work reliably, economically, and over the full life of the building. A webinar discussion between Diverso Energy and MCW underscored a clear lesson: geothermal cannot be designed in isolation. The ground loop, heat pumps, mechanical systems, auxiliary equipment, energy model, and operating strategy must be treated as one connected system.
Geothermal Is a Thermal Battery, Not a Standalone Piece of Equipment
A geothermal borefield does not have “capacity” in the same way a boiler, chiller, or heat pump does. It behaves more like a thermal battery. In the summer, the building rejects heat into the ground. In winter, it draws heat back out. Over time, the goal is to manage that balance, so the field remains within a healthy operating range.
This is especially important in large multifamily and mixed-use buildings, where a borefield may include dozens or hundreds of boreholes. Much of the energy transferred into the ground remains within the field. If the building is cooling dominant, the system needs a strategy for handling excess heat over time.
Successful geothermal design is not simply a matter of drilling enough boreholes. It requires a clear understanding of loads, temperatures, flow rates, auxiliary systems, and how the building is expected to perform in practice—not only in the model.
The Risk of Designing in Silos
A recurring issue in geothermal projects is that the ground loop and mechanical system are treated as separate scopes. A geothermal provider may focus on the borefield, while the mechanical engineer focuses on heat pumps, fan coils, risers, pumps, and heat exchangers. But these systems are deeply connected.
For example, a smaller borefield may appear to reduce capital cost. But if that decision forces lower operating temperatures, the impact can surface elsewhere: larger heat pumps, higher flow rates, larger piping, bigger pumps, increased electrical service, and higher operating costs. The cost has not been eliminated; it has been transferred.
That is why right-sizing matters. The strongest design is not automatically the smallest borefield or the most conservative mechanical plant. It is the solution that balances capital cost, efficiency, risk, peak capacity, long-term performance, and maintainability.
Glycol: Useful Tool or Long-Term Penalty?
Few geothermal design topics generate as much debate as glycol. In building loops, glycol can be added as an antifreeze solution, allowing systems to operate below the freezing point of water. This can expand the temperature range available to the system during cold conditions.
But glycol comes with trade-offs. It reduces heat transfer, increases viscosity, raises pumping energy, and can affect equipment capacity. In practice, that can mean larger terminal units, pumps, piping, and heat exchangers, along with higher operating cost and maintenance complexity.
The takeaway is not that glycol is inherently problematic. In certain applications, such as centralized plants or systems where glycol can be contained, it may be appropriate. But it should never be a default shortcut. Any design that relies on glycol must account for its full system impact.
The Incubation Period Matters
Another important concept is the “incubation period,” typically the first few years of operation. During this time, occupancy ramps up, systems are commissioned, loads stabilize, and the ground loop moves toward normal operating balance. Real operating data often shows a more gradual pattern than design assumptions suggest.
This period matters because some systems solve short-term concerns in ways that create long-term penalties. If glycol is added mainly to manage early low-temperature conditions, the building may carry the efficiency and maintenance penalty for decades. In some cases, a small auxiliary boiler or heat injection strategy may preserve long-term efficiency.
The objective is not to overbuild; it is to preserve flexibility. Monitoring, commissioning, and close coordination between the geothermal provider, mechanical engineer, owner, and operator are essential during the early years.
Auxiliary Systems Are Not a Failure
In the push toward decarbonization, some teams assume the goal must be eliminating boilers entirely. That may be possible in some projects, but it may not always be the best value. The last 10 to 20 percent of the load can be disproportionately expensive to electrify, particularly when domestic hot water or peak heating requirements are involved.
A pragmatic geothermal strategy may provide most space heating and cooling, meet ventilation loads, and preheat domestic hot water while retaining limited auxiliary capacity. This approach can achieve meaningful carbon and energy reductions without unnecessary complexity or cost.
For cooling-dominant buildings, domestic hot water preheat can be especially valuable. Instead of rejecting excess heat through a fluid cooler, a building can recover that heat for domestic hot water. This improves efficiency and helps manage borefield balance.
Energy Models Need Real-World Context
Energy modeling is essential, but not all models serve the same purpose. A model prepared for code compliance, financing, or incentives may not provide the operational insight needed to size and guarantee a geothermal borefield for decades.
Schedules, occupancy, internal gains, envelope assumptions, and user behaviour can all vary significantly. The best geothermal designs pair modeling with experience, historical performance, and a plan for adjustment during operation.
Collaboration Is the Real Best Practice
Geothermal design requires constant coordination between disciplines. Ground loop flow rates, building flow rates, heat exchanger temperatures, heat pump limits, boiler integration, domestic hot water strategies, structural constraints, drilling layout, and maintenance all interact.
True coordination is more than exchanging drawings. It means testing assumptions about temperatures, equipment performance, peak events, and how the system will be managed five, ten, or thirty years from now.
The Takeaway
Geothermal is one of the most effective tools available for reducing building energy use and carbon emissions, but it is not a plug-and-play solution. The most successful projects treat the borefield and building systems as one integrated design.
Right-sizing requires a full understanding of temperatures, loads, equipment performance, cost, risk, and long-term operation. Glycol, auxiliary boilers, domestic hot water heat recovery, and advanced heat pumps can all play a role—but only when applied with purpose and context.
The opportunity is not simply to make geothermal more common. It is to make geothermal better designed, better coordinated, and better understood—so buildings can achieve meaningful carbon reductions while remaining reliable, practical, and cost-effective for decades.
Considering geothermal for an upcoming development? Start with the whole building, not the borefield alone. Early collaboration can help identify the right balance between capital cost, operating efficiency, carbon reduction, and long-term asset value.